> A linear does not demand constant gain at any power level, within the
> context of this discussion
You have an interesting definition of linear then. An "amplifier for SSB"
may not demand constant gain, but by definition, a linear amplifier does. It
also requires linear phase but I seldom see a phase/amplitude AM/PM
conversion or group delay plot for an amateur PA.
The DAF circuit relies on the envelope of the drive signal. If it can peak
hold well enough then it will appear to be linear. Hence the good 2 tone
test results. It will also work well for speech as long as the speech has
sufficient energy to maintain the screen potential nearly constant. The IMDs
will be a function of the modulation envelope and the time constant of the
screen supply.
A better way is to use a detector that only biases the valve on when there
is a signal to amplify, but to ensure no nasty switch on transients, it
really needs a delay line. This is easy with DSP and is something that
modern rigs could usefully add to their feature list. The DSP could also do
useful speech filtering, pre-distortion and compression.
Mike
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