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

[Roger Graves]

Manfred, the distorted waveform output (observed before at amp output when driving the antenna either with or without a T form LPF) is fixed. Thank you for suggesting using a Pi form LPF. When I built and installed a Pi filter, the waveform at the output of the filter (antenna matching system input) was, as expected, a nice sine wave. The waveform at the input of the filter (amplifier output) was also a nice looking sine wave, not visibly different than the waveform at the LPF output. This was the case up to 200W into the antenna after which the amp output began to show some waveform distortion (flattening of the peaks). This was all at 137 kHz - I have not built a Pi form LPF for 475 kHz yet. Interestingly, the heatsink is cooler running at 200W output than at 125W output.

The turn-on transient was unchanged - no surprise there. I will work on trying your other suggestions later.

73,

Roger



> On May 28, 2018, at 10:05 AM, Manfred Mornhinweg <manfred at ludens.cl> wrote:
> 
> 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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