Intermodulation Analysis of Class C Transistorized Amplifiers with Applications to V.H.F. Amplifiers

Abstract : The generation of intermodulation distortion products in non-linear radio amplifier circuits has always constituted an interference problem of much concern. It can be especially troublesome in today's air-ground voice transmissions as the increasing density of air traffic dictates the use of more of the available broadcast frequency channels with decreasing bandwidth and separation between channels. The foregoing intermodulation distortion analysis involves transistorized class C amplifiers considered for use in the transmitter output stage of FAA established ground-to-aircraft transmission links. The type of intermodulation problem discussed in this paper occurs when one frequency (f sub 1) is introduced at the input of a transmitter amplifier and a second frequency (f sub 2) is introduced through the output of the amplifier. Since a class C transistorized amplifier is somewhat nonlinear, the two frequencies interact to produce a third intermodulation frequency (2 f sub 1 - f sub 2) which shows up in the output but are usually insignificant when compared with the amplitude of 2 f sub 1 - f sub 2.