Behavior of Noise Figure in Junction Transistors

It is well-known that generator resistance and dcemitter current are two major factors in determining the noise figure of junction transistors. Other important factors are base resistance, low-frequency alpha, and alpha cutoff frequency. A method of calculating the noise figure in terms of these parameters, the frequency variation of the noise figure, and the conditions for minimizing the noise figure are presented. The results provide means for specifying transistor and circuit parameters to meet noise requirements. In the absence of "excess" or 1/f noise, the noise figure as a function of frequency for the common base, common collector, and common emitter configurations is constant up to ¿1-a0 fa. Here a0 is the low-frequency alpha and fa the alpha cutoff frequency. Above this frequency the noise figure increases toward an asymptote of 6 decibels per octave. Calculations show that for minimum noise figure the base resistance and emitter current should be small, a0 should be close to one, fa should be large, and the driving source resistance has an optimum value. This work is based on a simplified version of a transistor noise equivalent circuit developed by van der Ziel.