"Applied Science, Faculty of"@en . "Electrical and Computer Engineering, Department of"@en . "DSpace"@en . "UBCV"@en . "Walton, John Thomas"@en . "2011-10-06T22:37:41Z"@en . "1964"@en . "Master of Applied Science - MASc"@en . "University of British Columbia"@en . "The design of a 16 word 25 bit tunnel-diode memory is described. The memory is word organized and employs destructive readout, the current change of state of the tunnel diodes being sensed. This arrangement requires only a resistor and tunnel diode for each bit stored.\r\nThe driver circuits for the memory serve three functions:\r\n1) to couple into the array the information to be stored,\r\n2) to supply dc biasing to the array and, 3) to sense the current transient on readout.\r\nLow impedance circuits are required, and two approaches are examined: the modified White emitter follower and the transformer coupled emitter follower. The former employs negative feedback to decrease its input impedance, while the latter employs a broadband transformer.\r\nThe design of the modified White circuit necessitates an examination of the properties of transistors in the 100 Megacycle frequency range. The characteristics of a few high frequency transistors are shown.\r\nThe transformer coupled circuit depends on the properties of the broadband transformer. These transformers are examined and a design technique for various current ratios is given.\r\nTwo sets of experimental results are described using 2X2 arrays to simulate the 16X25 memory. One employs 5 ma tunnel diodes, and the other 1 ma tunnel diodes. Using the 1 ma array with transformer input, successful operation with write pulses 10 nanoseconds wide is demonstrated."@en . "https://circle.library.ubc.ca/rest/handle/2429/37813?expand=metadata"@en . "A SMALL HIGH-SPEED TUNNEL-DIODE MEMORY by JOHN THOMAS WALTON B.A.Sc* The U n i v e r s i t y of B r i t i s h Columbia, 1 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE In the Department of E l e c t r i c a l Engineering We accept t h i s t h e s i s as conforming to the standards r e q u i r e d from candidates f o r the degree of Master of A p p l i e d Science Members of the Department of E l e c t r i c a l Engineering The U n i v e r s i t y of B r i t i s h Columbia MARCH 1964 In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department o r by h i s r e p r e s e n t a t i v e s . It i s understood t h a t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allowed without my w r i t t e n permiss ion . Department of E l e c t r i c a l Engineering The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada. Date March, 1964. ABSTRACT The design of a 16 word 25 b i t tunnel-diode memory i s d e s c r i b e d . The memory i s word organized and employs d e s t r u c t i v e readout, the current change of state of the tunnel diodes being sensed. This arrangement r e q u i r e s only a r e s i s t o r and tunnel diode f o r each b i t s t o r e d . The d r i v e r c i r c u i t s f o r the memory serve three functions} 1) to couple i n t o the array the i n f o r m a t i o n to be s t o r e d , 2) to supply dc b i a s i n g to the array and, 3) to sense the current t r a n s i e n t on readout. Low impedance c i r c u i t s are r e q u i r e d , and two approaches are examined: the modified White emitter f o l l o w e r and the t r a n s -former coupled emitter f o l l o w e r . The former employs negative feedback to decrease i t s input impedance, while the l a t t e r employs a broadband transformer. The design of the modified White c i r c u i t n e c e s s i t a t e s an examination of the p r o p e r t i e s of t r a n s i s t o r s i n the 100 Megacycle frequency range. The c h a r a c t e r i s t i c s of a few high frequency^ t r a n s i s t o r s are shown. The transformer coupled c i r c u i t depends on the p r o p e r t i e s of the broadband transformer. These transformers are examined? and a design technique f o r v a r i o u s current r a t i o s i s g i v e n . Two sets of experimental r e s u l t s are d e s c r i b e d using 2X2 arrays to simulate the 16X25 memory. One employs 5 ma tunnel diodes, and the other 1 ma tunnel diodes\u00E2\u0080\u009E Using the 1 ma array with transformer i n p u t , s u c c e s s f u l o p e r a t i o n with w r i t e pulses 10 nanoseconds wide i s demonstrated. i i ACKNOWLEDGEMENT The i n i t i a l work on t h i s t h e s i s p r o j e c t was s t a r t e d d u r i n g the Summer of 1962 while the author was employed at the Lawrence R a d i a t i o n Laboratory at Berkeley, C a l i f o r n i a . G r a t e f u l acknowledgement i s thus given to Mr. F.S\u00C2\u00AB Goulding, who i n i t i a l l y suggested the t o p i c , to Mr. L. S c o t t , and to Dr. L.B. Robinson, w i t h whom the author had many u s e f u l d i s c u s s i o n s . At The U n i v e r s i t y of B r i t i s h Columbia, the author i s g r a t e f u l f o r the encouragement and ideas given him by h i s s u p e r v i s o r * P r o f e s s o r P.K. Bowers, and f o r the a s s i s t a n c e given him by Dr# P. Noakes and other members of the department. F u r t h e r , he i s g r a t e f u l f o r the help he r e c e i v e d from h i s f e l l o w graduate students. Acknowledgement i s given to NRC f o r a Research A s s i s t a n t -ship from Block Term Grant BT-68 and f o r a Studentship h e l d i n 1962-3. i x TABLE OF CONTENTS Page Ab S \"b r c l C \"b \u00C2\u00AB..\u00C2\u00AB\u00C2\u00AB\u00C2\u00AB.. . . . . . . a . . . . . . . a a o a a a . a . a a a a . a a . . . LiS\"fc O f 111 U S t r a l ; ionS \u00E2\u0080\u00A2 a a . a . a a . e a . . . a a a a a a a \u00C2\u00AB . . \u00C2\u00AB \u00C2\u00BB * \u00C2\u00AB v L i S \"t O f Tab l e S ^ \u00E2\u0080\u00A2 a \u00C2\u00AB s . * a 6 a \u00C2\u00AB . . a . a a a a . a a a a a . a a a a a \u00C2\u00AB . \u00C2\u00AB \u00C2\u00AB V 1 1 1 AcknOWl 6CLgemen\"fc \u00C2\u00AB . \u00C2\u00AB o a o . a a a . a a a o o . a . . a a a \u00C2\u00AB \u00C2\u00AB a a . a s \u00C2\u00AB . . . 1 ^ 1 a I n't T O d .UC~bion \u00C2\u00A3 . e . . o \u00C2\u00AB a a a a a . a a a a \u00C2\u00AB a \u00C2\u00AB a a a a a a a a a a e \u00C2\u00AB * 1 1 \u00C2\u00AB1 De S i g n Problem a e a a a a \u00C2\u00AB . a a a a a a a a 8 \u00C2\u00AB a . \u00C2\u00AB a e e s * 1 1.2 Types of Memories ............. a \u00E2\u0080\u00A2 a . a . . . . \u00E2\u0080\u00A2 1 2. Tunnel\u00E2\u0080\u0094Diode Arrays . . . . . . . . . . . . . a * . . . a . a . . . . . 8 2.1 P h y s i c a l P r o p e r t i e s of Tunnel Diodes .... 8 2.2 The Tunnel Diode as a Memory C e l l ....... 8 2.3 Array C o n f i g u r a t i o n s .................... 12 2.4 DC Aspects of the Array ................. 14 2.5 Tunnel-Diode Switching C h a r a c t e r i s t i c s \u00E2\u0080\u00A2 \u00C2\u00AB 17 3. D r i v e r s f o r the Array .\u00C2\u00AB\u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00E2\u0080\u00A2..\u00C2\u00AB 20 3.1 D r i v e r Requirements ..................... 20 3.2 P o s s i b l e D r i v e r C i r c u i t s 22 4. The M o d i f i e d White E m i t t e r Follower . . . . . . . a . . 27 4.1 DC Co n s i d e r a t i o n s . . . a . . . . . . . . . . . . . . . . . . . 27 4.2 High\u00E2\u0080\u0094Frequency C o n s i d e r a t i o n s ........... 34 5. The Transformer Coupled E m i t t e r Follower ..... 44 5.1 DC Co n s i d e r a t i o n s ......................\u00C2\u00AB 44 5.2 HighMFrequency C o n s i d e r a t i o n s . a . . . . 46 o 6. Re s u l t s and Conclusions 50 6.1 Experimental Arrays ..................... 50 6.2 Comparison wi t h Other Devices 54 i i i Page 6.3 E x t e n s i o n of the Array ...... ............ 57 6.4 GoncilJlSiOnS . . . o o e o . e . e e . e o o o e o . o . a e . o . . . 58 Appendix I - Tunnel-Diode Switching C h a r a c t e r i s t i c s 60 Appendix I I - T r a n s i s t o r VHP Measurements ........ 64 A2.1 T r a n s i s t o r Parameters .................. 64 A2.2 C i r c u i t J i g Measurements ............... 71 Appendix I I I \u00E2\u0080\u0094 R e s i s t o r - C a p a c i t o r Performance . ... 73 A3al R e s i s t o r Measurements . . \u00C2\u00AB e .o ..<> o .. \u00C2\u00AB... a \u00E2\u0080\u00A2 73 A3.2 C a p a c i t o r Measurements ................. 75 Appendix IV \u00E2\u0080\u0094 Broadband Transformers ........ ..... 78 A4.1 I n t r O d U C t i O n o o A o . . . o \u00C2\u00AB o a c 0 \u00C2\u00AB . o . o s o o . o o a e . 78 A4.2 Operation of B i f i l a r Transformers ...... 79 A4.3 Autotransformer Representation ......... 79 A4.4 Transmission Line Representation ....... 83 A4.5 Experimental Work ...................... 86 A4.6 Conclusions . . . . . . . . . o o a o o A a o o o o o . . . . . . . 89 Appendix V \u00E2\u0080\u0094 MWEF Loop Gain . . . . . o . . . . . . . . . . . . . . . . . 90 Appendix VI \u00E2\u0080\u0094 2X2 Pulse Generator ................ 95 References . . . . . . . . . . . . . . . s o . . . . . . . . . . . . . . . . . . . . . . 100 i v LIST OF ILLUSTRATIONS Fi g u r e Page 1\u00C2\u00AB1 B i t Organized Array . . . . . . . . . . . . . . . . . a o . . . 3 1 . 2 Vo rd~ Organi zed. Array o . o . o o . . o o \u00C2\u00AB . . . o o a o o o \u00C2\u00AB 3 2.1 Nondestructive Voltage D e t e c t i o n . . . \u00C2\u00AB . \u00C2\u00AB . 0 10 2.2 T r a n s i s t o r ^ T u n n e l Diode C e l l . . . . . . . . . a . . . 10 2.3 D e s t r u c t i v e Voltage D e t e c t i o n ............ 11 2.4 Nondestructive Current D e t e c t i o n ......... 11 2.5 D e s t r u c t i v e Current D e t e c t i o n . . .... .\u00C2\u00AB ... . . . 12 2.6 P o s s i b l e A r r a y C o n f i g u r a t i o n s ....\u00C2\u00AB......\u00E2\u0080\u00A2 13 2.7 D e f i n i t i o n of Bin a r y States . . . . . a . . . . . . . \u00C2\u00BB 14 2.8 Tunnel\u00E2\u0080\u0094Diode Design C h a r a c t e r i s t i c s ...... 15 2.9 Tunnel\u00E2\u0080\u0094Diode E q u i v a l e n t C i r c u i t ........\u00C2\u00AB\u00C2\u00BB 17 2.10 Tunnel\u00E2\u0080\u0094Diode Switching Current . . . c o o . . . . * 18 3.1 Examination of Y D r i v e r Admittance ....... 21 3.2 Block Diagram of D r i v e r C i r c u i t s ......... 22 3.3 A Simple E m i t t e r Follower ...............o 23 3.4 Comparison of y ^ with Y^ . . a o . . . . . . . . . . . . 24 i. 3.5 E v o l u t i o n of the M o d i f i e d White Emitter F O 1 X O 3? 4 > \u00C2\u00AB a \u00C2\u00AB e # o o o e e o 0 0 0 0 0 0 0 o e o o o o o o e o o o a * 2 f) 3.6 X and Y M o d i f i e d White E m i t t e r Followers . 26 3.7 X and Y Transformer Coupled E m i t t e r F o i l OWe r S . \u00C2\u00AB . \u00C2\u00AB . o . a o . o . . o o . \u00C2\u00AB a o o o o o a . . . 0 0 . 0 0 0 26 4.1 The MWEF . . . . \u00C2\u00AB \u00C2\u00AB o \u00C2\u00AB . \u00C2\u00AB o . o o o . o . \u00C2\u00AB oo o . . . o . o o o \u00C2\u00AB . o 27 4.2 Power D i s s i p a t i o n as a F u n c t i o n of R 2 28 4.3 DC Output Voltage as a Fu n c t i o n of Temper-eft 113? 6 e \u00C2\u00AB 0 O \u00C2\u00AB \u00C2\u00AB \u00C2\u00AB # c o o 0 0 0 0 0 0 0 0 0 0 0 e o o o s 0 0 0 0 0 0 0 0 0 3 X 4.4 A Balance (X 3?cLlI* \u00C2\u00AB o a \u00C2\u00AB o o s o . 0 4 0 0 0 0 0 0 0 0 0 0 0 0 0 4 9 6 31 4.5 DC Compensated MWEF ........... o o . . . . . . . . . 32 4.6 Output Voltage S e n s i t i v i t y of the Y MWEF . 33 v F i g u r e Page 4.7 Output Voltage S e n s i t i v i t y of the X MWEF 33 4.8 The MWEF at High Frequencies ............ 35 4.9 T y p i c a l h ^ g Values ...................... 37 4*10 Current Gain f o r T d r i v e r of 1 ma Array . 39 4.11 L i m i t i n g Value of T d r i v e r Gain ......... 41 4.12 P o s s i b l e Detector C i r c u i t s .............. 42 5.1 X and Y TGEF . . . . . . o o o . . . . . . . . . . . . . . . . . . . 45 5.2 TCEF Admittance C h a r a c t e r i s t i c s ......... 47 5.3 Y. Drxver Gain . . . . . . . . . . . . o o a o o o o o o . . . . . . 48 6.1 Block Diagram of 2X2 Array .... ......\u00C2\u00AB\u00E2\u0080\u00A2 ... 51 6.2 Read\u00E2\u0080\u0094Write S i g n a l s f o r Si n g l e C e l l ...... 51 6.3 Read\u00E2\u0080\u0094Write S i g n a l s f o r 2X2 Array ........ 52 6.4 D r i v e r Arrangement f o r 5 ma Array ....... 52 6.5 Waveforms f o r 5 ma Array ................ 53 6.6 TCEF D r i v e r C i r c u i t s .................... 54 6.7 Waveforms f o r 1 ma Array . . . . . . . . . . . . . . 5 5 6.8 \"1\" and \"0\" Output Pulses ............... 55 6.9 F u l l \u00E2\u0080\u0094 W r i t e Pulse from Tektronix P u l s e r .. 57 A l . l Analogue Setup of Tunnel-Diode DEq ........ 61 A1...2 Analogue Setup f o r Generation of Input . * 62 A1.3 Tunnel\u00E2\u0080\u0094Diode Switching T r a j e c t o r i e s as Simulated on an Analogue Computer ....... 62 A2.1 Test Arrangement Block Diagram .......... 64 A2.2 Me asur ed h s o . o . o . o . o . o o o . 0 . 0 . 0 0 . 0 . 0 . . 65 f e A2.3 The [ y ] b Parameters f o r the 2N834 ....... 67 A2.4 The [y] Parameters f o r the 2N1143 ...*\u00C2\u00AB> 69 A2.5 Comparison of Gain from the y'.s and from Measurements . . . . e o . . o . o o o o . o . o . . . o . o o o . . 72 v i F i g u r e Page A3.1 R e s i s t o r Measurements 74 A3\u00C2\u00AB2 R e s i s t o r Mounting ..\u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00C2\u00AB..\u00C2\u00AB ...<>.... \u00E2\u0080\u00A2 74 A3\u00C2\u00AB3 C a p a c i t o r Measurements .-,. \u00E2\u0080\u009E \u00C2\u00BB\u00C2\u00BB<> .\u00C2\u00AB \u00C2\u00AB . 76 A3\u00C2\u00AB4 C a p a c i t o r Mounting ...\u00C2\u00AB.\u00C2\u00BB<,. \u00E2\u0080\u009E \u00E2\u0080\u009E o\u00C2\u00BB<> \u00E2\u0080\u009E <, \u00C2\u00BB \u00C2\u00AB \u00C2\u00BB\u00E2\u0080\u009E . o \u00C2\u00AB \u00C2\u00AB 75 A3\u00C2\u00AB5 C a p a c i t o r s i n P a r a l l e l 0000000000000000000 75 A4\u00C2\u00ABl 1:1 I n v e r t i n g Transformer o o o o o o o o o o o o o o o . 79 A4.2 Autotransformer E q u i v a l e n t of 1:1 Trans-A4.3 A 1:2 Current Transformer . o o o o o o o o o o o . . . . 80 A4.4 Examples of Autotransformer Design T G C lllT X C^ U G A o e e o o o e o o o o o o o o o o 00000000000040 8 1 A4.5 Response of 1:4 Transformer ...... . .. 82 A4.6 Response of 1:2 Transformer f o r DiffGrGn*fc f i n d i n g s o o o o oo o\u00C2\u00ABo o o o o o\u00C2\u00ABo o o o o o * \u00C2\u00AB 83 A4.7 1:2 Transformer with Transmission Line Ec^LlXVclX 6Il\"fc \u00C2\u00AB \u00C2\u00AB \u00C2\u00AB * o o o e * e oo ooeo oeoo o \u00C2\u00AB o o o o o o o \u00C2\u00AB \u00C2\u00AB 84 A4.8 T h e o r e t i c a l Response Curves <>... \u00C2\u00BBo .\u00C2\u00AB . \u00C2\u00AB \u00E2\u0080\u009E . . . 85 A4.9 Experimental Res u l t s o . o o o o o . o o o . o o ,<,<,. s \u00C2\u00AB . 87 A4.10 Comparison of T h e o r e t i c a l and Experimental Re S u i t s #. .\u00C2\u00AB\u00C2\u00AB..\u00C2\u00AB 0000 0000 00000000 0000 00000. 88 A 5 o1 S i m p l i f i e d MWEF . o o o o o o o o o o o e o o o o o o o o o o o . . 90 A5.2 MVEF f o r Gain C a l c u l a t i o n s .0000.00000. ... 91 A5.3 Current Gains from y Parameters . . . o o o o . . . 93 A6.1 2X2 Pulse Generator Block Diagram .....\u00E2\u0080\u00A2\u00C2\u00AB. 95 A6.2 A s t a b l e , Monostable. and B i s t a b l e C i r c u i t s 96 A6 o 3 PulS e Shape r . .000000000.00000000000000.00. 97 A6.4 OR and AND\u00E2\u0080\u0094GATES 0000 0.. o o o o o o o . \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00C2\u00AB \u00E2\u0080\u00A2 . \u00E2\u0080\u00A2 98 A6 o 5 Delay C i r C U i t e o o o . o o o 000000000000000000.0 98 A6.7 Waveforms from 2X2 Pulse Generator . . 0 , . . . 99 v i i LIST OF TABLES Table Page .2.1 Bias Arrangements ...o .oo... o\u00C2\u00AB. ...... .... 