AM Cross Modulation in Transistor RF Amplifiers

Cross modulation rejection vs bias variations in semiconductors is not a smooth function as it is with vacuum tubes. The reason for these large variations with bias point change has never been adequately explained. It is the purpose of this paper to introduce into the cross modulation analysis a component not previously considered as having an active part. This component is the detected audio component of the undesired signal. It will be shown how this audio component both increases and decreases the cross modulation rejection over normal bias ranges. Furthermore, it will also be shown that there exist very high cross modulation rejection points and that these points can be predicted from standard RF parameters. Through the use of audio circuit techniques these high rejection points can be made to move so that under severe cross modulation conditions a high rejection situation can be created.