FLATTERED BY THE opportunity to publish a project circuit, the designer is often beset by seemingly contradictory considerations. On the one hand, it is tempting to design a complex circuit as a demonstration of technical prowess; an amplifier with large numbers of esoteric components performing obscure functions. Such an amplifier might be a smorgasbord of electronic technique, featuring class A operation, cascoding, constant current sources, current mirrors, and extra-loop error correction. It would be fascinating to build and perhaps would also sound good…

The distortion is inherently lower without the need for cleaning up via feedback, thus class A lends itself well to low distortion performance in a simple circuit with low open loop gain. Fig. 2 shows the transfer curves for a push-pull emitter follower output stage operated in class A, B, and AB modes where the crossover distortion is apparent in the discontinuity of the curve. In class AB, this effect is alleviated by a small bias current, and then is eliminated in class A where the bias current is high. Fig. 3 shows the open loop output impedance of these stages where the class B amplifier is seen to rise abruptly at the discontinuity, whereas the class AB actually drops at the point where both halves conduct current. The class AB amplifier can be said to run in class A over this small region and will exhibit class A performance at small current levels. The class A curve can be observed to be the smoothest of the three in an effect which can be looked upon as the damping factor of the amplifier multiplied by the amount of feedback employed. Naturally, this kind of performance has a price tag, and with class A operation, the low efficiency causes considerable energy loss. Class A power amplifiers require large power supplies to handle this energy, but the task is not as enormous as might be imagined.

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