16 2.2 Current Requirements .................... 17. 2.3 Switching Current C h a r a c t e r i s t i c s ...... \u00E2\u0080\u00A2 19 4.1 R e s i s t o r Values i n Ohms ................. 29 4.2 Current and Power D i s s i p a t i o n Ranges .... 30 A2.1 Frequency at which j h^ g j = 1 ............ 66 v i i i 1. INTRODUCTION 1.1 Design Problem This t h e s i s w i l l be concerned with the development of a small high\u00E2\u0080\u0094speed storage u n i t . The s p e c i f i c a t i o n s f o r t h i s u n i t were decided upon while the author was employed at the Lawrence R a d i a t i o n Laboratory at Berkeley, C a l i f o r n i a . In general the s p e c i f i c a t i o n s f o r the store were t h a t i t was to have a r e a d -w r i t e time of l e s s than 100 nanoseconds and- to h o l d 16 words of 25 b i n a r y b i t s each. The intended use f o r the store was i n f a s t - c o i n c i d e n c e n u c l e a r i n s t r u m e n t a t i o n and the s p e c i f i c a t i o n s were determined to make the device compatible with e x i s t i n g equipment. Furthermore, thd storage i s to be only temporary as the i n f o r m a t i o n i s subsequently t r a n s f e r r e d to a slower permanent memory. The store thus i s r e q u i r e d to act as a b u f f e r between the randomly o c c u r r i n g s i g n a l s of the nucl e a r d e t e c t o r s and the slower permanent memory and,is consequently c a l l e d a b u f f e r s t o r e . The technology of d i g i t a l storage has advanced c o n s i d e r a b l y i n the past decade. In hand with t h i s t e c h n o l o g i c a l advance has been the advance i n terminology. Thus i n the f o l l o w -i n g s e c t i o n s of t h i s chapter a b r i e f review of a few of the aspects of d i g i t a l storage technology and terminology w i l l be gi v e n . Furthermore, j u s t i f i c a t i o n w i l l be shown f o r the design d e c i s i o n s t h a t r e s u l t i n the array of Chapter 2. 1 02 Types of Memories Memory arrays f o r the storage of d i g i t a l i n f o r m a t i o n are c l a s s i f i e d a c c ording to l ) the type of addressing scheme, 2) the method of readout* and 3) the type of memory c e l l used i n the a r r a y . The way i n which the l o c a t i o n i s determined f o r d i g i t a l i n f o r m a t i o n to be read i n t o or out of the memory-is c a l l e d a d d r e s s i n g . Two c o n c e p t u a l l y d i f f e r e n t modes e x i s t ; s e q u e n t i a l addressing, and c o n d i t i o n a l or random a d d r e s s i n g . With the former i n f o r m a t i o n i s ordered s e q u e n t i a l l y with r e s p e c t to time, while with the l a t t e r no such temporal o r d e r i n g i s necessary. Consequently, i n f o r m a t i o n s t o r e d i n a random addressing (random access) a r r a y can be readout i n a time sequence d i f f e r e n t than t h a t used f o r reading i n ; whereas with s e q u e n t i a l addressing the input and output sequences are always the same. Random access memories have been the subject of much developmental work i n the past decade. This has been prompted by the concept of the stored-program d i g i t a l computer where the prograjti and data are st o r e d i n the machine. With t h i s con-cept, access to any p a r t of the memory f a c i l i t a t e s the programming and the l o a d i n g of data. The i n f o r m a t i o n s t o r e d i n a d i g i t a l memory i s i n b i n a r y ( 0 , l ) coded form. A s i n g l e d i g i t of the b i n a r y code, (0 or l ) , i s c a l l e d a b i t and f o r a given memory a f i x e d number of b i t s comprise an e n t i t y c a l l e d a word. The b i n a r y coding r e q u i r e s elements having two d i s t i n c t s t a t e s and these elements are u s u a l l y c a l l e d memory c e l l s , c e l l s , or b i t s . The memory c e l l s can be wired i n two d i s t i n c t p a t t e r n s r e s u l t i n g i n the b i t - o r g a n i z e d a r r a y and the word-organized a r r a y . These are i l l u s t r a t e d i n Figure's 1 . 1 and 1 . 2 f o r equal 3 V A D D R E S S I N G - S E L E C T O R S fa = 4 .71 - Ul <\u00E2\u0080\u00A2 \ / r ^ ) c > <\u00E2\u0080\u00A2 \ f ) \ p I. f* > \ ) f ) K r ^ 1 f > > ^ / 1 f k /\u00E2\u0080\u00A2 ) \ \ t t i t > \ V. t ' ( ) <\u00E2\u0080\u00A2 1 ( ) \ > \ ' \ \ ( > \. ^ \ { \ / \u00E2\u0080\u00A2 \\ f > \ \ c J v * \ ) K \ t > \ \ f f ) \ f * \ \ ^ ) \ \u00E2\u0080\u00A21 r ) v. 1 f I ) ' c ) K 1 t ) y ) ( \ <. x . S f ) \ > C ) ^ \ r * \ \ \ ( ' \ XI6 J \u00E2\u0080\u00A2> / * \^ / ) \ 1 y > \ d ) \ >~ U- 16 V/OW.T3S W = <* B I T Lines F i g u r e 1*2. Word-Organized Array f o r N = 16 Words of\u00E2\u0080\u00A2w = 4 B i t s Each c a p a c i t i e s : N = 16 words of w = 4 b i t s each. In t h e - b i t organized array of F i g u r e 1,1, the memory c e l l s are arranged i n w pl a n e s . The f i r s t plane s t o r e s the f i r s t b i t of each word* the second plane the second b i t * and so on, To l o c a t e a word i n memory r e q u i r e s a coincidence between the X l i n e and T l i n e corresponding to the word. For example* to l o c a t e the word whose b i t s are stored i n the top l e f t corner of each plane, the X ( l ) l i n e and Y ( l ) l i n e must be a c t i v a t e d simultaneously by the addressing selectors,, The i n f o r m a t i o n i s then i n s e r t e d or withdrawn u s i n g the w b i t l i n e s . In the word-organized a r r a y of Figure 1.2, the b i t s of each word are st o r e d i n one h o r i z o n t a l row of c e l l s . The f i r s t b i t of each word i s s t o r e d i n the f i r s t column, the second b i t i n the second column* and so on. Here, however, no coincidence i s r e q u i r e d f o r l o c a t i n g a word i n the memory as the N X - l i n e s are t i e d d i r e c t l y to the N words i n storage. Thus a word i s l o c a t e d simply by a c t i v a t i n g i t s X l i n e * the i n f o r m a t i o n then being r e a d - i n or read\u00E2\u0080\u0094out on the v e r t i c a l b i t (or T) l i n e s * From the preceding i t i s seen that f o r N\u00E2\u0080\u0094word storage* a b i t - o r g a n i z e d memory r e q u i r e s 2jN addressing s e l e c t o r s while a word-organized memory r e q u i r e s N such s e l e c t o r s . -Consequently* f o r l a r g e systems* b i t o r g a n i z a t i o n i s u s u a l l y employed to reduce the number of addressing s e l e c t o r s . However, f o r small memories ( i . e . l e s s than 64 words^^) t h i s r e d u c t i o n does not warrant the i n c r e a s e d complexity r e q u i r e d f o r a c h i e v i n g the coinc i d e n c e s of the b i t - o r g a n i z e d a r r a y . As the work to be di s c u s s e d here i s concerned with the development of a small memory, the word-organized a r r a y i s p r e f e r r e d . 5 A second f e a t u r e i n the c l a s s i f i c a t i o n of an ar r a y i s the method of readout*, Readout i s the process of determining the i n f o r m a t i o n content of the memory c e l l s . Two techniques are p o s s i b l e : n o n - d e s t r u c t i v e and d e s t r u c t i v e readout. With non-d e s t r u c t i v e readout the memory c e l l s are u n a f f e c t e d by the read o p e r a t i o n . With d e s t r u c t i v e readout the c e l l s are a f f e c t e d as they are r e s e t to t h e i r quiescent s t a t e , the change of s t a t e , i f p resent, being d e t e c t e d . The type of readout d e s i r e d w i l l depend on the uses of the a r r a y . In some a p p l i c a t i o n s i t i s h i g h l y d e s i r a b l e to have no n \u00E2\u0080\u0094 d e s t r u c t i v e readout. However, with temporary storage d e s t r u c t i v e readout i s p r e f e r a b l e as i t e l i m i n a t e s a c y c l e r e q u i r e d f o r r e s e t t i n g the c e l l s . In the previous d i s c u s s i o n on word-organized a r r a y s , i t was po i n t e d out t h a t only the X l i n e s (see F i g . 2b) need be e x c i t e d to l o c a t e a word i n the a r r a y . This f e a t u r e , coupled with d e s t r u c t i v e readout* can l e a d to very short readout times as the c e l l s can be o v e r d r i v e n to achieve a r a p i d change of s t a t e * Thus i t i s seen t h a t a word-organized d e s t r u c t i v e r e a d -out a r r a y b e s t meets the requirements of a b u f f e r s t o r e . The t h i r d c h a r a c t e r i s t i c of a b i n a r y storage u n i t i s the memory c e l l . For each b i t to be s t o r e d , there must be a device e x h i b i t i n g two s t a b l e s t a t e s * These s t a t e s can be e i t h e r n a t u r a l states of the dev i c e * or s t a t e s induced by dc or ac b i a s i n g . Examples of the former are ferromagnetic ( f e r r i t e c ores, t h i n f i l m s ) and superconductive memories, while of the l a t t e r 5 t u n n e l - d i o d e and parametric memories. Of these devices the tunnel diode at the present time has 6 the f a s t e s t switching time* I t must he noted however t h a t the switching time alone does not determine the access time (time r e q u i r e d f o r a read-write c y c l e ) of a storage u n i t . A d d i t i o n a l time i s l o s t due to the time f o r propagation of i n f o r m a t i o n through the s t o r e , f o r a m p l i f i c a t i o n of s i g n a l l e v e l s . f o r r eading and w r i t i n g t r a n s i e n t s to subside and f o r addressing the memory matrix,, C o n s i d e r i n g the f i r s t two of these, along (2) with the switching time of the dev i c e , Rajachman v ' has shown that f o r memories of l e s s than 1024 words an access time of 10 nanoseconds i s u l t i m a t e l y p o s s i b l e f o r a tunnel-diode s t o r e . This i s a f a c t o r of t e n b e t t e r than that which could be achieved by a comparable s i z e d ferromagnetic or superconductive unit\u00C2\u00A9 However^ f o r l a r g e r storage u n i t s the magnetic devices tend to be s u p e r i o r as t h e i r smaller p h y s i c a l s i z e and methods of f a b r i c a t i o n begin to have a grea t e r i n f l u e n c e on the access time than does the switching time. Thus the tunnel diode i s s u i t a b l e f o r small high\u00E2\u0080\u0094speed memories and at present i s the only device capable of r e a d i l y a c h i e v i n g access times of l e s s than 100 nanoseconds. Prom the preceding d i s c u s s i o n i t i s thus apparent t h a t a word-organized, d e s t r u c t i v e readout, tunnel-diode memory would best meet the requirements of the b u f f e r s t o r e . The memories as shown i n F i g u r e s 1.1 and 1.2 c o n s i s t of the memory c e l l s and addressing s e l e c t o r s . The addressing s e l e c t o r s i n t u r n are composed of l o g i c c i r c u i t s , d r i v i n g c i r c u i t s , and a dc supply,, The l o g i c c i r c u i t s decode the command signalswhich give the address of the r e q u i r e d word. The d r i v e r c i r c u i t s i n s e r t (or withdraw) i n f o r m a t i o n i n t o (or from) the a r r a y , w h i l e t h e dc s u p p l y e s t a b l i s h e s t h e two r e q u i r e d s t a b l e s t a t e s o f t h e t u n n e l d i o d e . Of t h e s e , o n l y t h e d r i v e r c i r c u i t a n d t h e dc s u p p l y w i l l be c o n s i d e r e d i n t h i s t h e s i s -8 2. TUNNEL-DIODE ARRAYS This chapter \"will deal with the c h a r a c t e r i s t i c s of tunnel diodes and t h e i r a p p l i c a t i o n to d i g i t a l storage <> To t h i s end v a r i o u s c o n f i g u r a t i o n s of tunnel-diode c e l l s are examined* wi t h c o n c l u s i o n s drawn as to the optimum c o n f i g u r a t i o n from the p o i n t of view of minimum number of elements r e q u i r e d f o r a c e l l . The dc supply requirements f o r the array are then derived,, F i n a l l y , the s w i t c h i n g of a tunnel diode i s examined, and the bandwidth requirement of the a r r a y determined,, 2.1 P h y s i c a l P r o p e r t i e s of Tunnel Diodes In the i n v e s t i g a t i o n of the p r o p e r t i e s of h e a v i l y doped (3 V semiconductor pn j u n c t i o n diodes, E s a k i x i n 1957 developed a diode which e x h i b i t e d a v o l t a g e \u00E2\u0080\u0094 s t a b l e negative conductance r e g i o n . This c h a r a c t e r i s t i c r e s u l t s from a t u n n e l i n g of e l e c t r o n s across the pn j u n c t i o n . As t u n n e l i n g i s a quantum mechanical process, the e l e c t r o n s t r a v e r s e the junction,-) at the speed of l i g h t . Consequently the diode i s an extremely high-frequency d e v i c e , i t s frequency response i n f a c t being l i m i t e d at s e v e r a l Kilomegacycles by i t s j u n c t i o n c a p a c i t a n c e . F u r t h e r , as the diode does not depend on m i n o r i t y c a r r i e r s , i t s behaviour should be f a i r l y w e l l independent of temperature, r a d i a t i o n e f f e c t s and surface contamination,, 2.2 The Tunnel Diode as a Memory C e l l In u s i n g tunnel diodes as b i n a r y c e l l s there are b a s i c a l l y (4) three approaches. These are the Goto P a i r v , v o l t a g e d e t e c t i o n , 9 and c u r r e n t detection,, The s t a t i c c h a r a c t e r i s t i c s of the l a t t e r are shown i n Fi g u r e s 2*1 and 2 . 2 . I t should be noted that the c o n f i g u r a t i o n s d i s c u s s e d here are the b a s i c memory c e l l and would be repeated throughout the memory. The Goto P a i r was r e p o r t e d e a r l y i n the development of tunnel-diode memory u n i t s and presents the f e a t u r e s of non\" d e s t r u c t i v e readout and l o g i c g a i n ( i . e . a small input s i g n a l causes a large o u t p u t ) . The Goto P a i r has r e a l a p p l i c a t i o n as a memory c e l l only where l o g i c g a i n i s r e q u i r e d , and con-sequently w i l l not be used here,, Voltage d e t e c t i o n of the s i n g l e diode element a l s o lends i t s e l f to e i t h e r d e s t r u c t i v e or nondest r u c t i v e readout. Several schemes have been proposed f o r determining the voltage s t a t e of the tunnel diode i n a nondestructive manner. Among these are a combination of a diode and tunnel diode shown i n Fig u r e 2 . 1 a ^ ^ * and the tunnel d i o d e - t r a n s i s t o r combination of Figure 2 . 2 . Figure 2\u00C2\u00ABlb shows the l o a d l i n e s f o r the operations of reading and w r i t i n g as w e l l as the quiescent s t a t e of the diode c e l l . To determine the state of the tunnel diode nortde s t r u c t i v e l y , a small p r e c i s e current pulse i s a p p l i e d to the read p i n . In an array t h i s p i n i s al s o connected to an output t r a n s i s t o r . The current that flows i n t h i s t r a n s i s t o r depends on the state of the tunnel d i o d e 0 The a p p l i e d current pulse here must be small so that i t does not cause the tunnel diode to switch. The or d i n a r y pn diode of Figure 2 . 