For
on-off keying of a switch mode PA you really should be using
supply line control. Switching the drive then trying to
reduce rise /fall times for keyclick elimination by
controlling drive amplitude(something that seems to be done in
some designs) is really asking for trouble. Even
hard-switching drive gives its problems , as we see here
repeatedly
Instead,
why not a simple emitter follower in the positive supply, then
you can apply waveform shaping to the base drive at low
current, where you've got full control. Even a 20A supply
could be controlled this way with a suitably rated
transistor. Quite likely have to be a Darlington. There
will be a slight fixed drop across it, say 1V at saturation /
full power, so a few watts of dissipation, but it's only
operating non saturated for a short period during the rise /
fall times.
A power
FET could be used, but you'd need a drive voltage Vgs higher
than the supply, albeit only a few mA capability; nothing a
small switcher module couldn't manage.
