UBC Theses and Dissertations
Noise figure of the transistor amplifier Hatton, Walter Lewis
The noise figure of the transistor amplifier is of special interest since the large quantity of excess noise present limits the usefulness of the transistor as a low-noise amplifier. While the equivalent noise voltages generated in the emitter and collector leads vary inversely with frequency, the noise figure does not. This variation from the inverse frequency characteristic is produced by the decrease of the effective gain at higher frequencies. The subject of this thesis is the investigation of this variation. The variation of the noise figure with frequency has been measured for several different transistors, with varying emitter and collector currents. Equivalent diagrams have been suggested which explain the variation of the noise figure with frequency if transit time dispersion is negligible. For low frequencies, the noise figure is given by, (Formula omitted) Variation of the noise figure due to the stray capacities is given approximately by (Formula omitted) The variation, due to transit time effects , ignoring transit time dispersion , i s (Formula omitted) The operating conditions for minimum noise figure of the transistor amplifier are dependent on the frequency. At low frequencies, a low value of emitter current should be used, and the lowest possible value of collector voltage which is compatible with the desired gain. At higher frequencies a low value of emitter current should also be used but the value of the collector voltage is determined by the frequency. The higher the frequency, the higher will be the required collector voltage. The best noise figures at higher frequencies will be obtained with transistors with the following characteristics: (i) ʳm large (ii) close spacing of point contacts (iii) high value of resistivity (iv) small transit time dispersion. Equivalent diagrams have been suggested which parially account for the variation of the noise figure with frequency. It has been suggested that transit time dispersion would explain the remaining deviation of the noise figure from the inverse frequency characteristic.
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