1 not only i s o l a t e s the tunnel diode from the r e s t of the array during the switc h i n g p e r i o d , but i t a l s o provides a n o n l i n e a r l o a d l i n e to accentuate the d i f f e r e n c e i n current l e v e l s of the two p o s s i b l e v o l t a g e s t a t e s . The scheme here i s 10 Figure 2\u00E2\u0080\u009E1, Nondestructive Voltage D e t e c t i o n c l a s s i f i e d as v o l t a g e d e t e c t i o n as i t i s the v o l t a g e across the tunnel diode which d e f i n e s the c u r r e n t flow i n the pn diode* \NR.irE PIN \u00E2\u0080\u00A2o READ PIN F i g u r e 2*2* T r a n s i s t o r - T u n n e l Diode C e l l The c i r c u i t of F i g u r e 2\u00E2\u0080\u009E2 i s i n essence the same as t h a t of F i g u r e 2\u00C2\u00BB1 with the t r a n s i s t o r r e p l a c i n g the pn diode. How-ever w i t h t h i s , the i n t e r r o g a t i o n of the tunnel diode would he made on the c o l l e c t o r of the t r a n s i s t o r thus e l i m i n a t i n g the need f o r a c a r e f u l l y r e g u l a t e d read pulse amplitude,, For d e s t r u c t i v e \" (6 ) readout of the v o l t a g e s t a t e , the c i r c u i t of Figure 2\u00C2\u00AB3 could be used, the i n d u c t o r i n p a r a l l e l with the tunnel diode d e t e c t i n g the v o l t a g e change. -o -o O U T P U T F i g u r e 2*3* D e s t r u c t i v e Voltage D e t e c t i o n The preceding c i r c u i t s have det e c t e d the s t a t e of the tunnel diode by observing i t s v o l t a g e c o n d i t i o n s C i r c u i t s can a l s o be designed to determine the c u r r e n t s t a t e of the tunnel diode i n e i t h e r a d e s t r u c t i v e or n o n d e s t r u c t i v e manner, The c i r c u i t of Figure 2*4 would enable a n o n d e s t r u c t i v e determination of the c u r r e n t s t a t e to be made, This c i r c u i t i s the c u r r e n t (7) e q u i v a l e n t of Figure 2,2* The c i r c u i t shown i n Figure 2*5 has a p p l i c a t i o n i n d e s t r u c t i v e readout employing current d e t e c t i o n . This c i r c u i t employs the i n d u c t o r to d e t e c t the c u r r e n t change of s t a t e * The c i r c u i t as shown i n Figure 2.5 c o n s i s t s of a tunnel F i g u r e 2.4. Nondestructive Current D e t e c t i o n 12 diode, a r e s i s t o r and an i n d u c t o r . I f , however, the c i r c u i t i s used i n a word-organized a r r a y (Figure 1.2), the i n d u c t o r need not be repeated f o r each c e l l of the a r r a y . This i s apparent i O U T P U T F i g u r e 2.5* D e s t r u c t i v e Current D e t e c t i o n since any change i n the tunnel\u00E2\u0080\u0094diode current must a l s o occur i n the T l i n e s of the a r r a y * Thus, f o r \" a word\u00E2\u0080\u0094organized cu r r e n t d e t e c t i n g a r r a y , the b a s i c memory c e l l c o n s i s t s of j u s t a r e s i s t o r and a tunnel diode* This c e l l has the l e a s t number of elements of the f o r e g o i n g * and consequently i s the one t h a t i s used i n the subsequent work* 2.3 A r r a y C o n f i g u r a t i o n s There are two c o n f i g u r a t i o n s t h a t could be used with c u r r e n t d e t e c t i o n i n a word-organized a r r a y . They are shown i n F i g u r e 2*6. For the arrangement of F i g u r e 2.6a (the E s a k i (8) A r r a y ) the X and X l i n e s r e q u i r e the same p o l a r i t y pulses f o r w r i t i n g . Furthermore,only one p o l a r i t y of dc b i a s i s i r e q u i r e d . However,on readout only one h a l f of the c u r r e n t change would flow i n the X l i n e s and t h i s i s a s e r i o u s drawback as the c u r r e n t change between s t a t e s i s small (e.g. about 0*5 ma f o r a 1 ma tunnel d i o d e ) * The c o n f i g u r a t i o n of Figure 2.6b r e q u i r e s two d i f f e r e n t dc supply v o l t a g e s and the d r i v e r s r e q u i r e opposite p o l a r i t y pulses f o r w r i t i n g . However, the e n t i r e c u r r e n t change now reaches the I l i n e s on readout. This i s the arrangement used i n the a r r a y . W 'BIT (Yl DRIVERS X2 X3 X5 > W O R D 5 (a) IS (b) F i g u r e 2\u00C2\u00BB6i, P o s s i b l e Array C o n f i g u r a t i o n s 2.4 The DC Aspects of the Array The memory to be designed here i s t h a t of 16 words of 25 b i t s each. One of the design q u a n t i t i e s to be determined i s the dc supply requirements of the a r r a y . For t h i s , the tunnel-diode dc c h a r a c t e r i s t i c s are now examined. The tunnel\u00E2\u0080\u0094diode c h a r a c t e r i s t i c , shown i n Fig u r e 2.7* has three p o i n t s of i n t e r s e c t i o n with the quiescent l o a d l i n e : A* B, and C. Of these, only A and B are stable, s t a t e s , C (9) being u n s t a b l e . ' Thus the s t a t e s A and B can be used to correspond to the (0,1) coding of the b i n a r y system. In s t a t e B the t o t a l power d i s s i p a t e d i n the diode and r e s i s t o r i s l e s s than i n s t a t e A. Thus s t a t e B w i l l be used f o r the 0 s t a t e , W R I T E t\u00C2\u00A3>r\V L I N E W R . \ T B C I N E F i g u r e 2.7* D e f i n i t i o n of Bi n a r y States since the diode spends most of i t s time i n t h i s s t a t e , being r e t u r n e d to i t by the read o p e r a t i o n . The diode then i s normally i n the B s t a t e * and the a p p l i e d v o l t a g e , V , must be 15 decreased to w r i t e a \"1\" i n t o the c e l l . The w r i t e procedure i n v o l v e s a coincidence between the X and T l i n e s of the word. This coincidence i s shown i n Figure 2.7 by drawing i n the h a l f -w r i t e and f u l l - w r i t e l o a d l i n e s . The read o p e r a t i o n i n v o l v e s i the X l i n e alone and i s shown as the read l o a d l i n e i n the f i g u r e . Since w i t h c u r r e n t d e t e c t i o n the l a r g e s t p o s s i b l e c u r r e n t change between s t a t e s A and B i s r e q u i r e d f o r ease i n d e t e c t i o n , the load l i n e should be as steep as the tunnel-diode c h a r a c t e r i s t i c s w i l l a l l o w . In t h i s r e s p e c t Germanium diodes are s u p e r i o r to S i l i c o n and Gallium Arsenide types. S i l i c o n diodes have a lower peak c u r r e n t to v a l l e y c u r r e n t r a t i o than Germanium, and Gallium Arsenide diodes have e x h i b i t e d r e l i a b i l i t y problems. Thus, Germanium tunnel diodes were used i n t h i s d e s i g n . Figure 2.8 shows the boundaries of the v a r i a t i o n i n the LN3713 1N3717 v l 58 mv 58 mv V 2 395 mv 395 mv V 3 315 mv 315 mv V 4 72 mv 72 mv h .975 ma 4. 58 ma .140 ma . 60ma H .075 ma .350 ma X4 1.075 ma 4.82 ma Figure 2.8. Tunnel-Diode Design C h a r a c t e r i s t i c s peak and v a l l e y c h a r a c t e r i s t i c s , d e f i n e d i n Figure 2.7, of the General E l e c t r i c 5 ma and 1 ma tunnel diodes. These boundaries are used to determine the l o a d i n g r e s i s t o r , R^, the quiescent a p p l i e d v o l t a g e , ^ aQ* ^ e V T ^ ^ e s i g n a l amplitude, V w , and the read s i g n a l amplitude, v r , a l l of which are i n t e r r e l a t e d . Thus,using F i g u r e 2.8 the f o l l o w i n g four equations can be w r i t t e n ; V a 0 + * VaO = + V l V a 0 - V2 - & VaO = + V 2 V a 0 - Vw + ^ V a 0 = H\ + v 3 V a 0 + V r - * VaO = URL + V where the values of i ^ , x^j e t c , are the values given f o r the boundary p o i n t s , and ^>VaQ i s the v a r i a t i o n i n the supply v o l t a g e . I f S^ aQ i s allowed to be 20 mv ( c f . S e c t i o n 4 . l ) , a ^ then the value s of Table 2.1 r e s u l t f o r the 1 ma and 5 ma diodes. I t should be noted that the values l i s t e d f o r v are r . minimum* A small v i s d e s i r a b l e so th a t the read s i g n a l does JEDEC NO. Peak Current (ma) Load Resistance (ohms) V a 0 < m v ) v (mv) wv ' v r(mv) 1N3713 1 645 666 322 86 1N37T7 4.7 130 676 334 77 Table 2.1. Bias Arrangements not a p p r e c i a b l y redude the s i g n a l from the tunnel diode. This i s examined f u r t h e r i n S e c t i o n 2.5. 17 The c u r r e n t r e q u i r e d f o r s t a t e s A and B i s obtained by-drawing the c a l c u l a t e d l o a d l i n e s on the dc c h a r a c t e r i s t i c s of the 1 ma and 5 ma tunnel d i o d e s . The r e s u l t s of t h i s are gi v e n i n Table 2.2 f o r a 16 word. 25 b i t a r r a y . Peak Current (ma) \"A\" State Current / Diode (ma) \"B\" State Current / Diode (ma) X Supply Current Range (ma) % Supply Current Range (ma) 1 0*975 0.48 12-24.6 7.7-15.6 4.7 4.58 2.8 70-115 44.8-73.4 Table 2*2. Current Requirements 2.5 Tunnel-Diode Switching C h a r a c t e r i s t i c s The tunnel diode and l o a d r e s i s t o r can be represented by the e q u i v a l e n t c i r c u i t of Fi g u r e 2.9^\"^.' Here L represents S the t o t a l s e r i e s inductance of the c i r c u i t , R the t o t a l s e r i e s s 777 Figure 2.9. Tunnel-Diode E q u i v a l e n t C i r c u i t r e s i s t a n c e , C the junction, c a p a c i t a n c e , and f ( v ) the s t a t i c i - v c h a r a c t e r i s t i c s of the tunnel diode. This c i r c u i t i s d e s c r i b e d by the f o l l o w i n g d i f f e r e n t i a l equations; 18 - r r = V - R l - v dt a s Cdv . \u00E2\u0080\u009E/ N dt = 1 \" f ( v ) where i s the dc b i a s v o l t a g e and v o ( t ) i s the input s i g n a l . a Using a Donner Analogue Computer these equations were solved f o r the change i n current t h a t . o c c u r s when the tunnel diode i s switched by a step input from, the \"1\" s t a t e to the \"0\" s t a t e . The d e t a i l s of t h i s computation are g i v e n i n Appendix I . Fi g u r e 2.10 shows the s w i t c h i n g c u r r e n t as a f u n c t i o n of time, with the magnitude of the : i n p u t , YQ , as a parameter. In each case here the r i s e time of the input was the same, about 15 nanoseconds. I f these curves are now approximated by s t r a i g h t l i n e s as shown on the 0.1 v o l t curve, then the v a l u e s S W I T C H IN& C U R R B M T v.o T I M E \M M A N O S E C O N D S Figure 2.10. Tunnel\u00E2\u0080\u0094Diode Switching Current l i s t e d i n Table 2.3 r e s u l t . From t h i s t a b l e i t i s seen that as the input s i g n a l v o l t a g e i s i n c r e a s e d , the switching time^T i s decreased. However^so a l s o i s the output s i g n a l c u r r e n t . The former e f f e c t i s due to the f a c t t h a t the rate of change of the tunnel-diode v o l t a g e i s p r o p o r t i o n a l to the d i f f e r e n c e between the s i g n a l c u r r e n t and the tunnel-diode c h a r a c t e r i s t i c The l a t t e r e f f e c t i s due to the input s h i f t i n g the second stable p o i n t , B. Input AI = Max I - Min I (ma) T ( nanoseconds) 0.1 0.54 3,3 0.2 0.44 3.0 0.3 0.40 2.5 Table 2.3. Switching Current C h a r a c t e r i s t i c s The bandwidth r e q u i r e d to tra n s m i t the cur r e n t s i g n a l shown i n Fig u r e 2.10 i s of the order of 0.35/T where T i s d e f i n e d i n the f i g u r e . Thus^using the values given i n Table 2 i t i s seen t h a t a 3 db bandwidth of the order of 110 - 140 Megacycles i s r e q u i r e d f o r t r a n s m i s s i o n of the c u r r e n t s i g n a l , with some response up to 300 Megacycles, to ensure smooth r o l l - o f f . 20 3. DRIVERS POR THE ARRAY In t h i s chapter the c h a r a c t e r i s t i c s r e q u i r e d of the d r i v e r c i r c u i t s are g i v e n . Prom these the c o n c l u s i o n i s drawn t h a t the d r i v e r s should be some form of an e m i t t e r - f o l l o w e r c i r c u i t . Two v a r i a t i o n s of an e m i t t e r \u00E2\u0080\u0094 f o l l o w e r c i r c u i t are proposed f o r f u r t h e r study. These are i n v e s t i g a t e d i n Chapters 4 and 5. 3.1 D r i v e r Requirements The fundamental requirement of the d r i v e r c i r c u i t s i s the c o u p l i n g i n t o the a r r a y of the b i n a r y i n f o r m a t i o n to be s t o r e d . Por the tunnel\u00E2\u0080\u0094diode a r r a y t h i s means that the d r i v e r s should have a low output impedance. Any impedance due to the d r i v e r s w i l l have the e f f e c t of decreasing the steepness of the load l i n e on the tunnel-diode c h a r a c t e r i s t i c , and thus r e q u i r e a l a r g e r d r i v e v o l t a g e . Therefore, f o r w r i t i n g i n t o the a r r a y , the d r i v e r s should be v o l t a g e sources ( i . e . have zero output impedance)\u00E2\u0080\u00A2 A second requirement w i l l apply to the Y d r i v e r s only* they should act as d e t e c t o r s . That i s the Y d r i v e r s should be designed to have some means of d e t e c t i n g the c u r r e n t t r a n s i e n t which occurs i f the tunnel diode switches d u r i n g readout. This means the i n p u t admittance l o o k i n g from the array i n t o the Y d r i v e r should be g r e a t e r than the t o t a l admittance of the other c e l l s t i e d to that d r i v e r . Figure 3.1 shows what i s i n v o l v e d . Consider a c u r r e n t change in,say,the (XI, Y l ) c e l l of the f i g u r e . When t h i s c u r r e n t step reaches the p o i n t A, i t sees the 21 W O R D X> W O R D X7 F i g u r e 3 . 1 . E x a m i n a t i o n o f Y D r i v e r A d m i t t a n c e i n p u t a d m i t t a n c e -of t h e Y d r i v e r , y ^ , s h u n t e d b y t h e a d m i t t a n c e o f t h e o t h e r c e l l s t i e d t o t h e Y I l i n e . U s i n g t h e l o a d r e s i s t o r s d e r i v e d i n S e c t i o n 2 . 4 and n e g l e c t i n g t h e i m p e d a n c e o f t h e t u n n e l d i o d e s t h e m s e l v e s , t h e a d m i t t a n c e o f t h e o t h e r c e l l s t i e d t o t h e Y I l i n e w o u l d . b e o f t h e o r d e r o f 1 5 x l / 6 8 0 = 22 m mhos f o r t h e 1 ma a r r a y a n d 1 5 x 1 / 1 3 0 = 115 m mhos f o r t h e 5 ma a r r a y . (The a c t u a l m e a s u r e d a d m i t t a n c e i s g i v e n i n F i g u r e 3 . 4 ) . I d e a l l y , f r o m t h e p o i n t o f v i e w o f e l i m i n a t i n g c r o s s t a l k , ( i . e . s w i t c h i n g o f c e l l s o t h e r t h a n t h e d e s i r e d o n e s ) t h e i n p u t a d m i t t a n c e s h o u l d be i n f i n i t e . T h u s , f o r i n s e r t i n g a n d w i t h d r a w i n g i n f o r m a t i o n f r o m t h e a r r a y t h e d r i v e r a d m i t t a n c e s h o u l d be s e v e r a l h u n d r e d m m h o s . F i n a l l y , i t w o u l d be d e s i r a b l e t o h a v e t h e d r i v e r c i r c u i t s s u p p l y t h e dc b i a s i n g f o r t h e a r r a y . M o s t t r a n s i s t o r s r e q u i r e 5 -20 ma dc b i a s f o r opt imum o p e r a t i o n , a n d t h i s c u r r e n t c a n t h e n be u s e d f o r t h e a r r a y b i a s i n g . T h i s scheme p l a c e s some dc r e s t r i c t i o n s on t h e t r a n s i s t o r s t o be u s e d , b u t a p p e a r s t o be t h e m o s t e f f i c i e n t f r o m t h e p o i n t o f v i e w o f t o t a l c u r r e n t r e q u i r e d f o r the memory. T h i s , however, w i l l r e q u i r e that each d r i v e r have good dc s t a b i l i t y and i n a c h i e v i n g t h i s s t a b i l i t y (see S e c t i o n 4 . l ) some current e f f i c i e n c y i s l o s t . Furthermore, f o r some of the d r i v e r s , p a r a l l e l t r a n s i s t o r s w i l l be needed to meet the cu r r e n t demands. I t i s , t h e r e f o r e , not c e r t a i n t h a t t h i s c r i t e r i o n w i l l l e a d to more economical design than one i n which a l l or p a r t of the dc b i a s i n g i s obtained from a separate supply. B i a s i n g of the ar r a y r e q u i r e s that a p o t e n t i a l d i f f e r e n c e , , e x i s t between the X and T l i n e s . To conserve c u r r e n t , the p o t e n t i a l of the 25 Y l i n e s was made zero. The p o t e n t i a l of the 16 X l i n e s was then set at -V , t h i s being d i c t a t e d by the type o f . t r a n s i s t o r employed f o r the Y d r i v e r . The d r i v e r c i r c u i t s can thus be represented i n diagram form as shown i n Figure 3*2* O V J T f g T T F R O M WE.VtOF?Y COIAMAtlP C I R C U I T S X DJ5WBR S I G N A L S T O D C S U P P L Y <~^0 V O L T S j COMM4.V1C1 F R O I ^ L O & l t cvvuruiTS D R I V E I ? OUTPUT P R f l W CELLS -j^t\u00E2\u0080\u0094 -5te.MA.LS TO cei-L<> D C su^PWY ( a V O U T S ) F i g u r e 3.2. Block Diagram of D r i v e r C i r c u i t s 3.2 P o s s i b l e D r i v e r C i r c u i t s The requirement of hig h input admittance l e d to the c o n s i d e r a t i o n of e m i t t e r - f o l l o w e r c i r c u i t s . A simple emitter f o l l o w e r i s shown i n F i g u r e 3.3. I f the r e s i s t a n c e of the base, , i s small then the input admittance i s e s s e n t i a l l y t h a t of I N P U T F R O M SVM5VLE ceUL F i g u r e 3.3. A Simple E m i t t e r Follower a grounded-base stage, y^]-,* Figure 3.4 the magnitude of the i n p u t admittance to the grounded-base stage of a t y p i c a l VHF t r a n s i s t o r i s shown as a f u n c t i o n of frequency. For comparison, the input admittance to 15 tunnel diode c e l l s , y^ , i s a l s o shown. From the f i g u r e i t i s seen t h a t the input admittance to the grounded-base stage i s comparable at h i g h f r e q u e n c i e s to the admittance of the 15 diode c e l l s f o r the 1 ma a r r a y , and i s about h a l f t h a t of the 5 ma a r r a y . In the 5 ma case i t would be p a r t i c u l a r l y expedient to i n c r e a s e the input admittance of the d r i v e r . Even with the 1 ma a r r a y about h a l f of the h i g h f r e -quency components are l o s t . A t t e n t i o n was t h e r e f o r e turned to methods of i n c r e a s i n g the input admittance of the simple emitter f o l l o w e r . .One c i r c u i t t h a t was examined was a t r a n s i s t o r v e r s i o n of the White Cathode F o l l o w e r ^ of F i g u r e 3.5a. A second c i r c u i t examined was the transformer coupled emitter f o l l o w e r c i r c u i t of F i g u r e 3.7. 