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Walter Maxwell · June 16th 08, 04:11 AM posted to rec.radio.amateur.antenna

"Owen Duffy" wrote in message
...
(Richard Harrison) wrote in news:26406-
:

...
Class C amplifiers are used lawfully in great abundance. That is proof
enough that they are relatively free from distortion. Pulses in plate
current don`t prevent the output of the Class C amplifier from becoming
a pure sinusoid.

... a very long dissertation on Class C amplifiers snipped.

Richard, analysis of the Class C amplifier excited with a constant
amplitude single frequency sine wave is revealing about their transfer
linearity.

I do not disagree that a Class C amplifier excited with a constant
amplitude single frequency sine wave driving a resonant load produces a
low distortion constant amplitude single frequency sine wave output.

But the absence of harmonic distortion in such an amplifier is not
evidence that the amplifier transfer characteristic is linear. You may be
able to use harmonic distortion to detect non-linearity in, for example,
audio amplifiers... but not in RF amplifiers with a resonant load... for
the reasons set out in your quotation.

A Class C amplifier is unsuited to amplfying SSB telephony because it is
manifestly non-linear. In fact, a Class C amplifier is so non-linear that
it is well suited to use as a relatively efficient harmonic multiplier.

Class B and AB RF amplifiers are extremely sensitive to non-linearity in
the region near cut-off and must have sufficient idle current in every
active device (which means conduction ange is 180°) so that distortion
products are sufficiently low. This means that the theoretical conduction
angle of 180° for Class B is just not realisable because of distortion,
much less 120°.

Owen

Sorry about the 'long dissertation on Class C amps', Owen, but I thought it
appropriate to include it in view of Richard's similar discussion on the
automotive engine analogy to the RF tank circuit. I'll try to keep my comments
shorter from now on.

Walt, W2DU