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

[Roger Graves]

Manfred,

Thank you for the helpful followup.

I will try lower value gate resistors. That will also have the advantage that I will be able to drive to higher output on 160 I think. (My 1W drive is not sufficient on 160.)

I can substitute braid for the wire I used for the gate and drain leads for lower inductance.

Yes, as far as I could tell, the 5023 is just a 5022 with 0.23 Ohm Rds(on) instead of 0.22.

I will try adding the negative feedback if other changes do not eliminate the transient.

I had wondered if the asymmetrical waveform related to un-matched FET’s. I do have separate bias pots and the bias is adjusted for the same resting current in each FET. As you inferred, the bias voltage is not equal (0.3V different I think). I could try what you suggest in setting the bias current to be unequal to try to get the waveform more symmetrical. I have ordered some more of the mosfets and can try to select a better matched pair.

The APT502X series is obsolete and hard to find. Can you suggest a possibly better replacement that can work on 48V and provide 200W? I found the relatively low transconductance and high power dissipation ratings of the APT units attractive. The power output increases linearly with input up to the 1dB compression point at close to 200W. 

I have not used any compensation capacitance across the output transformer primary. The measured Z of the transformer was better on 160 with 500pF across it, but I was concerned the 500V silver micas (two 249 pF in parallel) might not survive. Do you think C across either the primary or secondary would be beneficial for either the transient oscillation or the harmonic distortion?

73,
Roger


> On May 27, 2018, at 6:43 PM, Manfred Mornhinweg <manfred at ludens.cl> wrote:
> 
> 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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