24 F i g u r e 3.4. C o m p a r i s o n o f w i t h y ^ F i g u r e 3.5 shows t h e e v o l u t i o n o f t h e m o d i f i e d W h i t e e m i t t e r f o l l o w e r (mwef) f r o m t h e W h i t e C a t h o d e F o l l o w e r a n d t h e W h i t e e m i t t e r f o l l o w e r ( w e f ) shown i n F i g u r e 3.5b. The m w e f ^ 1 2 ^ i s s i m p l e r t h a n t h e w e f , a n d h a s an a d d e d a d v a n t a g e : t h e b i a s c u r r e n t s i n QI a n d Q2 m u s t b o t h f l o w i n t h e l o a d . T h u s , f a i r l y l a r g e ( i . e . 20 - 100 ma) q u i e s c e n t c u r r e n t s c a n be o b t a i n e d f r o m t h e mwef . 25 W h i t e C a t h o d e W h i t e E m i t t e r M o d i f i e d W h i t e F o l l o w e r F o l l o w e r E m i t t e r F o l l o w e r F i g u r e 3 .5 . E v o l u t i o n o f M o d i f i e d W h i t e E m i t t e r F o l l o w e r A s w i l l be shown i n S e c t i o n 4.2, t h e i n p u t a d m i t t a n c e t o t h e mwef i s a p p r o x i m a t e l y ( l + h f ^ ^ i b i ' w h e r e n f e 2 ^ s c o m m o n - e m i t t e r s h o r t - c i r c u i t c u r r e n t g a i n o f Q2, a n d y ^ i i s \"the g r o u n d e d - b a s e i n p u t a d m i t t a n c e o f QI. S i n c e t h e Y d r i v e r a c t s as t h e d e t e c t o r , i t s h o u l d h a v e a l a r g e i n p u t a d m i t t a n c e o v e r t h e b r o a d e s t b a n d w i d t h p o s s i b l e . , T h e r e f o r e . , t h e Q2 t r a n s i s t o r o f t h i s d r i v e r s h o u l d be t h e b e s t . A s a t t h e p r e s e n t t i m e PNP t r a n s i s t o r s h a v e b r o a d e r b a n d w i d t h s t h a n N P N , Q2 o f t h e Y d r i v e r s h o u l d be P N P . H e n c e t h e Y a n d X m w e f 1 s a r e as shown i n F i g u r e 3.6. The o u t p u t s i g n a l f r o m t h e d r i v e r i s t a k e n f r o m t h e c o l l e c t o r o f Q2 o f t h e Y mwef b y u s i n g e i t h e r a t r a n s f o r m e r o r a l u m p e d d e l a y l i n e * C h a p t e r 4 d e a l s w i t h t h e s e c i r c u i t s i n d e t a i l . The s e c o n d m e t h o d i n v e s t i g a t e d o f i n c r e a s i n g t h e i n p u t 2fe O U T t P U T T O A R R A Y (a) Y mwef F R O M K R R A Y / F i g u r e 3.6, X and Y M o d i f i e d White Em i t t e r Followers admittance i s shown i n Figure 3,7. This uses the simple emitter r f o l l o w e r c i r c u i t and a broadband transformer. Here the i n p u t -2 admittance i s i n c r e a s e d by n , where n i s the e f f e c t i v e turns r a t i o of the transformer* These c i r c u i t s are i n v e s t i g a t e d i n Chapter 5 , with d e t a i l s of the transformers given i n Appendix IV, -*v, O U T P U T F R O M A R R A Y Figure 3.7. X and Y Transformer-Coupled E m i t t e r F o l l o w e r s ( t c e f ) 27 4. THE MODIFIED WHITE EMITTER FOLLOWER This chapter w i l l d e al with the modified White emitter f o l l o w e r (mwef) as shown i n Fi g u r e 4,1\u00C2\u00BB The chapter begins w i t h a d i s c u s s i o n of the dc c h a r a c t e r i s t i c s of the mwef. The high-frequency c h a r a c t e r i s t i c s are then examined, the bandwidth l i m i t a t i o n s of the c i r c u i t being i n d i c a t e d . 4,1 DC C o n s i d e r a t i o n s As shown i n Table 2\u00C2\u00BB2 of Chapter 2, the cur r e n t requirements of the array v a r y over a wide range. The power d i s s i p a t e d i n the t r a n s i s t o r s of a d r i v e r w i l l thus be dependent on the b i a s c u r r e n t r e q u i r e d by the a r r a y . This power dependence w i l l now be examined f o r the mwef. Figur e 4 \u00E2\u0080\u009E 1 0 The MWEF High loop c u r r e n t g a i n i n the c i r c u i t of Figure 4,1 r e q u i r e s R 1 \u00C2\u00BB R 2 \u00C2\u00B0 However, good dc s t a b i l i t y of the Q 2 t r a n s i s t o r l i m i t s R-^ \u00E2\u0080\u0094 IOR20 Hence i n the f o l l o w i n g R-^ = 10R2\u00C2\u00BB Since R^>-R2\u00C2\u00AB and since both t r a n s i s t o r s r e q u i r e b i a s c u r r e n t s of the same order of magnitude, V, should be grea t e r t h a n V2* H o w e v e r , a v a i l a b l e power s u p p l i e s l i m i t e d t h e s i z e of t h e r a t i o V j A g t o V 1 / V 2 = 2 . t i POVYEFS. D I S S I P A T I O N iH C I I L U l V V A T T i 500, F i g u r e 4 . 2 . Power D i s s i p a t i o n as a F u n c t i o n o f R 2 F i n a l l y , t h e t r a n s i s t o r s u s e d h a d a minimum o p e r a t i n g c o l l e c t o r v o l t a g e , V * o f a b o u t 6 v o l t s . F o r s m a l l power d i s s i p a t i o n t h e n , t h e maximum o u t p u t c u r r e n t , l Q m > s h o u l d o c c u r when V = V \u00E2\u0080\u009E T h u s , w i t h R, = 1 0 R o , V , = 2 V 0 , a n d I n = I - a t c c m > 1 2 1 2 ' 0 0m V c = V c m , t h e t r a n s i s t o r power d i s s i p a t i o n c a n be e x p r e s s e d as P t = - 9 1 I 0 . B 2 [ X 0 m ~ X o ] + T0 V c m F r o m t h i s i t i s s e e n t h a t t h e l o w e s t p o w e r d i s s i p a t i o n o c c u r s when R 2 = 0 . H o w e v e r , f o r t h e t r a n s i s t o r s u s e d , t h e b a s e r e s i s t a n c e , ^ , was a b o u t 70 o h m s . H i g h - f r e q u e n c y g a i n r e q u i r e s 29 Rj> ( i . e . R^ i s by\u00E2\u0080\u0094passed at high f r e q u e n c i e s ) . Hence R-^ / should be of the order of 1 Kohm which means R2 should be about 100 ohms. The e f f e c t of R^ on the power d i s s i p a t i o n i s shown i n Fig u r e 4.2 f o r the 1 ma T mwef. As i s seen the v o l t a g e V 2 i s dependent of the R,^ v a l u e . The v o l t a g e i s , of course, a l s o dependent on the value of the maximum output c u r r e n t . For the four d r i v e r c i r c u i t designs r e q u i r e d ( i . e . two f o r the 5ma ar r a y and two f o r the 1 ma a r r a y ) , w i t h R^ = 100 ohms, the four v a l u e s range from 6.8 to 15 v o l t s . Consequently, f o r design convenience was chosen as 12 v o l t s (and hence V^ = 24 v o l t s ) . The r e s u l t i n g r e s i s t o r v a l u e s are giv e n i n Table 4.1, while the current and power d i s s i p a t i o n ranges are given i n Table 4.2. 1 ma tunnel diode 5 ma tunnel diode Xmwef Tmwef Xmwef Ymwef R l 2.7K 4.7K 680 IK R 2 330 470 68 100 Table 4.1. R e s i s t o r Values i n Ohms From Table 4.2 i t i s seen that i n the 5 ma array (both i n the X and T mwefs) Q2 must be a p a r a l l e l combination of t r a n s i s t o r s to s a t i s f y the power d i s s i p a t i o n l i m i t of about 180 m i l l i w a t t s (see Appendix I I ) . 30 D i s s i p a t i o n i n M i l l i w a t t s D r i v e r I 0(ma) Ql Xmwef 1 ma Max 15.6 31 75 Min 7.7 25 44 Xmwef 1 ma Max 24 .6 52 120 Min 12.0 41 67 Ymwef 5 ma Max 73.4 146 3 52 Min 44.8 118 260 Xwmef \u00E2\u0080\u00A2 5 ma Max 115 220 186 550 405 Min 70 Table 4.2. Current and Power D i s s i p a t e d Ranges One f u r t h e r dc c o n s i d e r a t i o n i s the s e n s i t i v i t y of the output v o l t a g e of the mwef to changes i n temperature and supply v o l t a g e . This aspect w i l l now be examined: the r e s u l t s g i v e n here w i l l a l s o apply to the t c e f of Chapter 5. Examination of the mwef c i r c u i t shows that the output v o l t a g e i s d e f i n e d by a source v o l t a g e plus the base\u00E2\u0080\u0094emitter j u n c t i o n v o l t a g e of Q^* Since t h i s j u n c t i o n v o l t a g e i s known to vary at approximately \u00E2\u0080\u00942mv/\u00C2\u00B0C f o r Ge and -2,3mv/\u00C2\u00B0C f o r S i , ^ 1 \" ^ the output v o l t a g e w i l l be temperature dependent. Fi g u r e 4.3, shows t h i s f o r both the X and T mwefs. Also shown i s the temperature dependence of the output v o l t a g e f o r anemitter f o l l o w e r . The mwef v a r i a t i o n i s grea t e r at higher temperatures duq to c o l l e c t o r leakage c u r r e n t e f f e c t s . The r e s u l t s of 31 Fi g u r e 4.3. DC Output Voltage as a F u n c t i o n of Temperature Fi g u r e 4.3 c l e a r l y c a l l f o r some scheme to reduce the output v o l t a g e v a r i a t i o n . To t h i s end the balanced\u00E2\u0080\u0094pair c i r c u i t of Figu r e 4.4 was used. Examination of t h i s c i r c u i t shows that F i g , 4.4. A Balanced P a i r 33 i f i s d r i v e n from a v o l t a g e source v n , then the volt a g e at the base of Q2\u00C2\u00AB, v 2 , i s given by the f o l l o w i n g ; KT T I e 2 ^ KT , Ieo2 ^_ v~ = \u00E2\u0080\u0094 - I n 7 \u00E2\u0080\u0094 + \u00E2\u0080\u0094 In ^ + v, , 2 q I , q I , l y u e l ^ eo1 where I i s the emitter leakage c u r r e n t . T h u s , i f equal c u r r e n t s flow i n both t r a n s i s t o r s and the t r a n s i s t o r s are i d e n t i c a l , then v\u00C2\u00BB = v, independent of temperature. (a) (b) Figur e 4\u00E2\u0080\u009E5. DC Compensated MWEF The e f f e c t of t h i s s t a b i l i z i n g network can be analyzed by c o n s i d e r i n g the v o l t a g e g a i n around the loop of Q^-Q^a The negative feedback of the loop reduces any e r r o r v o l t a g e by a f a c t o r of l/(l + where A v i s the magnitude of the loop v o l t a g e g a i n . This loop gain was c a l c u l a t e d and checked e x p e r i m e n t a l l y . For the T mwef the g a i n was found to be of the order of 100, while f o r the X mwef of the order of 65. The d i f f e r e n c e i n 33 Fig u r e Output Voltage S e n s i t i v i t y of the I MVEF Fig u r e 4,7. Output Voltage S e n s i t i v i t y of the X MWEF 34 these two gains i s due to the d i f f e r e n c e s i n the h ^ of the t r a n s i s t o r s employed. As shown i n Figure 4.3 the \"compensated\" output c h a r a c t e r i s t i c s are much more s a t i s f a c t o r y , though v a r i a t i o n s are s l i g h t l y l a r g e r than the v o l t a g e g a i n c a l c u l a t i o n s would p r e d i c t . This could he due to an unbalance i n the c h a r a c t e r i s t i c s of the t r a n s i s t o r s i n the dc p a i r . The v a r i a t i o n of the output v o l t a g e f o r changes i n l o a d c u r r e n t and f o r changes i n supply v o l t a g e was a l s o examined. For l o a d c u r r e n t changes from 5 ma to 30 ma i n both the X and Y mwef c i r c u i t s the output was observed to change l e s s than 3 mv. The s e n s i t i v i t y of the mwefs to supply-voltage changes i s shown i n F i g u r e s 4.6 and 4.7* The extreme s e n s i t i v i t y of the output v o l t a g e to v a r i a t i o n s i n the \u00E2\u0080\u009412 v o l t supply i n Fig u r e 4.7 i s due to the f a c t t h a t the base vol t a g e of i s d e f i n e d by a simple r e s i s t a n c e c h a i n . This s e n s i t i v i t y due to the -12 v o l t supply could be e l i m i n a t e d i f a zener diode were used to d e f i n e the v o l t a g e on the base of Q^. The Y mwef does not e x h i b i t t h i s dependence as the base of there i s t i e d to ground. The preceding d i s c u s s i o n on dc s t a b i l i t y a p p l i e s p r i n c i -p a l l y to the d r i v e r s f o r the 1 ma a r r a y , although the v a r i a t i o n s f o r the 5 ma mwefs were e s s e n t i a l l y the same. Thus^from the r e s u l t s i t i s seen that a + 10 mv t o l e r a n c e can be achieved* and i n f a c t can be b e t t e r e d i f the vo l t a g e on the base of i s a c c u r a t e l y d e f i n e d . 