ABSTRACT
FETs are often sloppily described as square-law devices. The notion of synthesising a linear transfer characteristic by a combination of square laws. The idea of combining square laws to get a linear result surfaced again in Ian Hegglun's design for Linear Audio. Michael William's basic circuit illustrates, and the problem to overcome in applying it for audio power is that the wanted output is the difference of two currents whereas hard-bitten amplifier designers are more used to a low impedance voltage output. The more usual emitter follower type of opamp output is usually called a common-collector (CC) stage. It is highly probable that the widest application of these voltage-efficient common emitter (CE) configurations is in the headphone amplifiers of personal stereos. At high frequencies the distortion is easily measurable, and almost all of its results from crossover effects in the output stage. Since negative feedback (NFB) typically falls with frequency, these high-order harmonics receive much less linearisation.
