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

[Manfred Mornhinweg]

Roger,

> 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.)

Ops... We have a misunderstanding there. I was talking about gate LOAD 
resistors - the ones that connect from the gates to RF ground. Now I 
realize that you are talking about resistors connected in series with 
the gates.

With such big FETs I would use resistors of roughly 10-20 ohm from each 
gate to ground (through a larguish bypass capacitor), making sure that 
the bias supply has an impedance not over a few hundred ohm and that it 
is coupled to the gates in such a way that the bias supply's internal 
resistance loads the gates fom DC up to beyond the frequency where the 
reactance of the mentioned bypass caps has become irrelevant. This gives 
you gate loading at a few hundred ohm from DC to some low frequency, and 
of 10 to 20 ohm from there on and into the VHF range.

The gate series resistance for such a big FET is typically just 1 to 3 
ohm. 50 ohm gate series resistance is what I use for 6 watt FETs (like 
the RD06) in the HF range. At your low frequency the resistance can be 
higher than at HF, but what you have, for your FETs having  huge 
capacitances, is far too high.

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

That would be an improvement. Anyway I wonder how much effect it's 
causing, given that you are running this at relatively high drain 
impedance and very low frequencies. Still, just to be on the safe side, 
I would still use RF construction techniques, with wiring lengths 
approaching zero as much as possible.

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

OK. I have a bag full of APT5020, which I got for free. Given their 
extremely high capacitances, I'm using them in DC applications only! 
Definitely not at RF. Even in switching applications that need to run at 
50kHz or so, I prefer buying more modern, better suited FETs, than using 
those 5020.

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

OK. It also aids linearity. As soon as you find you have any spare gain, 
you should use it up in negative feedback, instead of an attenuator.

> The APT502X series is obsolete and hard to find.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.

Have a look at the IXFQ20N50P3, AOK42S60L, IXFQ30N60X, and IXFH26N50P3. 
They are all current and available at Digikey and other distributors. 
Each of them replaces the APT5023 with considerable advantages, either 
in most or in all areas. Compare the output and reverse capacitances of 
these to the APT5023's ones...

Any of these will provide far more gain.

All four are rated at 500-600V, which is really a lot of overkill for a 
48V supply. But in the optimal range, say, 150-200V, there isn't much to 
choose from. Those lower voltage, high power FETs have extremely high 
current and low RDSon ratings, which brings along extreme 
transconductance and very high capacitances. So it's probably better to 
stay with the 500-600V FETs. These should also be more resistant to 
hotspotting in linear operation.

 > The power output increases
> linearly with input up to the 1dB compression point at close to 200W.

In fact with 48V and a 3:6 turns output trafo you should be getting 
closer to 300W at the -1dB point.

> 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?

Probably not.

A properly compensated transformer has a pretty flat response up to a 
certain frequency, then falls off rapidly. Without compensation it will 
start falling off at a lower frequency, but less abruptly. I would think 
that when operating with 2MHz as the top frequency, compensation won't 
be needed. But it really depends on the details of transformer 
construction. If you find that your transformer works well on the VLF 
bands but struggles on 160m, consider adding properly calculated 
compensation capacitors. The measurable symptom of a transformer that's 
not able to cope with the frequency is that the secondary voltage no 
longer is in the correct ratio to the primary voltage (bad coupling 
factor). In that case one would add the correct amount of compensation 
capacitance ON BOTH WINDINGS, thus absorbing the unwanted leakage 
inductance, that causes the poor coupling, into a PI-type low pass 
section, whose cutoff frequency hopefully ends up above the highest 
frequency of operation. That's all there is about compensating a 
transformer. It's not directly related to linearity, although the 
frequency characteristics of the transformer do affect the harmonic 
structure of the signal.

Since compensation capacitors will shift existing resonances, stability 
can change. But it could change for the better or for the worse. 
Typically for the worse, though, given the fact that adding reactance 
tendsa to raise the Q of those resonances.

Manfred

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