4.2 High-Frequency C o n s i d e r a t i o n s \ \ In S e c t i o n 2.5 the bandwidth requirement of the t u n n e l -diode a r r a y was given as about 300 Megacycles. As l i t t l e i n f o r m a t i o n i s a v a i l a b l e about the performance of t r a n s i s t o r s , r e s i s t o r s , and c a p a c i t o r s i n t h i s frequency range, i t vas found necessary to make measurements on the elements to determine t h e i r VHP c h a r a c t e r i s t i c s * The r e s u l t s of these measurements are given i n Appendices II and I I I . i n the 50-350 Megacycle frequency range the elements a r e . s u b j e c t to the e f f e c t s of le a d inductance and p a r a s i t i c c a p a c i t a n c e . This complicates the c i r c u i t r e p r e s e n t a t i o n of these passive elements. With t r a n s i s t o r s the p i c t u r e i s even more i n v o l v e d as lumped element approximations to the t r a n s i s t o r equations become poor and as e f f e c t s of p a r a s i t i c capacitances begin to predominate. Thus^ adequate e q u i v a l e n t c i r c u i t r e p r e s e n t a t i o n i n t h i s frequency range i s cumbersome. Hence^in the f o l l o w i n g no attempt w i l l be made to give an e q u i v a l e n t c i r c u i t f o r the network. looked at as a grounded-base stage with feedback, the feedback loop being a grounded-emitter a m p l i f i e r . Prom the r e s i s t o r \u00E2\u0080\u0094 c a p a c i t o r measurements i t i s seen that As i s seen i n Fig u r e 4.8a, the mwef c i r c u i t can be 777 (a) (b) Fi g u r e 4.8. The MWEF at High Frequencies 36 Assuming that the admittance l/R-^ can be neglected and that C i s a short c i r c u i t * then the mwef can be drawn as shown i n F i g u r e 4.8(b). F u r t h e r , i f i t i s assumed t h a t each t r a n s i s t o r , i s working i n t o a short c i r c u i t , then the loop c u r r e n t g a i n , A T, i s A I = 3 V I 1 = ~ k f e k f b * and the input admittance T^ , i s I + I I ^ y 1 2 _ i (]_ + _2 ) C ~ V - v v I ' \u00C2\u00B0 v l v l x l = y i b ( l - h f b h f e > =\u00E2\u0080\u00A2 ^ i b ( l y ^ y ^ The r a t i o 1-^ /V^ = y ^ since i t was assumed t h a t the output of Q-^ i s shorted by the input of Q^. An exact expression f o r the loop g a i n i s d e r i v e d i n Appendix V where i t i s shown t h a t there i s a d i f f e r e n c e of about 4 db and 35 degrees between the two expressions at 350 Megacycles. However 5 f o r an i n i t i a l understanding of the problems a s s o c i a t e d with t h i s c i r c u i t the approximation i s e a s i e r to examine, and hence w i l l be used i n the f o l l o w i n g . The preceding thus shows that the input admittance to the i grounded-base stage i s i n c r e a s e d by 1 + A T. Hence^A T should be as l a r g e as s t a b i l i t y c o n s i d e r a t i o n s w i l l allow. I t i s seen from Equation ( l ) that.the input admittance i s zero when Aj = -1* Thus^ i n the complex plane- the c u r r e n t g a i n must avoid the (\u00E2\u0080\u00941,0) p o i n t i f the network i s to be s t a b l e . N y q u i s t ^ ^ has extended t h i s concept of a v o i d i n g the (-1,0) I 3 7 p o i n t by s t a t i n g .that f o r a s i n g l e - l o o p s t a b l e a m p l i f i e r , the loop gain when p l o t t e d i n p o l a r form must not e n c i r c l e the (-1,0) p o i n t . This statement j>hen leads to the B o d e ^ 1 ^ c r i t e r i o n f o r s i n g l e - l o o p a m p l i f i e r s : the loop g a i n must be l e s s than u n i t y when the phase reaches -180 degrees. Thus,to determine the s t a b i l i t y of the mwef the loop g a i n , A T = -h\u00E2\u0080\u009E, h,, ' 1 ib fe - h ^ e must be examined. Shown i n Figure 4.9 i s a Bode p l o t of the t y p i c a l measured h\u00E2\u0080\u009E valu e s f o r the 2N1141 PNP t r a n s i s t o r and the \ N d t a FREQ MC I Figure 4.9 . T y p i c a l h\u00E2\u0080\u009E Values 2N2369 NPN t r a n s i s t o r . These are the common-emitter t r a n s i s t o r s f o r the T and X d r i v e r s r e s p e c t i v e l y . As can be seen from the f i g u r e the h ^ ' s a r e s i m i l a r : both are i n t h e i r 6 db/octave c u t - o f f r e g i o n , w i t h a phase l a g of gr e a t e r than 90 degrees. In the f o l l o w i n g only the ex p e r i m e n t a l l y measured r e s u l t s f o r the T d r i v e r of the 1 ma a r r a y w i l l be given, since t h i s was the most c r i t i c a l d esign problem. The Y d r i v e r c i r c u i t to be examined i s shown i n Fig u r e 4.8 with QT the 2N2369 t r a n s i s t o r and Q 2 the 2N1141. The short - c i r c u i t c u r r e n t gains of these t r a n s i s t o r s were measured, the 2N2369 as a grounded-base a m p l i f i e r , and the 2N1141 as a grounded-emitter. The product, h f e n f k \u00C2\u00BB \u00C2\u00B0f these measurements i s shown i n Fig u r e 4.10. Also shown i n the f i g u r e i s the a c t u a l measured s h o r t \u00E2\u0080\u0094 c i r c u i t c u r r e n t g a i n of the* Y d r i v e r . The d i f f e r e n c e of about 80 degrees at 350 Megacycles between these two curves i s due,to two causes. F i r s t , i n the a c t u a l c i r c u i t there i s a 50 ohm r e s i s t o r on the base of Q N f o r impedance matching of the input s i g n a l to the a r r a y . This caused, an a d d i t i o n a l phase l a g of 35 degrees. Secondly,the assumption made i n d e r i v i n g the g a i n e x p r e s s i o n as ~ n f ^ n f e w a s \"that Q-^ worked i n t o a short c i r c u i t . In f a c t the input impedance of Q 2 i s about 70 ohms - comparable to the Q-^ output impedance. This causes about a 40 degree a d d i t i o n a l phase l a g at 350 Megacycles. Thus 3the combination of these two p e r t u r b a t i o n s to the i d e a l mwef accounts f o r the measured phase: l a g and the r e s u l t a n t i n s t a b i l i t y of the network. A t t e n t i o n was thus turned to s t a b i l i z i n g t h i s network. Bode v ' shows that an i d e a l feedback a m p l i f i e r has a c u t - o f f 39 C-tAIN I N d b 5 0 ^OO \" 2 . 0 0 \"300 -4-00 FREQ IN MC F i g u r e 4 . 1 0 . Current Gain f o r T D r i v e r of 1 ma Array 40 c h a r a c t e r i s t i c o f 12 d b / o c t a v e . F u r t h e r , h e i n d i c a t e s how t h e a m p l i f i e r b a n d w i d t h may be m a x i m i z e d p r o v i d e d t h a t t h e a c t u a l a m p l i f i e r h a s a n a s y m p t o t i c g a i n r o l l - o f f o f g r e a t e r t h a n 12 d b / o c t a v e . A s t h e g a i n r o l l - o f f h e r e i s o f t h e o r d e r o f 6 d b / o c t a v e , t h i s p r o c e d u r e c a n n o t be e m p l o y e d . Commonly u s e d b r o a d b a n d i n g t e c h n i q u e s a r e e q u a l l y i n a p p l i c a b l e as t h e y a r e c o n c e r n e d w i t h e x t e n d i n g o n l y t h e g a i n m a g n i t u d e t o t h e c u t \u00E2\u0080\u0094 o f f r e g i o n . No a t t e n t i o n i s p a i d t o t h e p h a s e c h a r a c t e r i s t i c w h i c h h e r e w i l l be a t l e a s t 90 d e g r e e s when t h e b r o a d b a n d e d g a i n c h a r a c t e r i s t i c i n t e r s e c t s t h e c u t - o f f a s y m p t o t e . C o n s e q u e n t l y t h e o n l y a t t a c k a p p l i c a b l e i s t h a t o f a t t e n u a t i o n o f t h e g a i n . T h i s c a n be done b y s e v e r a l m e t h o d s . The s i m p l e s t m e t h o d i s t o r e d u c e t h e g a i n o f t h e t r a n s i s t o r b y u s i n g a n e m i t t e r r e s i s t o r . T h i s was u s e d w i t h t h e 1 ma m w e f . A l t e r n a t i v e l y , g a i n c a n be r o l l e d - o f f a t t h e c o l l e c t o r o f b y u s i n g a s e r i e s - R C n e t w o r k . W i t h t h e 5 ma mwef b o t h t e c h n i q u e s w e r e u s e d as t h e s t a b i l i z i n g e m i t t e r r e s i s t o r e x c e e d e d t h e dc b i a s e m i t t e r r e s i s t o r . F o r t h e T d r i v e r a l i m i t i n g v a l u e o f t h e e m i t t e r r e s i s t a n c e was f o u n d e x p e r i m e n t a l l y t o be a b o u t 220 o h m s . F o r v a l u e s s m a l l e r t h a n t h i s , t h e c i r c u i t was u n s t a b l e . The c l o s e d - l o o p g a i n was d e t e r m i n e d b y m e a s u r i n g t h e i n p u t a d m i t t a n c e w i t h t h e l o o p o p e n , YQ , a n d t h e n c l o s e d , Y C \u00E2\u0080\u009E The l o o p g a i n t h u s / c a l c u l a t e d f r o m Y \u00C2\u00A3 = YQ(1 + A T ) i s shown i n F i g u r e 4.11 (, ( s e e A p p e n d i x V ) . A s c a n be s e e n f r o m t h e f i g u r e , t h e d r i v e r i s o n l y c o n d i t i o n a l l y s t a b l e . The i n c r e a s e i n g a i n a t h i g h f r e q u e n c i e s i s due t o c u r r e n t f e e d t h r o u g h t h e b a s e - c o l l e c t o r j u n c t i o n 41 d e p l e t i o n capacitance of Q-. Fu r t h e r , a ga i n margin of 4 db G A I N IN d b -\"2 - 4 -a / / / / / / / / / P H A S E ^ / \ \ -* \ P>14AS'= is* \u00E2\u0080\u0094 1 0 0 -\40 -\ao ->oo -.-8o 5 0 TOO 1O0 3 00 -4-00 FREQ \U V\C Figure 4.11. L i m i t i n g Value of T D r i v e r Gain at 200 Megacycles i s inadequate, as the gain of i n d i v i d u a l t r a n s i s t o r s of the same type can vary by t h i s amount. The r e s u l t s of F i g u r e 4.11 thus represent a bound on the upper l i m i t of allowable loop g a i n . As was i n d i c a t e d i n F i g u r e 3.7 the output from the ar r a y i s obtained from the Q 2 c o l l e c t o r of the T d r i v e r . The c u r r e n t . 42 i c , t h a t flows there i s giv e n by A I i = ( ')im \"> c y A T where Aj i s the loop g a i n , y^ the admittance of the 15 other diode c e l l s t i e d to the X d r i v e r ^ and the t o t a l tunnel\u00E2\u0080\u0094diode switching c u r r e n t . ThuSjusing Figure 4.1I wit h about an 8 db gain margin, the c o l l e c t o r c u r r e n t w i l l be at l e a s t 12 db down at 150 Megacycles r e g a r d l e s s of what form the a c t u a l d e t e c t o r c i r c u i t takes* S e v e r a l arrangements are p o s s i b l e f o r the d e t e c t o r . A simple r e s i s t o r , R^, i n the c o l l e c t o r as shown i n Figu r e 4.11a would be s u i t a b l e provided i t d i d not s e r i o u s l y (a) (b) Fig u r e 4.12. P o s s i b l e Detector C i r c u i t s a f f e c t the dc b i a s i n g of Q^* The e f f e c t of on the dc b i a s i n g of Q 2 can be e a s i l y e l i m i n a t e d by us i n g a shorted delay l i n e as shown i n Fi g u r e 4.12(b). This was the arrangement used wi t h the f i r s t a r r a y d i s c u s s e d i n Chapter 6. In both F i g u r e s 4.12(a) and (b) the assumption i s made th a t the output i s f e d i n t o a r e l a t i v e l y high-impedance a m p l i f i e r , such as ah emitter f o l l o w e r . In both cases i t i s the v o l t a g e across Ry. t h a t i s d e t e c t e d . As the impedance on the c o l l e c t o r of must be small ( i . e . should be l e s s than 300 ohms f o r high f r e q u e n c i e s as l / v o e \u00E2\u0080\u0094 300 ohms) the output v o l t a g e w i l l be cor r e s p o n d i n g l y s m a l l . Por the 5 ma ar r a y i t was of the order of 100 mv, while f o r the 1 ma ar r a y i t was about 20 mv. Broadband transformers were a l s o examined f o r p o s s i b l e a p p l i c a t i o n i n the d e t e c t i n g c i r c u i t . High-frequency transformers are l o s s y and. t h e i r presence i n the c o l l e c t o r c i r c u i t of causes a l a r g e phase s h i f t i n the loop g a i n , thus r e q u i r i n g the g a i n to be f u r t h e r attenuated. I t i s thus seen that the mwef high\u00E2\u0080\u0094frequency response i s h i g h l y dependent on the h ^ g of the t r a n s i s t o r . As c u r r e n t l y a v a i l a b l e t r a n s i s t o r s are i n t h e i r 6 db/octave c u t -o f f r e g i o n f o r the 100 Megacycle frequency range, i t i s d i f f i c u l t to o b t a i n a s t a b l e c i r c u i t and s t i l l maintain any loop g a i n . For bandwidths below 50 Megacycles the c i r c u i t does have a p p l i c a t i o n . Thus., c i r c u i t s t h a t r e q u i r e t r a n s i s t o r feedback to improve t h e i r i nput admittance are not very s a t i s f a c t o r y at hi g h f r e q u e n c i e s . Consequently the use of passive elements was i n v e s t i g a t e d . Out of these c o n s i d e r a t i o n s came the t r a n s -former coupled emitter f o l l o w e r of the next chapter. 44 5. THE TRANSFORMER COUPLED EMITTER FOLLOWER In t h i s chapter the c h a r a c t e r i s t i c s of the transformer coupled e m i t t e r f o l l o w e r ( t c e f ) are d i s c u s s e d . This c i r c u i t employs a broadband transformer, the p r o p e r t i e s of which are examined i n Appendix IV. I t i s shown i n t h i s chapter t h a t , with the help of these transformers, an o v e r a l l a m p l i f i e r bandwidth of 250 Megacycles i s p o s s i b l e with an input admittance of g r e a t e r than 15 m i l l i m h o s * The t c e f c i r c u i t s c onsidered are only those f o r the 1 ma a r r a y n 5ol DC Co n s i d e r a t i o n s In Chapter 4 i t was shown that i f the d r i v e r s supply the dc b i a s i n g , then both the X and Y mwefs had p a r a b o l i c d i s s i p a t i o n curves. Here, only the Y t c e f w i l l e x h i b i t a p a r a b o l i c power dependence as the X t c e f i s j u s t an em i t t e r f o l l o w e r . As with R-^ of the mwef, R^ of Fig u r e 5.1 should be about 1 K\u00E2\u0080\u0094ohm. Thus u s i n g an a n a l y s i s s i m i l a r to that of S e c t i o n 4.1 wit h R^ = 1.2K, V = 24 v o l t s , the maximum power d i s s i p a t i o n i s about 127 m i l l i w a t t s . This can be handled by a s i n g l e t r a n s i s t o r . The maximum X t c e f t r a n s i s t o r d i s s i p a t i o n i s about 148 m i l l i w a t t s with a c o l l e c t o r supply v o l t a g e of -6 v o l t s . This power can again be handled by a s i n g l e t r a n s i s t o r . As was mentioned i n Chapter 4, the dc output v o l t a g e from an emitter f o l l o w e r c i r c u i t e x h i b i t s a temperature dependence. Hence the same dc compensation i s r e q u i r e d f o r the t c e f c i r c u i t as was r e q u i r e d f o r the mwef. The complete t c e f X and Y d r i v e r 45 c i r c u i t s a r e as shown i n F i g u r e 5 . 1 . OUT t F R O M (a) T D r i v e r (b) X d r i v e r F i g u r e 5 . 1 . X a n d T T C E F The t e m p e r a t u r e d e p e n d e n c e o f t h e dc o u t p u t v o l t a g e f r o m t h e s e c o m p e n s a t e d d r i v e r s i s s i m i l a r t o t h a t shown i n F i g u r e 4 . 4 , A c t u a l l y i t i s s l i g h t l y l e s s t h a n t h a t shown f o r t h e mwef as t h e r e a r e no l e a k a g e c u r r e n t e f f e c t s o f a s e c o n d t r a n s i s t o r t o c o n t e n d w i t h h e r e . The t r a n s f o r m e r shown i s a 1 : 2 b r o a d b a n d c u r r e n t t r a n s -f o r m e r ( see A p p e n d i x I V ) . W i t h t h e t c e f t h e dc b i a s i n g o f t h e a r r a y c o u l d be done t h r o u g h one arm o f t h e t r a n s f o r m e r * The t c e f ' s w o u l d t h e n be ac c o u p l e d t o t h e a r r a y . W i t h t h i s , t r a n s i s t o r s w i t h l o w e r power d i s s i p a t i o n b u t w i d e r b a n d w i d t h s c o u l d t h e n be u s e d . H o w e v e r , f o r c o n v e n i e n c e t h e same b i a s a r r a n g e m e n t was u s e d as w i t h t h e m w e f . T h i s i s shown i n F i g u r e 5 . 1 . 5.2 High-Frequency C o n s i d e r a t i o n s I f one assumes t h a t Q3 of Figure 5.1 i s a grounded-base stage, and t h a t i n the case of the Y d r i v e r the e f f e c t of the \u00E2\u0080\u00A2 l o a d may be1 n e g l e c t e d , then the input admittance to the t c e f 2 i s g i v e n by N y ^ , here N i s the e f f e c t i v e turns r a t i o of the transformer, and y ^ i s the grounded-base s h o r t - c i r c u i t i n p u t 2 admittance* I d e a l l y here N =4, and consequently the input admittance to the d r i v e r should be 4 y ^ as shown i n F i g u r e 5.2. The X d r i v e r c h a r a c t e r i s t i c s alone w i l l be considered, as the measured admittance to the two d r i v e r s was p r a c t i c a l l y the same. F i g u r e 5,2 shows the admittance c h a r a c t e r i s t i c s to the X d r i v e r with Q3 a 2N2369 t r a n s i s t o r . Also shown i s the admittance of the other tunnel-diode c e l l s t i e d to the X d r i v e r , X^, and the e f f e c t i v e turns r a t i o of the transformer. The f i g u r e demonstrates t h a t there i s only a s l i g h t improvement i n the admittance at f r e q u e n c i e s above 100 Megacycles. F u r t h e r i t i s seen t h a t the transformer turns r a t i o does not approach i t s i d e a l value of 2 u n t i l the frequency drops to about 20 Megacycles,, This, however, rep r e s e n t s an improvement over the mwef where the input admittance was a c t u a l l y decreased at f r e q u e n c i e s above 50 Megacycles by the presence of the feedback 1 o op. The next f e a t u r e to be examined i s the o v e r a l l d r i v e r c u r r e n t g a i n . With t h i s the Q4 t r a n s i s t o r i s a PNP 2N1141, Fi g u r e 5,3 shows the g a i n f o r the X d r i v e r f o r v a r i o u s values of l o a d r e s i s t a n c e on the c o l l e c t o r of Q4. The input admittance f o r the v a l u e s of load r e s i s t o r s considered was p r a c t i d a l l y the same. Also shown i n the f i g u r e i s the o v e r a l l g a i n when the 47 transformer 1 was e l i m i n a t e d * The d i f f e r e n c e between the short V8Q. ifeo. 140 \ 7 A \00 -4-0. 