Asimov wrote:
"The maximum power transfer 50% efficiency figure is only valid for a
"linear" amplifier termed Class A (current flows through the whole
cycle)."

Maximum power transfer does not care how many degrees of a cycle the
power flows. Nor does it care whether the source is electronic. Whether
it applies depends entirely on the behavior of the source and load.

It`s true that current in a linear Class A amplifier flows continuously
and does not vary in average amplitude over a complete cycle.

When there`s no signal input to a Class A amplifier, its power input is
volts x amps and its efficiency is zero. Maximunm undistorted output
requires 1/2 the d-c input to the stage. As this power exits the stage,
it cools as the remaining dissipation is only 1/2 the no-signal
dissipation.

An amplifier can be a linear source without operating under Class A
conditions. Higher efficiency is the main reason for not operating in
Class A. By turning the amplifier devices off
for part of each cycle, efficiency can be raised significantly beyond
50%.

Best regards, Richard Harrison, KB5WZI