"Walter Maxwell" wrote in
:
....
Owen, as I view your last paragraph above it seems apparent that you
do not believe Richard's and my position that the output of a Class C
amplifier can be linear. We're talking about at the 'output', not the
'thruput'. How can you refute the evidence of a nearly pure sine wave
at the output terminals of the pi-network?

Walt, you have posted this twice.

There are subtle word shifts here, you are saying "a Class C
amplifier can be linear" rather than is (always) linear.

It is true that a Class C amplifier with resonant load and a constant
amplitude sine wave drive may appear linear when comparing Vout to Vin.

But, as I explained earlier, if you vary the drive amplitude, it is
clearly not linear... in typical cases output will cease below about 25%
of the drive level required for maximum output.

Further, if you drive it with a complex waveform, it is clearly non
linear at any drive level.

Richard's solution to detecting RF PA distortion by monitoring harmonics
is an interesting one, because it suffers the disadvantage of output
filtering masking the harmonics (unless the monitor point was prior to
filtering).

The most widely accepted test for linearity (Vout/Vin) of an RF PA is the
'two tone test', where the drive is a complex waveform (the sum of two
equal amplitude sine waves quite close in frequency) and at least some of
the distortion products due to third order and fifth order etc transfer
terms appears in-band in the output after all output filtering, and where
they can be reliably compared in amplitude to the desired signals. A
Class C RF PA will not appear to be linear under such a test at any drive
level.

I suspect that the issue of transfer linearity is a red herring to your
proposition about the Thevenin equivalent of an RF PA, but if you do
depend on arguing that the transfer characteristic of a Class C RF PA is
linear, I think you are on shaky ground.

Owen