20 SO -40 50 bo \"IO SO VOO \"2.00 TWO 400 Figure 5.2* TCEF Admittance C h a r a c t e r i s t i c s - c i r c u i t load curves i s a l s o shown. This represents the l o s s i n c u r r e n t gain through the transformer, which f o r a 1:2 t r a n s -former should be =-6db\u00C2\u00AB As can be seen from the f i g u r e the transformer response i s at best -5.7db\u00C2\u00BB For comparison the response curve f o r the 1:2 transformer of Appendix IV i s a l s o g i v e n . This transformer was terminated i n 47 ohms. Agreement between these two curves improves as the input impedance of the t r a n s i s t o r approaches the 47 ohm l e v e l . F i g u r e 5.3. Y D r i v e r G a i n 49 Thus Fig u r e 5.3 demonstrates t h a t i t i s p o s s i b l e to achieve bandwidths of the order of 250 Megacycles . Now with the a c t u a l memory the c u r r e n t that flows i n the c o l l e c t o r of Q4 i s given by-5 i A . (1 + y A / y D ) where A i s the o v e r a l l system g a i n , y^ the admittance of the 15 other c e l l s t i e d to the y d r i v e r , y^ the admittance to the X d r i v e r , and i ^ , the t o t a l t u n n e l diode switching current,, Since A ^ ( nf e)g4\u00C2\u00BB a Q4 t r a n s i s t o r with a higher gain would give an even b e t t e r bandwidth. This aspect i s considered f u r t h e r i n Chapter 6. 6. RESULTS AND CONCLUSIONS 50 In t h i s chapter the experimental r e s u l t s obtained u s i n g the c i r c u i t s of Chapters 4 and 5 to d r i v e the a r r a y w i l l be examined. The speed of the a r r a y i s compared wi t h that of e x i s t i n g f e r r i t e \u00E2\u0080\u0094 c p r e arrays and with tunnel-diode l o g i c . Some i n d i c a t i o n i s a l s o given as to the l i m i t of the s i z e of an a r r a y t h a t can be b u i l t ^ u s i n g current d e t e c t i o n . F i n a l l y , a t t e n t i o n i s drawn to areas which c o u l d warrant f u r t h e r inve s t i g a t i o n . 6.1 Experimental Arrays I n i t i a l i n v e s t i g a t i o n on the tunnel-diode array was c a r r i e d out u s i n g two d r i v e r s , X and Y, and a s i n g l e diode c e l l . The r e s u l t s of t h i s i n d i c a t e d that current d e t e c t i o n was p o s s i b l e . However,to determine i f the s w i t c h i n g of one tunnel diode would a f f e c t another i n the a r r a y , a 2x2 matrix was b u i l t f o r both the 5 ma and 1 ma tunnel diodes. This array was b u i l t on a p r i n t e d - c i r c u i t board w i t h a minimum amount of copper removed. The arrangement of the a r r a y i s shown i n F i g u r e 6,1. The presence of the other 14 words and 23 b i t s was simulated u s i n g an RC combination on each l i n e as shown. Only the r e s u l t s f o r the 2x2 array w i l l be given, as the performance of the a r r a y i s of more i n t e r e s t than that of a s i n g l e diode c e l l . F u r t h e r , the d i f f e r e n c e i n the output s i g n a l was s l i g h t : the r i s e time and pulse height of the output being about 2 nanoseconds slower and 5 mv smaller f o r the array than f o r the s i n g l e c e l l . 51 X2 L-AAA.-rAA-'V I \u00E2\u0084\u00A2 ! ^ - xA/V\rj-*V\AA--2 v \u00E2\u0080\u00A2MAArrjVVNA-Figu r e 6.1. Block Diagram of 2x2 Array The pulse sequence r e q u i r e d f o r w r i t i n g i n t o and r e a d i n g out of a s i n g l e diode c e l l i s shown i n F i g u r e 6.2. Y IT i rr W , i \u00C2\u00A3 R E A D F i g u r e 6.2. Read\u00E2\u0080\u0094Write S i g n a l s f o r S i n g l e C e l l For d r i v i n g the 2x2 a r r a y the pulse sequence of F i g u r e 6.3 was used. Note that the X h a l f \u00E2\u0080\u0094 w r i t e pulse i s wide enough to allow each of the two c e l l s on the X l i n e to be set s e p a r a t e l y . This permits the presence of c r o s s - t a l k to be determined. A pulse generator was designed to produce the pulse sequence of F i g u r e 6.3 (see Appendix VI) \u00C2\u00BB With t h i s the X h a l f - w r i t e pulses 52 were about 100 nanoseconds wide while the Y h a l f - w r i t e and X read pulses were about 30 nanoseconds wide. \u00C2\u00A5 \u00C2\u00A5 ' \"WRITE | R E A D F i g u r e 6.3. Read-Write S i g n a l s f o r 2x2 Array The exact c o n f i g u r a t i o n s f o r the X and Y d r i v e r s employed with the 5 ma ar r a y are shown i n Figure 6.4. The output, shown i n Figure 6.5, i s a pulse r a t h e r than a step due to the presence -2N8\"M-- 1 1 V (a) Y MWEF D r i v e r (b) X MWEF D r i v e r Figure 6.4. D r i v e r Arrangement f o r 5 ma Array 53 of the shorted delay l i n e on the output. These output pulses had about a 15 nanosecond r i s e time and were about 35 nanoseconds wide. F u r t h e r ^ i t should be noted here that when the diode c e l l does not switch ( i . e . when a \"0\" i s p r e s e n t ) , the output pulse i s i n i t i a l l y of the opposite p o l a r i t y to the pulse when the diode switches. 1'. O o A-, O U T \r \ 5 c \u00E2\u0080\u0094 \u00E2\u0080\u0094t/s*>. W R 1 ( T E \u00C2\u00BB E S | i i i \u00E2\u0080\u00A2 ij i 1 i 1 1 I C O M V ! ! i i 1 I N j j ( i T I M E F i g u r e 6.5. Waveforms f o r 5 ma Array When the mwef i s used with the 1 ma tunnel-diode a r r a y , output pulses of the order of 20 m i l l i v o l t s are produced. This was considered inadequate as i t was intended t h a t t h i s s i g n a l should d r i v e a tunnel-diode gate f o r which 100 m i l l i v o l t s was r e q u i r e d to ensure r e l i a b l e o p e r a t i o n . The exact c o n f i g u r a t i o n s f o r the X and Y d r i v e r s f o r the 1 ma a r r a y are shown i n F i g u r e 6.6, The output waveforms are shown i n F i g u r e 6.7. These f i g u r e s i l l u s t r a t e the output f o r three d i f f e r e n t storages* The waveforms have a r i s e time of about 10 nanoseconds* This i s shown i n F i g u r e 6.8, which also shows (a ) Y T C E F D r i v e r (b) X T C E F D r i v e r F i g u r e 6,6, T C E F D r i v e r C i r c u i t s t h e o u t p u t s i g n a l when a \"0\" i s p r e s e n t . The o u t p u t h e r e i s a p u l s e due t o t h e RC e m i t t e r t i m e c o n s t a n t w h i c h was c h o s e n t o d i f f e r e n t i a t e t h e c u r r e n t s t e p i n p u t f r o m t h e a r r a y . T h u s , u s i n g t h e t c e f c i r c u i t i t i s p o s s i b l e t o a t t a i n t h e d e s i r e d o u t p u t s i g n a l l e v e l o T h i s d e m o n s t r a t e s t h e s u p e r i o r i t y o f t h e t c e f c i r c u i t o v e r t h e mwef f o r t h i s a p p l i c a t i o n , 6,2 C o m p a r i s o n w i t h O t h e r D e v i c e s To e v a l u a t e t h e f o r e g o i n g r e s u l t s , a c o m p a r i s o n s h o u l d be made w i t h t h e o p e r a t i n g s p e e d s of b o t h f e r r i t e c o r e s a n d t u n n e l \u00E2\u0080\u0094 d i o d e l o g i c . P r e s e n t l y a v a i l a b l e f e r r i t e c o r e s h a v e a b o u t 20 n a n o s e c o n d r i s e t i m e s when d r i v e n b y a h e a v y c u r r e n t p u l s e ( i . e . 350 ma a n d 1Q0 n a n o s e c o n d s w i d e ) . The a r r a y s b u i l t w i t h t h e s e c o r e s h a v e a r e a d - w r i t e c y c l e t i m e o f a b o u t 250 n a n o s e c o n d s . kVOLT-A.G.E F i g u r e 6 \u00E2\u0080\u009E 7 . ' W a v e f o r m s f o r 1 ma A r r a y IO MS' F i g u r e 6 , 8 . \" 1 \" a n d \" 0 \" O u t p u t P u l s e s 56 By comparison the tunnel-diode a r r a y has about a 1G nanosecond r i s e time when d r i v e n by a 100 m i l l i v o l t , 30 nano-second wide p u l s e . Furthermore^as shown i n F i g u r e 6.7 the tunnel\u00E2\u0080\u0094diode a r r a y has about a 200 nanosecond read-write c y c l e time. This does not represent the l i m i t . The long c y c l e time here r e s u l t s from the t r a n s i e n t due to the h a l f - w r i t e pulse oh the X l i n e s . For t e s t purposes these pulses were chosen to be about 100 nanoseconds wide. I f both h a l f - w r i t e pulses were made 30 nanoseconds wide (as i s the Y h a l f - w r i t e p u l s e ) , then the read-write c y c l e time would be about 60 nanoseconds\u00C2\u00AB To determine whether the a r r a y would accept pulses s h o r t e r than t h i s , a T e k t r o n i x P r e t r i g g e r Pulse Generator was used to produce the e n t i r e w r ite p u l s e . This pulse was of the order of 10 nanoseconds wide and i s shown i n Figure 6.9. This then i n d i c a t e s that a read\u00E2\u0080\u0094write c y c l e time of the order of 2Q nanoseconds might u l t i m a t e l y be p o s s i b l e . This corresponds to an o p e r a t i n g frequency of 50 Megacycles which i s about a f a c t o r of 10 f a s t e r than f e r r i t e - c o r e a r r a y s . This speed should i n t u r n be compared with present t r a n s i s t o r - t u n n e l diode l o g i c speeds which are r e p o r t e d to be (16) of the order of 250 Megacycles . However^transistor l o g i c (17) has a present l i m i t of about 100 Megacycles , and the a r r a y speed would be adequate f o r t h i s . One f i n a l aspect should perhaps be noted i n the comparison of tunnel\u00E2\u0080\u0094diode with f e r r i t e - c o r e memories. At the present time,, tunnel diodes are about 100 times as expensive as f e r r i t e 57 \ 1 j i i 1 < i j 1 i | r lOO MV T I N E V G U f - . ' * * \" -F i g u r e 6.9. F u l l - W r i t e Pulse Produced Using a Tektronix Pulse Generator and J e r r o l d RF Attenuator c o r e s . Consequently the f a c t o r of 10 i n c r e a s e i n speed must be weighed a g a i n s t a f a c t o r of 100 increase i n c o s t . This i s one reason f o r l i m i t i n g the s i z e of tunnel\u00E2\u0080\u0094diode memories. The aspect of memory s i z e i s considered f u r t h e r i n the next s e c t i o n . 6.3 E x t e n s i o n of the Array The extent to which the s i z e of the a r r a y can be i n c r e a s e d depends on the c i r c u i t s used to driv-e i t . The t c e f c i r c u i t i s more f l e x i b l e than the mwef as i t s o v e r a l l system ga i n can be a l t e r e d without a f f e c t i n g i t s s t a b i l i t y . Consequently i t alone w i l l be considered here. The a r r a y s that have been d i s c u s s e d are of 16 word 25 b i t c a p a c i t y . I f the requirement that the d r i v e r s supply the de b i a s i n g i s removed, then l a r g e r a r r a y s become p o s s i b l e . The 58 extension, however, i s l i m i t e d by the admittance of the array i t s e l f . As the a r r a y s i z e i s i n c r e a s e d i t s impedance becomes sm a l l e r , causing a g r e a t e r mismatch f o r the t r a n s f o r m e r s . The transformers used r e q u i r e d about a 40 ohm load f o r matching. Consequently^array s i z e s v a s t l y d i f f e r e n t than that designed would perhaps r e q u i r e transformers with d i f f e r e n t c h a r a c t e r i s t i c impedances. This a p p l i e s p a r t i c u l a r l y to the X d r i v e r s . Por the Y d r i v e r , however, there i s the a d d i t i o n a l requirement t h a t i t d e t e c t the s w i t c h i n g c u r r e n t . The c u r r e n t t h a t flows i n the Y d r i v e r i s dependent on the number of c e l l s shunting i t . Thus any i n c r e a s e i n word c a p a c i t y w i l l r e s u l t i n l o s s of s i g n a l c u r r e n t . This of course can be countered by u s i n g a higher g a i n t r a n s i s t o r i n the grounded-emitter stage, or by decreasing the amount of d i f f e r e n t i a t i o n ( i . e . extending the band width to lower f r e q u e n c i e s ) i n t h i s stage. The s i z e of the a r r a y i s a l s o dependent on the type of tunnel diode used. Por diodes with peak c u r r e n t s g r e a t e r than 1 ma the mismatch on the transformer w i l l occur f o r smaller s i z e d a r r a y s . This would have a tendency to reduce the number of b i t s , but the number of words c o u l d be i n c r e a s e d due to the l a r g e r s w i t c h i n g c u r r e n t . Use of diodes with peak c u r r e n t l e s s than 1 m could i n c r e a s e the b i t c a p a c i t y , but not the number of words* 6*4 Conclusions The f e a s i b i l i t y of d e t e c t i n g the c u r r e n t change of s t a t e has been demonstrated i n t h i s t h e s i s . The method i s a p p l i c a b l e , however,only to small s i z e a r r a y s as the shunting admittance of 59 the a r r a y i t s e l f causes s i g n a l l o s s e s . Consequently,driver c i r c u i t s are r e q u i r e d which have a high input admittance r e l a t i v e to the a r r a y admittance. The i n v e s t i g a t i o n of h i g h input admittance c i r c u i t s l e d to the c o n s i d e r a t i o n of the mwef and the t c e f c i r c u i t s . Con-s i d e r a t i o n of the mwef showed t h a t i t s a p p l i c a t i o n here was l i m i t e d due to the i n a b i l i t y to s t a b i l i z e the network i n the 100 Megacycle frequency range and s t i l l m aintain loop g a i n . The t c e f c i r c u i t , on the other hand, employed a b i f i l a r wound 1:2 t r a n s -former to achieve an improved i n p u t admittance. This l e d to an i n v e s t i g a t i o n of a few of the p r o p e r t i e s of b i f i l a r wound t r a n s -formers. D e s i r e f o r good c o u p l i n g and the d e s i r e f o r broadband response are not compatible, and r e q u i r e that a compromise be made. A technique f o r d e s i g n of these transformers was developed. Throughout t h i s work d i f f i c u l t i e s were encountered r e s u l t i n g from an e a r l y assumption that i t would be economical to supply the dc to the a r r a y through the d r i v e r c i r c u i t s . The i n i t i a l d r i v e r , c i r c u i t considered was the mwef which employs two t r a n s i s t o r s and could be made to handle the cur r e n t r e q u i r e d . However,as the t c e f c i r c u i t appears to be su p e r i o r i t would, i n fu t u r e i n v e s t i g a t i o n s , be wise to d i s c a r d the b i a s i n g aspect f o r the d r i v e r s , and i n s t e a d ac couple them to the a r r a y . A d d i t i o n a l work c o u l d be done on i n c r e a s i n g the bandwidth of the d r i v e r c i r c u i t s . T h i s coupled w i t h the use of f a s t e r tunnel diodes could perhaps l e a d to access times of the order of 10 nanoseconds. More work co u l d a l s o be done on the broadband t r a n s f o r m e r s . E x t e n s i o n of the b i f i l a r technique to t r i f i l a r and higher order windings may be v e r y p r o f i t a b l e . 