Distortion due to the mismatch of transistors in push-pull audio-frequency amplifiers

In a class-B push-pull amplifier having balanced transformers, evenorder harmonic distortion can be caused by differences in the characteristics of the two transistors. By considering the voltage gain of the two halves of the amplifier it is possible to obtain general expressions from which distortion caused by any particular characteristic can be calculated. The paper is primarily concerned with audio-frequency amplification. It is necessary to consider harmonics outside this range because of the possibility of their giving rise to intermodulation products in the a.f. range. The parameters giving rise to distortion can be divided into the frequency-independent and frequency-dependent types. With regard to the frequency-independent parameters, the only significant factors are the current gain factor, the variation of basecollector current gain factor with emitter current and the extrinsic base resistance. In the case of the frequency-dependent parameters, which, in general, only become important at the higher audio frequencies, the significant factors are the collector capacitance, the variation of current gain with frequency and the carrier-storage effect. In order to obtain low distortion, some matching of transistor characteristics is necessary. Methods are described whereby this can be achieved by fairly simple measurements. Most of the work described in the paper is based on a low-power junction-type transistor. However, the methods used are quite general, and therefore applicable to other types of transistors.