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Fig 1: The incremental gain of a CFP output stage as output voltage changes. SPICE (Simulation
Program with Integrated Circuit Emphasis) for 8 and 4 Ohm loads.
There has always been a desire for a compromise between the efficiency of Class B and the
linearity of Class A and the most obvious way to make one is to turn up the quiescent current of a
Class B stage giving what is called Class AB operation. As this is done, an area of Class A
operation, with both output transistors conducting, is created around the zero-crossing.
In fact as this area widens as the quiescent current increases, until ultimately it encompasses the
entire voltage output range of the amplifier, there is thus an infinite range of positions between the
two extremes of Class B and Class A, and this range of modes of operation is referred to as Class
AB.
Unfortunately, while Class AB would seem to be a perfect compromise between Class A and Class
B operation, it does have some hidden issues.
It can be shown [Ref 1] that if Class AB is used to trade-off between efficiency and linearity, its
performance is certainly superior to B below the AB transition level, operating as it does in this
region in pure Class A. This can have very low THD indeed, at less than 0.0006% up to 10 kHz [Ref
3].
However, once the signal exceeds the limits of the Class A region, the THD worsens and does so
somewhat abruptly due to the gain-changes when the output transistors turn on and off. Linearity is
in fact inferior not only to Class A but also to optimally-biased Class B. This is not always fully
appreciated. The effect is sometimes called "gm-doubling".
Class AB distortion can be made very low by good design, but remains at least twice as high as for
the equivalent Class B situation. The bias control of a Class B amplifier actually does not give a
straightforward trade-off between power dissipation and linearity at all levels; this is often not well