60 APPENDIX I TUNNEL-DIODE SWITCHING CHARACTERISTICS The d i f f e r e n t i a l equations governing the sw i t c h i n g of the tunnel diode are as given i n S e c t i o n 2.5: L | r = V - V - RI dt a c | = I - f (v) = 1 - 1 ' \u00E2\u0080\u00A2 Time s c a l i n g these by s e t t i n g t = CT/b r e s u l t s i n L b 5 ? = V a - V - R I and s e t t i n g L = 5 x l 0 ~ 9 i l , C \u00E2\u0080\u00A2= 5 x l O ~ 1 2 c , b = 2 x l 0 1 0 , the d i f f e r e n t i a l equations become 20 1 ^1 _ v - V - RI .02 c |5 = I ~ I' ' d j which when the ranges of the v a r i a b l e V and I are examined can be w r i t t e n i n terms of machine parameters i , v as 4000 1 | i = 5 0 x l 0 3 (v - v) - 200 Ri 10 3 c = 200 i - 200 i ' 3 where i = 50 x 10 I and v = 200 x V. S u b s t i t u t i n g i n the values of R, JL, and c as R = 680, 1 = 1 , and c = 1 y i e l d s 0.1 |j = 0.25 (v - v). - 0.68 i 2.5 j j j = 0.1 i - 0.11' The d i o d e f u n c t i o n g e n e r a t o r a s s o c i a t e d w i t h t h e D o n n e r C o m p u t e r was s e t up t o g e n e r a t e t h e i ' f u n c t i o n , w h i c h i s t h e t u n n e l - d i o d e s t a t i c i - v c u r v e . T h u s t h e a n a l o g u e c i r c u i t i s as shown i n F i g u r e A l . l . to io is \u00E2\u0080\u00A2O -0.681 -o-GENERATOR A.1 F i g u r e A l . l . A n a l o g u e S e t u p o f T u n n e l - D i o d e D E q . The i n p u t v o l t a g e was g e n e r a t e d b y s o l v i n g a n e q u a t i o n o f t h e f o r m + a X = K . d t T h i s e q u a t i o n * w h e n w r i t t e n i n t e r m s o f m a c h i n e p a r a m e t e r s , becomes , \u00E2\u0080\u00A2 v- . \u00E2\u0080\u0094 x dx 0 &3 140 w h e r e x = 200X, V Q = 200V n where V n i s t h e a m p l i t u d e o f t h e s t e p 0 0 62 f u n c t i o n as d e f i n e d i n Figure 2.11, and 0 = 2 x l 0 ^ t . The c i r c u i t arrangement then i s as i n Fig u r e Al<>2. ;, Figure A1.2, Analogue Setup f o r Generation of Input F i g u r e A1.3. Tunnel-Diode Switching T r a j e c t o r i e s as Simulated on an Analogue Computer 63 The switching t r a j e c t o r i e s are shown i n Figure A1.3 f o r the three input s i g n a l s considered. Figure 2.11 shows the same t r a j e c t o r i e s p l o t t e d a g a i n s t time. I t should a l s o be noted here that while the F i g u r e A1.3 has 1 ma as i t s peak current the r e s u l t s obtained here apply almost e q u a l l y to the 5 ma tunnel diode. That t h i s i s so can be seen from the f a c t t h a t the d i f f e r e n t i a l equations are not a f f e c t e d by the s u b s t i t u t i o n of R' = R/m, L' = L/m, C' = mC, and i ' = mi. Since f o r the 5 ma tunnel diode Rr- \u00E2\u0080\u0094 l/5R n , 5 ma ' 1 ma' C r \u00E2\u0080\u0094 5CT , and ic \u00E2\u0080\u0094 5 i n the only approximation i n v o l v e d 5 ma 1 ma 5 ma 1 ma J rr i s f o r L. T h i s r e s u l t s as the inductance a s s o c i a t e d with the tunnel diode i s due to the leads and hence i s independent of the tunnel diode s i z e . However as the inductance considered i n the switching of the 1 ma case i s small i t does not p l a y a very important r o l e i n determining the o v e r a l l switching time. Thus to an accuracy of about 10% the 1 ma curves can be used to represent the 5 ma tunnel diode. 64 APPENDIX I I TRANSISTOR VHP MEASUREMENTS In t h i s appendix the high-frequency performance of s e v e r a l t r a n s i s t o r types i s examined* Prom t h i s examination some i n d i c a t i o n i s given of the v a l i d i t y of the measurements made on the s h o r t \u00E2\u0080\u0094 c i r c u i t gains of the mwef and t c e f . The measurements on the t r a n s i s t o r parameters were r e q u i r e d f o r an understanding of the performance of the mwef. The t r a n s i s t o r s , because of the intended a p p l i c a t i o n of t h i s c i r c u i t , were r e q u i r e d to have at l e a s t a 300 m i l l i w a t t d i s s i p a t i o n i n f r e e a i r . This power c o n d i t i o n e l i m i n a t e d many of the higher frequency t r a n s i s t o r s c u r r e n t l y a v a i l a b l e . The measurements were c a r r i e d out us i n g the General Radio 1607-A T r a n s f e r F u n c t i o n and Immittance B r i d g e . The 'block diagram of the t e s t arrangement i s shown i n Fig u r e A2.1. GR U n i t O s c i l l a t o r s 50-250 Mc 250-920 Mc GR 1607-A Bridge and Mounts SIGNAL FREQUENCY Input Supply Bias Supplie s GR U n i t Detector and IF A m p l i f i e r Output Supply GR Un i t O s c i l l a t o r 65-500 Mc LOCAL OSCILLATION Fi g u r e A2.1. Test Arrangement Block Diagram A2.1 T r a n s i s t o r Parameters The t r a n s i s t o r s l i s t e d i n Table A2.1 were examined. 65 SO \00 7.00 30O FV-eeouEHCY IN MC F i g u r e A2.2. M e a s u r e d h. 66 Quantity j Manuf a c t u r e r T r a n s i s t o r JEDEG No. Type T y p i c a l f. T from Manu. Average f from T Measurements 6 Motorola 2N834 NPN 3 50 Mc 360 Mc 6 T.I. 2N1143 PNP 300 Mc 350 Mc 2 F a i r c h i l d 2N2369 NPN 625 Mc 450 Mc 2 T.I. 2N1141 PNP 480 Mc 550 Mc Table A2.1* Frequ ency at Which h\u00E2\u0080\u009E J i e := i (t> The measured h\u00E2\u0080\u009E T s of these t r a n s i s t o r s are shown i n Figure A2.2< f e Two sets of parameters are c o n v e n t i o n a l l y used w i t h t r a n s i s t o r s : the h y b r i d parameters, and the admittance para-meters. In the a n a l y s i s of the mwef loop g a i n , the admittance parameters were found more convenient to use than the h y b r i d . Consequently o n l y the admittance parameters were measured \u00E2\u0080\u0094 both f o r the grounded\u00E2\u0080\u0094base and grounded-emitter a m p l i f i e r s . The Jjyj k parameters can be r e a d i l y obtained from the y g parameters and v i c e v e r s a , and i n f a c t t h i s was done as a check on the accuracy of the measurements. For the t r a n s i s t o r s s t u d i e d there was found to be about a 10$ d i f f e r e n c e between the two methods of f i n d i n g [ y ] b # The admittance parameters were obtained f o r the b i a s c o n d i t i o n s e x i s t i n g when the 1 ma mwef Y - c i r c u i t has an output c u r r e n t of 7.7 ma. The r e s u l t s of these measurements are shown i n F i g u r e s A2\u00E2\u0080\u009E .3 and A2.4. The admittance parameters shown are i n d i c a t i v e of the orders of magnitude t h a t were observed f o r a l l t r a n s i s t o r s c o n s i d e r e d . The admittance parameters are roughly independent of 6 7 \u00C2\u00AB \u00C2\u00BB \u00C2\u00BB *\u00E2\u0080\u00A2 \u00E2\u0080\u00A2OO TOO 1)00 -400 F i g u r e A2 03(b)\u00E2\u0080\u009E y Q b f o r the 2N834 F i g u r e A2.3(d). y r b f o r the 2N83.4 69 IN M I L L i M H O S . \u00E2\u0080\u00A220.4 F i g u r e A 2 \u00E2\u0080\u009E 4 ( b ) 0 y f o r the 2N1143. 70 Figure A2,4(d). y r e f o r the 2 N 1 1 4 3 71 c o l l e c t o r v o l t a g e f o r v o l t a g e s g r e a t e r than 5 v o l t s . The parameters are more dependent on c o l l e c t o r c u r r e n t . In p a r t i c u l a r , the forward transadmittance, y^, changes about 50% f o r a change i n cu r r e n t from 5 to 15 ma. A2.2 C i r c u i t J i g Measurements As i n d i c a t e d i n Chapters 4 and 5 measurements were made on the gain of the two d r i v e r c i r c u i t s considered. The c i r c u i t s were b u i l t on p r i n t e d c i r c u i t boards with minimum copper removed. These were then mounted on the GR 1607-P601 Ungrounded Component Mount. With the measurements of the mwef c i r c u i t i t was found necessary to add an a d d i t i o n a l low\u00E2\u0080\u0094pass f i l t e r on the supply l i n e s to the Bridge to prevent the c i r c u i t from o s c i l l a t i n g . Using the measurements of the previous s e c t i o n , the short - c i r c u i t c u r r e n t gain f o r the mwef was c a l c u l a t e d from Equation A5.3 of Appendix V. A comparison of t h i s r e s u l t with the r e s u l t obtained from the measurements u s i n g the mwef c i r c u i t j i g i s shown i n Figure A2.5. As can be seen from the f i g u r e , the experimental curve i s about 20% above the c a l c u l a t e d curve. This e r r o r i s t o l e r a b l e since t r a n s i s t o r s of the same type can va r y as much as 50% i n t h e i r c h a r a c t e r i s t i c s . 72 F i g u r e A2.5\u00C2\u00BB C o m p a r i s o n o f G a i n F r o m y ' s a n d F r o m M e a s u r e m e n t s 73 APPENDIX I I I RESISTOR - CAPACITOR PERFORMANCE In t h i s appendix the r e s u l t s of measurements made i n the 50 to 350 Megacycle frequency range on l/2 watt carbon r e s i s t o r s and 75 v o l t ceramic c a p a c i t o r s are examined. A3.1 R e s i s t o r Measurements The r e s u l t s of the r e s i s t o r measurements are shown i n F i g u r e A3.1. A r e s i s t o r can be represented as an i n d u c t o r i n s e r i e s with a p a r a l l e l r e s i s t a n c e - c a p a c i t a n c e c i r c u i t . This can be s i m p l i f i e d to a s e r i e s r e s i s t a n c e - i n d u c t a n c e c i r c u i t , or to a p a r a l l e l r e s i s t a n c e - c a p a c i t a n c e combination, depending on the value of the r e s i s t o r . For the r e s i s t o r s measured the s e r i e s - R L combination was found to h o l d f o r values l e s s than 100 ohms, while the p a r a l l e l - R C h e l d f o r values g r e a t e r than 100 ohms. The range of valu e s of the p a r a s i t i c inductance and capacitance was 5-12 nanohenries f o r the inductance, and 0.5-1 p i c o f a r a d f o r the c a p a c i t a n c e . The measurements were made u s i n g the General Radio 1607-A B r i d g e . The components were mounted on the Component Mount with the arrangement shown i n Fig u r e A3.2. \u00E2\u0080\u0094H U { * - \"Oio C O P P E R . i-uas, Figure A3.2* R e s i s t o r Mounting OtiMS, 75 A3.2 C a p a c i t o r Measurements The r e s u l t s of these measurements are shown i n Figure A3.3. A ca p a c i t o r , can be represented as a s e r i e s inductance-capacitance c i r c u i t . Examination of the inductance showed that i t was a t t r i b u t a b l e to the leads and was of the order of 10 nanohenries f o r a l l the c a p a c i t o r s considered. The measurements were done u s i n g the General Radio 1607-A B r i d g e . The c a p a c i t o r s were mounted as shown i n Fig u r e A3.4 on the Component Mount. 0\ b C O P P B R . LUG-F i g u r e A3.4. C a p a c i t o r Mounting A technique employed i n high\u00E2\u0080\u0094frequency work i s that of p a r a l l e l i n g c a p a c i t o r s i n order to achieve a lower l e a d inductance. This can,however, cause c o m p l i c a t i o n i f a c a p a c i t o r which looks i n d u c t i v e i s shunted by one which i s s t i l l c a p a c i t i v e . T h i s produces a p a r a l l e l - L C c i r c u i t which w i l l resonate. At the resonant frequency the c i r c u i t w i l l have a high impedance. Fi g u r e A3.5 shows what i s i n v o l v e d . J 1 1 F i g u r e A3.5* C a p a c i t o r s i n P a r a l l e l rzi IN OHMS n 1 , 1 1 T A N K C ^ C U l T / / 1 1 1 1 / \ s \ \ \ \ / \ / ' \ \">?* 1 \ 1 \ owe \ \ \ \ / / \ / \ / \ \ \ \ \ \ \ \ / y V /\ \ \ / \ / i \ / \u00E2\u0080\u00A2 \ \ / y / \u00E2\u0080\u00A2 / / / / r V / / \ V \ 5-0 lOO \"500 F i g u r e A3\u00C2\u00AB3. Cap a c i t o r Measurements 77 The resonant frequency f o r t h i s c i r c u i t i s given as 1/2TC J (L-J^ + L 2 ) C 1 C 2 / ( C 1 + C 2 ) . The frequency response f o r a 50 pF combination i s shown i n Figure A3.3. 78 APPENDIX IV BROADBAND TRANSFORMERS A4*l I n t r o d u c t i o n This appendix deals w i t h the p r o p e r t i e s of broadband transformers i n the 50 to 450 Megacycle frequency range. T h e o r e t i c a l and experimental r e s u l t s are given which i n d i c a t e the range of a p p l i c a t i o n . Conventional transformers are c h a r a c t e r i z e d by having a leakage inductance, a magnetizing inductance, and an i n t e r -(18) winding c a p a c i t a n c e v \u00E2\u0080\u00A2 \u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00E2\u0080\u00A2 The bandwidth of these devices i s l i m i t e d by the leakage inductance and the i n t e r w i n d i n g capacitance* The transformers d e s c r i b e d here are b i f i l a r wound. This means that each t u r n i s p a i r e d with another c a r r y i n g , i d e a l l y , an equal and opposite c u r r e n t . The i n t e r w i n d i n g capacitance thus becomes a component of the c h a r a c t e r i s t i c impedance of the r e s u l t i n g t r a n s m i s s i o n l i n e . Hence }the input and output windings can be c l o s e l y spaced together to o b t a i n good c o u p l i n g and s t i l l achieve a wide bandwidth. The winding arrangement f o r a 1:1 i n v e r t i n g transformer i s shown i n F i g u r e A4.1. The primary i s the s o l i d l i n e , while the secondary i s d o t t e d . For h i g h e r r a t i o t r a nsformers, as w i l l be i n d i c a t e d , the primary and secondary are not d i s t i n c t . The simple 1:1 transformer, however, i s considered f o r an i n i t i a l e x p l a n a t i o n of the o p e r a t i o n of these transformers. F i g u r e A 4 . 1 1 : 1 I n v e r t i n g Transformer A 4 . 2 Operation of B i f i l a r Transformers The o p e r a t i o n of the f o r e g o i n g 1 : 1 transformer and other b i f i l a r transformers can be e x p l a i n e d by assuming the current flow components shown i n the f i g u r e . For the symmetrical components, i g , the magnetic f i e l d s almost c a n c e l , and the inductance i s s m a l l . With the unsymmetrical (i- u) component, the magnetic f i e l d s add, and the mutual inductance w i l l be a p p r e c i a b l e . As can be seen from the f i g u r e , the e f f e c t of the un-symmetrical c u r r e n t flow i s to reduce the output c u r r e n t . How-ever, f o r an output v o l t a g e there must be some(i u) c u r r e n t flow ( i . e . there must be a net f l u x c i r c u l a t i n g i n the c o r e ) . Consequently, from the p o i n t of view of o b t a i n i n g an output v o l t a g e without a p p r e c i a b l y d i s t u r b i n g the symmetrical ( i ) s c u r r e n t s , the inductance to the ( i u ) c u r r e n t s should be l a r g e . Thus a lar g e number of turns would be d e s i r a b l e . However,as w i l l be shown i n S e c t i o n A 4 . 4 , a long transformer l i n e l ength i s d e t r i m e n t a l to the transformer high\u00E2\u0080\u0094frequency response. A 4 . 3 Autotransformer Representation I f the assumption i s made th a t the mutual reactance i s s u f f i c i e n t to ensure that only symmetrical c u r r e n t s flow, then 80 the o p e r a t i o n of the transformer of Fig u r e A4.1 can be represented u s i n g the autotransformer of Figure A4.2. Fi g u r e A4.2. Autotransformer E q u i v a l e n t of 1:1 Transformer This r e p r e s e n t a t i o n of the transformer i n terms of an autotransformer becomes quite u s e f u l i n the design of higher r a t i o d e v i c e s . An example which i l l u s t r a t e s t h i s i s the 1:2 cu r r e n t transformer of Figure A4\u00C2\u00AB3. The a c t u a l w i r i n g arrange-ment i s shown i n F i g u r e A4.3a\u00C2\u00BB while the autotransformer e q u i -v a l e n t i s shown i n Figure A4*3b\u00C2\u00AB In bothjthe symmetrical current flow i s i n d i c a t e d . r e s u l t i n g autotransformer e q u i v a l e n t c i r c u i t , i t i s easy to design a transformer of any d e s i r e d c u r r e n t r a t i o , i n v e r t i n g or non-(a) (b) F i g u r e A4.3. A 1:2 Current Transformer Using the concept of symmetrical c u r r e n t flow and the 81 i n v e r t i n g , A few f u r t h e r examples are given i n Figure A4.4, (c) 1:4 N o n i n v e r t i n g Cascaded (d) 1:4 N o n i n v e r t i n g D i s t r i b u t e d i F i g u r e A4.4, Examples of Autotransformer Design Technique I t should be noted t h a t each of these transformers would be wound on one f e r r i t e cor'e. When the p e r m e a b i l i t y of t h i s core i s h i g h , the same flux' threads each t u r n . Consequently, i n the case of higher c u r r e n t r a t i o transformers, the voltage induced per t u r n i n each stage should be the same. For example, consid e r the 1:4 transformer of F i g u r e A4.4c - i f the f i r s t stage has 2N t u r n s , the second should have N t u r n s . E x p e r i m e n t a l l y t h i s was found to be necessary f o r optimum o p e r a t i o n below 100 Megacycles. However,above 100 Megacycles t h i s was not r e q u i r e d . In p a r t i c u l a r , two 1:4 transformers were wound, one with 4 turns on each stage, and the second with 8 turns and then 4 t u r n s . The responses f o r these were measured u s i n g the General Radio 1607-A i i ! 