TABLE OF CONTENTS The almost universal choice in semiconductor power amplifiers is for a unity-gain output stage, and specifically a voltage-follower. Output stages with gain are not unknown see Mann[1] for a design with ten times gain in the output section but they have significantly failed to win popularity. Most people feel that controlling distortion while handling large currents is quite hard enough without trying to generate gain at the same time.
In examining the small-signal stages, we have so far only needed to deal with one kind of distortion at a time, due to the monotonic transfer characteristics of such stages, which usually (but not invariably[2]) work in Class A. Economic and thermal realities mean that most output stages are Class B, and so we must now also consider crossover distortion (which remains the thorniest problem in power amplifier design) and HF switchoff effects.
We must also decide what kind of active device is to be used; JFETs offer few if any advantages in the small-current stages, but power FETS in the output appear to be a real possibility, providing that the extra cost proves to bring with it some tangible benefits.
The most fundamental factor in determining output-stage distortion is the Class of operation. Apart from its inherent inefficiency, Class-A is the ideal operating mode, because there can be no crossover or switchoff distortion. However, of those designs which have been published or reviewed, it is notable that the large-signal distortion produced is still significant.
