[Amps] No class F amplifier for now...

[Manfred Mornhinweg]

Several people on this forum have asked me to keep them current on my 
solid state class F amplifier project with envelope restoration.

Unfortunately I have bad news.

After building a scale model of it, building a setup to accurately 
measure IMD and other parameters, and trying everything I can think of, 
the conclusion is that I cannot achieve sufficiently low distortion with 
this approach.

The main problem is that the internal capacitances of a MOSFET, and 
specially the reverse transfer capacitance, increase dramatically at low 
drain voltage, and this causes a strong transfer of drive power to the 
output. As a result, at a certain drive level, typically a few percent 
of the maximum amplitude, the true-phase coupled drive power cancels 
with the opposed-phase output power, generating a null in the output. 
Above that drive level, the amplifier works as expected, while below 
this level the drive feedthrough predominates and causes reversed-phase 
output.

That phase reversal, coupled with the very distorted amplitude response 
in the crossover region, makes the IMD performance very poor, with the 
third order IMD products being only about 14dB down from each tone, in a 
two-tone test signal. It's so bad that the transmission even is audibly 
distorted, with a raspy sound.

It might be possible to obtain acceptable performance by combining class 
AB at low drive levels with class F at high levels, but biasing is 
extremely critical in this case because for class F a floating load is 
required, which leads to the MOSFETs easily saturating or cutting off 
when in class AB mode, and thus causing strong cross-over distortion. 
Even if this problem can be controlled, the elegance of pure class F 
would be lost, which in turn makes me loose interest in developing such 
a beast.

That's it, folks. Sorry for nurturing big expectations. At least it was 
a great learning experience.

73,
Manfred, XQ2FOD.



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