Optimum Noise Figure of Transistor Amplifiers

Noise figure of transistor amplifiers is commonly optimized by transforming of source impedance. Where transformers are not feasible, this method is inapplicable. This paper shows that noise figure can also be optimized by proper adjustment of emitter current with the source impedance remaining fixed. Moreover, for a rather wide range of source impedances, transformation affords very little noise improvement once the current has been optimized. Circuit design equations and curves are presented.