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The extra current therefore flows through Re1, and the extra voltage drop across it means the
output voltage must go negative before the current through Re1 stops and that in Re2 starts. In
other words, the crossover point when Q3 hands over to Q4 has been moved to a point negative of
the 0V rail; for the rest of this article we refer to this as the “transition point”. For output levels
below transition no crossover distortion is generated. The resulting change in the incremental gain
of the output stage is shown in Fig 4.
The displacer current could of course be constant, or, as we then developed in more sophisticated
implementations could even vary with the signal…
We now have before us the intriguing prospect of a power amplifier with three output devices, which
if nothing else is novel. The operation of the output stage is inherently asymmetrical, and in fact
that is its raison d’être, but this should not cause undue alarm. Circuit symmetry is often touted as
being a pre-requisite for either low distortion or healthy operation in general, but this has no real
basis in fact. A perfectly symmetrical circuit may have no even-order distortion, but it may still have
any amount of odd-order non-linearity, such as a cubic characteristic. Odd-order harmonics are
normally considered more dissonant and often more noticeable and deleterious to perceived sound
quality in listening tests, so circuit symmetry in itself is not a useful goal.