82 T r a n s f e r F u n c t i o n B r i d g e , a n d a r e shown i n F i g u r e 4 . 5 . As c a n be s e e n t h e r e s p o n s e f o r t h e e q u a l t u r n t r a n s f o r m e r i s b e t t e r above FREQUENCY IN (MC , , 1 . i 1 \u00E2\u0080\u00A2*\u00E2\u0080\u0094 *-\u00E2\u0080\u00A2SO \00 ISO 200 TLSP 30O 350 -400 F i g u r e A 4 . 5 . R e s p o n s e o f l s 4 T r a n s f o r m e r 100 M e g a c y c l e s . T h i s i n d i c a t e s t h a t a t t h e s e f r e q u e n c i e s t h e f l u x must r e t u r n t h r o u g h t h e a i r s u r r o u n d i n g t h e t r a n s f o r m e r r a t h e r t h a n t h r o u g h t h e f e r r i t e . F r o m t h e s e r e s u l t s i t i s i n d i c a t e d t h a t a s h o r t l i n e l e n g t h i s d e s i r a b l e f o r h i g h - f r e q u e n c y a p p l i c a t i o n s . To d e t e r m i n e t h e e f f e c t o f l i n e l e n g t h on t h e t r a n s f o r m e r p e r f o r m a n c e i t i s c o n v e n i e n t t o u s e a t r a n s m i s s i o n l i n e e q u i v a l e n t c i r c u i t (19) f o r t h e t r a n s f o r m e r . A4\u00C2\u00AB4- T r a n s m i s s i o n L i n e R e p r e s e n t a t i o n F r o m t h e a s p e c t o f a c h i e v i n g a m a g n e t i z i n g i n d u c t a n c e , a l a r g e number o f t u r n s i s d e s i r a b l e , u n t i l t h e c a p a c i t a n c e a s s o c i a t e d w i t h t h e t u r n s b e g i n s t o l i m i t t h e r e s p o n s e \u00E2\u0080\u009E The e x p e r i m e n t a l r e s u l t s o f v a r y i n g t h e number o f t u r n s on a 1:2 t r a n s f o r m e r a r e shown i n F i g u r e A4.6. H e r e i t i s s e e n FREQUENCY IN MC H 1 1 1 1 1 1 1 5 0 t O O \ 5 Q ^ 0 0 - Z S O - 5 0 O - 5 S O 4 0 0 F i g u r e A4.6* R e s p o n s e o f 1:2 T r a n s f o r m e r f o r D i f f e r e n t W i n d i n g s t h a t t h e 4 - t u r n t r a n s f o r m e r i s b e t t e r t h a n t h e 8 - t u r n f o r f r e q u e n c i e s g r e a t e r t h a n 400 M e g a c y c l e s . T h i s r e s u l t f u r t h e r 84 i n d i c a t e s t h a t f o r high\u00E2\u0080\u0094frequency performance the l i n e l e n g t h should be s h o r t . The e f f e c t of l i n e l e n g t h and i n t e r w i n d i n g capacitance can best be s t u d i e d i f the transformers are viewed as a t r a n s m i s s i o n l i n e . I f the assumption i s made t h a t the cur r e n t s f l o w i n g are symmetrical, then the b i f i l a r windings can be approximated by l o s s l e s s t r a n s m i s s i o n l i n e s . In p a r t i c u l a r ^ the t r a n s m i s s i o n l i n e e q u i v a l e n t f o r the 1:2 transformer i s shown i n F i g u r e A4.7. The output c u r r e n t (1^ + I^) can be obtained as \u00C2\u00A9 + v, - \u00C2\u00AE T7T Figure A4.7. 1:2 Transformer w i t h Transmission Line E q u i v a l e n t e (1 + co s K 2RT + (R + 2RT )cos0Q. L s L y + (R LR + Z j p s i n J0 where (3 i s the phase constant f o r the l i n e , ZQ i t s c h a r a c t e r i s t i c impedance, and JL i t s e f f e c t i v e l e n g t h . Maximum power t r a n s f e r occurs when L = 0, and 4R = R,. Further, an optimum value f o r g L \u00E2\u0080\u00A2> ZQ can be found by minimizing the c o e f f i c i e n t of the s i n (3JL term as ZQ = 2R^. S u b s t i t u t i n g the values of ZQ and R^ and n o r m a l i z i n g the equation^ (I, + 1^(1,+ I 0 ) computed. These are p l o t t e d i n Fig u r e A4.8. values were 85 I n F i g u r e 4 . 4 , two w i r i n g a r r a n g e m e n t s were shown f o r a 1 : 4 t r a n s f o r m e r . N o r m a l i z e d c u r r e n t e x p r e s s i o n s w e r e d e r i v e d f o r t h e s e u s i n g t h e same a p p r o a c h as f o r t h e 1 : 2 t r a n s f o r m e r . I n t h e c a s e o f t h e c a s c a d e d 1 : 4 a r r a n g e m e n t , t h e e x p r e s s i o n was e v a l u a t e d f o r e q u a l t u r n s on b o t h s t a g e s , a n d f o r t h e r a t i o o f 2 t u r n s t o 1 . T h e s e r e s u l t s a r e shown i n F i g u r e A 4 . 8 , NORMALIZED (MAfiHlTUPef' F i g u r e A 4 . 8 . T h e o r e t i c a l R e s p o n s e C u r v e s I t i s s e e n f r o m t h i s f i g u r e t h a t t h e 1 : 4 t r a n s f o r m e r s do n o t h a v e a s b r o a d a b a n d w i d t h as t h e 1 : 2 . T h i s r e s u l t s f r o m m i s m a t c h i n g b e t w e e n t h e v a r i o u s s e c t i o n s o f l i n e s u s e d t o compose t h e s e t r a n s f o r m e r s . F u r t h e r , t h e 1 : 4 t r a n s f o r m e r w i t h e q u a l 8 6 t u r n s on e a c h s t a g e i s s u p e r i o r t o t h e one w i t h t h e r a t i o o f 2 t o l i F i n a l l y , t h e d i s t r i b u t e d t h r e e - s t a g e t r a n s f o r m e r a p p e a r s t o be b e t t e r t h a n t h e two\u00E2\u0080\u0094stage w i t h t h e r a t i o o f 2 t o 1 t u r n s . I n a l l o f t h e s e i t i s s e e n t h a t i t i s d e s i r a b l e t o h a v e as s h o r t a n e f f e c t i v e l i n e l e n g t h as p o s s i b l e , A4<>5 E x p e r i m e n t a l Work T h r e e 1:4 a n d s e v e r a l 1:2 t r a n s f o r m e r s were b u i l t . The t r a n s f o r m e r s were c o n s t r u c t e d on 0.2\" d i a m e t e r f e r r i t e c o r e s . Two d i f f e r e n t t y p e s o f f e r r i t e c o r e s were e m p l o y e d . One was s p e c i f i e d f o r a p p l i c a t i o n s b e l o w 100 K i l o c y c l e s , w h i l e t h e o t h e r was i n t e n d e d f o r a p p l i c a t i o n b e l o w 10 M e g a c y c l e s . H o w e v e r , i n t h e f r e q u e n c y r a n g e o f i n t e r e s t t h e r e was p r a c t i c a l l y no d i f f e r e n c e i n t h e r e s p o n s e o f t r a n s f o r m e r s wound o n t h e two d i f f e r e n t f e r r i t e s . T h i s , o f c o u r s e , i s due t o t h e f a c t t h a t a t h i g h f r e q u e n c i e s t h e p e r m e a b i l i t i e s o f b o t h f e r r i t e s w o u l d be p r a c t i c a l l y u n i t y . The b i f i l a r l i n e was made b y t w i s t i n g two p i e c e s o f #34 w i r e t o g e t h e r . T h i s l i n e when c o a t e d w i t h G l y p t a l V a r n i s h h a d a c h a r a c t e r i s t i c i m p e d a n c e o f a b o u t 80 ohms, f r o m 50 M e g a c y c l e s t o 500 M e g a c y c l e s . F o r c o m p a r i s o n w i t h t h e t h e o r e t i c a l c u r v e s o f S e c t i o n A4.4 t h e n o r m a l i z e d e x p e r i m e n t a l c u r v e s f o r t h r e e o f t h e t r a n s -f o r m e r s a r e shown i n F i g u r e A4.9. The l o a d i m p e d a n c e f o r t h e r e s p o n s e c u r v e s shown was 47 o h m s . The e f f e c t o f h i g h e r i m p e d a n c e s i s t o r e d u c e t h e c u r r e n t o u t , w h i l e l o w e r i m p e d a n c e i n c r e a s e s i t . The c h o i c e o f 47 ohms was d i r e c t e d a t s i m u l a t i n g t h e l o a d t h e t r a n s f o r m e r s w o u l d be w o r k i n g i n t o i n t h e i n t e n d e d 87 c i r c u i t a p p l i c a t i o n o f C h a p t e r 5* \.o. .fe. -. MORMAL\ZED(GAIN> 1.2. F R E Q U E N C Y \N M C F i g u r e A4.9. E x p e r i m e n t a l R e s u l t s The c u r v e s f o r t h e 1*4 t r a n s f o r m e r shown i n F i g u r e A4.9 i n d i c a t e t h a t t h e t w o - s t a g e t r a n s f o r m e r i s b e t t e r t h a n t h e \u00E2\u0080\u00A2 d i s t r i b u t e d t y p e - T h i s i s c o n t r a r y t o t h e r e s u l t o f S e c t i o n 4.5, b u t c o u l d be e x p l a i n e d b y t h e i n c r e a s e d c o m p l e x i t y o f the w i r i n g f o r t h e d i s t r i b u t e d f o r m . T h i s p o i n t was n o t p u r s u e d f u r t h e r as t h e r e s u l t s i n d i c a t e d t h a t t h e h i g h e r c u r r e n t - r a t i o t r a n s -f o r m e r s do n o t h a v e as w i d e a b a n d w i d t h as t h e 1:2. A d i r e c t c o m p a r i s o n b e t w e e n t h e t h e o r e t i c a l a n d e x p e r i m e n t a l r e s p o n s e i s n o t p o s s i b l e w i t h o u t k n o w i n g t h e |3 L t e r m o f t h e e x p e r i m e n t a l r e s u l t s . F o r t h e 1:2 t r a n s f o r m e r , t h e pi t e r m c a n be r e a d i l y o b t a i n e d f r o m t h e s h o r t - c i r c u i t a d m i t t a n c e m e a s u r e m e n t s . The s h o r t - c i r c u i t a d m i t t a n c e i s g i v e n a t t h e 88 g e n e r a t o r e n d b y y _ c o s 3 sc - 3 Z Q s i n 3S2. ' K n o w i n g ZQ f o r t h e b i f i l a r w i n d i n g s , ^ 3 t- c a n be c a l c u l a t e d . T h i s c o u l d be done f o r t h e 1 : 4 t r a n s f o r m e r as w e l l , b u t t h e s h o r t c i r c u i t a d m i t t a n c e e x p r e s s i o n i s c o n s i d e r a b l y more c o m p l i c a t e d . F i g u r e A 4 . 1 0 t h u s shows t h e d i r e c t c o m p a r i s o n b e t w e e n t h e t h e o r e t i c a l a n d e x p e r i m e n t a l v a l u e s f o r t h e 1 : 2 t r a n s f o r m e r s . F i g u r e A 4 . 1 0 . C o m p a r i s o n o f T h e o r e t i c a l a n d E x p e r i m e n t a l R e s u l t 89 A4.6 Conclusions The cu r r e n t r a t i o s f o r the experimental transformers are of course l o v e r than the t h e o r e t i c a l ones as the c u r r e n t flow i n the b i f i l a r l i n e i s not e n t i r e l y symmetrical. However^ as S e c t i o n 4.4 i n d i c a t e d ^ t h e l i n e l e n g t h cannot be i n c r e a s e d to improve t h i s symmetrical c u r r e n t flow without decreasing the bandwidth. Thus> f o r these transformers a compromise must be made between cu r r e n t r a t i o and bandwidth. For the 1:2 transformer used with the t c e f of Chapter 5t an 8-turn transformer was found most s a t i s f a c t o r y f o r the frequency range of i n t e r e s t * 90 APPENDIX V MWEF LOOP GAIN In t h i s appendix the loop g a i n i s d e r i v e d i n terms of the y parameters. A comparison i s made between the exact expression, the s h o r t - c i r c u i t loop g a i n , and the i d e a l i z e d loop g a i n . C o n s i d e r i n g the c i r c u i t shown i n Fig u r e A5.1 the network r admittance matrix j Y i can be r e a d i l y obtained from V,. m Figure A5.1. S i m p l i f i e d MWEF ( Y i b + y Q e ) ( y r b + y f e ) (y*>, + y \u00E2\u0080\u009E J (yn-h + y,-J or ..(A5.1) To o b t a i n the loop g a i n the c i r c u i t of Figure A5 ..2 i s used. 91 Figure A5.2, m irr MWEF f o r Gain C a l c u l a t i o n s ^ i b 0 \" V l \" = y f b (y + y- ) w oe J l e y J re X V 2 . 0 ^oe 73 S e t t i n g V 3 = V 1 , and I 2 = 0, the c l o s e d - l o o p c u r r e n t g a i n i s h.1 _ y o e ( y 0 b + y i e } \u00E2\u0080\u00A2 y f e ( y f b + O T 1 V 1 l J y^(y.-h - y ^ y \u00E2\u0084\u00A2 + y r J (A5.2) i b V J o b ' \" i e ' \" r b ' T b \"re' S e t t i n g = 0 the s h o r t - c i r c u i t loop c u r r e n t gain i s ~ y f e y f b 1. ^ s c y i b ( y 0 b + y i e } - W f b ..(A5.3) Assuming t h a t y \u00C2\u00B1 e > y Q b and t h a t y l b y i e > y r b y f b ) t h e a P P r o x i m a t e e x p r e s s i o n ~ y f e y f b y i b y i e ..(A5.4) of Chapter 4 r e s u l t s . These three Equations (A5.2, 5\u00C2\u00BB3 and 5,4) 92 were evaluated u s i n g the y parameter values of Appendix II and are shown i n Figure A5.3.. As can be seen from the f i g u r e ^ t h e s h o r t - c i r c u i t loop g a i n and c l o s e d - l o o p gain are p r a c t i c a l l y i d e n t i c a l . However., the simple approximation of Chapter 4 i s i n e r r o r about 4 db and 40 degrees at 350 Megacycles. As the loop gain i s determined i n Chapter 4 by measuring the input admittance w i t h the loop open and then c l o s e d , the v a l i d i t y of t h i s procedure w i l l now be examined. With the loop c l o s e d the input admittance i s given by \u00E2\u0080\u009E _ A ( y j b + y 0 e ) ( y 0 b + y i e } - ( y f b + y r e ) ( y r b + y f e > c = A n = y o b y i e where A = det JY j and A - ^ i s the 1,1 minor of [ I j . Now i f the input admittance i s then measured with the loop open and the output shorted, t h i s i s T - \u00E2\u0080\u0094\u00E2\u0080\u0094 i { y i b(y. 0b + y i e ) - yr b y f b y o b + y\u00C2\u00B1e Now I 0 ( l + A I)=Y C, or (1 + A T ) = - . ...(A5.5) y i b ( y 0 e + y\u00C2\u00B1e ) - W f b S o l v i n g f o r A T y o e ( y o b + y i e 5 *- y f b y f e - y r e ( y r b + y f e ) A I = y i b i y o b + y i e 5 \" y r b y f b (3-AIU IN ells 94 Comparing t h i s with A5.2 i t i s seen that the expressions are p r a c t i c a l l y i d e n t i c a l since y ^ > y r g and y \u00C2\u00A3 e > y r b (see Appendix II ) \u00E2\u0080\u00A2 Hence A-j- = I ^ / l | and the g a i n obtained from the admittance measurements i s the closed\u00E2\u0080\u0094loop g a i n . One f u r t h e r p o i n t should perhaps be noted. From A5.5 an a l t e r n a t e approach to network s t a b i l i t y can be seen. With Equation 5.1 a necessary and s u f f i c i e n t c o n d i t i o n f o r the e x i s t e n c e of [ v ] with [ i ] = 0 i s t h a t det [ l ] = A = 0. From Equation A5.3 i f A = 0 then 1 + A-j- = 0 and hence Aj = -1. This c o n d i t i o n was d e r i v e d i n Chapter 6 by c o n s i d e r i n g the input admittance to the mwef. 95 APPENDIX VI 2X2 PULSE GENERATOR This appendix d e s c r i b e s the pulse generator that was designed to d r i v e the 2x2 a r r a y of S e c t i o n 6.1. To achieve the d e s i r e d r i s e times of about 20 nanoseconds 5' 2N708 t r a n s i s t o r s were employed almost e x c l u s i v e l y throughout the generator. This t r a n s i s t o r has a t y p i c a l f^, of about 4 0 0 Megacycles. The b l o c k diagram f o r the device i s shown i n Figure A6.1. The numbers t h a t appear i n t h i s f i g u r e correspond to s i m i l a r numbers i n the f a l l o w i n g f i g u r e s . Figure A6.1. 2X2 Pulse Generator Block Diagram The a s t a b l e , b i s t a b l e , and monostable c i r c u i t s are the 96 (a) Astable 33K ^ FROM 2 o o F < O R c H I J (V) ( y \u00E2\u0084\u00A2, p. 138. 8. Rajachman, op. e i t * ^ p. 125. 9. Salama, C.A.T., \"The S t a t i c and Dynamic C h a r a c t e r i s t i c s of Series\u00E2\u0080\u0094Connected Tunnel Diodes and t h e i r A p p l i c a t i o n s i n D i g i t a l C i r c u i t s \" , M,A.Sc. Thesis j, U#B.C, p. 17, December 1962. 10. Salama, C.A.T., op. c i t . , p. 17. I I . F a r l e y , Elements of Pulse C i r c u i t s , London, Metheun and Co., 1953. 12. Goulding, F.S., P r i v a t e Communication. 13. Joyce, M.V., and Clar k e * K.K 0, T r a n s i s t o r Network A n a l y s i s * Reading* Mass., Addison-Yesley, I960;, p. 23. 14. Bode, H.Y., Network A n a l y s i s and Feedback A m p l i f i e r Design, P r i n c e t o n * Van Nostrand, 1945, p. 151. 1.5. Bode, H.V., op. cit-\u00C2\u00AB-* Chapter 18. 1.6. Meyers* P., \"Trends on Nanosecond Switching\", Ele c t r o n i c s , V o l * 3 6 | No. 38j, pp. 35-38, September 20, 1963. 17. Meyers, P., op. c i t * * p. 36. 101 18. Millman, J . , and Taub, H., Pulse and D i g i t a l Ci: New Xork, McGraw-Hill, 1956, Chapter 9. 19. R u t h r o f f , C.L., \"Some Broad-band Transformers\", PIRE, V o l . 47 No. 8. pp. 1337-1342, August 1959. "@en . "Thesis/Dissertation"@en . "10.14288/1.0104938"@en . "eng"@en . "Electrical and Computer Engineering"@en . "Vancouver : University of British Columbia Library"@en . "University of British Columbia"@en . "For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use."@en . "Graduate"@en . "A small high-speed tunnel-diode memory."@en . "Text"@en . "http://hdl.handle.net/2429/37813"@en .