[Amps] Linearization problem

[Steve Thompson]

> And a bonus question: Do you think that the phase distortion in
> such a class-A-AB-C-E amplifier will be bad enough to still
> cause poor IMD performance, even while the envelope linearity
> is excellent?
>

Yes. Around 1980 I prototyped an amplifier aimed at improving 
efficiency in a military hf manpack. The output stage was a 
wideband push pull affair which behaved like class B at low drive 
levels, moving towards class D at full output. A 'polar loop' 
(Petrovic and Gosling, Bath University) applied amplitude feedback 
via the PA bias supply and phase feedback via the synth PLL. With 
either not connected, the output spectrum was very wide - as best 
I remember, both loop bandwidths needed to be some 10's kHz to get 
acceptable results.

Later the design evolved to use 'cartesian loop' feedback with I/Q 
channels which are commonplace in modern systems but I think it 
got shelved. System integration onto chips needed to develop 
another decade or so to make it a practical alternative in that 
environment.

Steve