Behavior of noise figure in junction transistors over their useful frequency spectrum

Abstract

It is known that the driving source resistance and DC emitter current are two major factors in determining the noise figure of junction transistors. Other factors are the 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 will be presented. For the common base, common collector, and common emitter configurations the noise figure as a function of frequency is constant up to a given frequency value. Above this frequency, it increases toward an asymptote of 6 db per octave. Calculations show that for minimum noise figure the base resistance and emitter current should be small, low-frequency alpha should be close to one, the alpha cutoff-frequency should be large, and the driving source resistance has an optimum value. Experimental verification of the results to frequencies beyond the alpha cutoff-frequency will be presented. The work is based on a simplified version of a transistor noise equivalent circuit developed by A. van der Ziel (Theory of Shot Noise in Junction Diodes and Junction Transistors, Proceedings of rhe IRE, November, 1955, Vol 43, pp 1639-1646).