Cross modulation and nonlinear distortion in RF transistor amplifiers

In order to avoid untractable calculations, the transistor four-pole is assumed to be short-circuited for ac at its output. Furthermore, the internal impedance of the signal source is assumed to be zero. First the nonlinear distortion effects in a grounded base intrinsic transistor are calculated. Then, the formulas are reverted to a grounded emitter intrinsic transistor, taking into account the extrinsic base lead resistance. They are confirmed by measurements of third harmonic distortion and of cross modulation. The measured curves of cross modulation vs collector bias current show a sharp minimum. This unexpected effect is explained by an extension of the theory, which takes into account previously neglected terms. The explanation is successfully tested by experiments. Comparisons with cross modulation in amplifier tubes are made.