[Amps] new 2200m/630m amplifier - distorted waveform output

[Manfred Mornhinweg]

Roger,

> I will follow your advice and work on eliminating the turn-on transient. 
> There are 56 Ohm gate resistors

That's quite high for such large FETs. I would use much lower ones. Note 
that I'm assuming that these FETs are similar to APT5020 and 5022 ones. 
I couldn't find a datasheet for the 5023.

 > and a small amount of gate to drain
> capacitance “negative feedback" (gate and drain wiring is twisted pair). 

That gives you just a few pF, which is swamped by the FET's internal 
drain-gate capacitance. But the inductance of such wires is significant, 
and can resonate the existing FET capacitances, causing the 
oscillations! You should strive for minimum inductance in the  gate and 
drain wiring, even in your relatively high impedance, very low frequency 
amplifier.

A somewhat effective feedback would take a resistor of a few hundred 
ohm, in series with a capacitor of 100nF or so, for each FET.

> The screenshots that I attached to my post email did not appear on the 
> amps website. I have uploaded them to:
> http://qsl.net/v/ve7vv//Temp/Waveform at antenna.png 
> http://qsl.net/v/ve7vv//Temp/Waveform at dummy load.png 

I can see that your two FETs are very dissimilar. One comes on much more 
softly than the other. I suspect that you have a single bias 
potentiometer for both, and that the two FETs have significantly 
different threshold voltages. You should either use matched FETs, or 
have a separate bias setting for each, and set the two idling currents 
for the most symmetric waveform.

Other than that, the waveform at the antenna shows much stronger 
harmonic voltage, because the antenna offers essentially no load at the 
harmonic frequencies. Of course you shouldn't be putting that signal 
into the antenna, without a low pass filter! Although the antenna will 
be a poorer radiator at harmonics than at the fundamental, it's still 
good enough to cause QRM to other services.

Manfred

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