White paper
Fig 7: The concept of push-pull crossover displacement. The control circuitry implements a scaling
factor of -a.
The use of push-pull displacement is analogous to the use of push-pull current sources in Class A
amplifiers, where there is a well-known canonical sequence of increasing efficiency, illustrated in Fig
8. [Ref 5] This begins with a real resistance giving 12.5% efficiency at full power, moves to a
constant-current source with effective infinite impedance giving 25%, and finally to a push-pull
controlled current-source, giving 50%. In the last case the sink transistor acts in a sense as a
negative resistance, though it is more usefully regarded as a driven source (VCIS) than a pure
negative resistance, as the current does not depend on rail voltage. In each move the efficiency
doubles. Note that these efficiency figures are ideal, ignoring circuit losses, and that efficiency is
