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

[Manfred Mornhinweg]

Roger,

> 1) When PTT is asserted (with no RF drive), which results in a relay
> applying the gate bias and doing the antenna change over, there is a
> turn on transient visible on an oscilloscope. There are two short
> bursts of oscillation (at max power) each at about 100 kHz. This is
> over by about 1 msec after which it seems stable.
> 
> Should I worry about this? If so, can you suggest a cure?

I would worry about every sort of instability.

During bias change conditions, a FET can run through several instability 
conditions, related to the changing internal capacitances. The best I 
can suggest to cure this is to improve the overall stability of the 
amplifier, for example by heavy, broadband gate swamping. Broadband 
means all the way from DC to beyond the highest frequency at which the 
FET might try to oscillate. That's probably ten megahertz or so for the 
FET you are using. Good groundplane construction is important, very 
short source leads, good bypassing, etc. Negative feedback, directly 
from drain to gate through a resistor and a capacitor, is very useful 
too in aiding stability. Even a weak negative feedback helps.

> 2) The output waveform is close to a sinewave (with no LPF) when
> driving a 50 Ohm dummy load. However, when driving my antenna,
> especially on 137 kHz, the waveform is distorted.  > On 137 kHz the
 > antenna load is 50 Ohm and close to resistive (at 137 kHz) as best as
 > I can measure.

FETs are inherently nonlinear, even more so the switching FETs, and 
specially when operated at low bias. So a distorted waveform is to be 
expected. If the distortion is moderate, you might not immediately see 
it on a scope, but it's still there.

FETs, just like bipolar transistors and pentodes, perform as current 
sources. If you have a small amount of distortion in the combined drain 
current, and apply this to a load resistor, the voltage you see on your 
scope only has the same small distortion, which might be too small to 
notice. But your antenna, despite offering a clean load on the operating 
frequency, surely has a high reactive impedance on the harmonics. So the 
small harmonic currents cause large voltage drops across the antenna, at 
harmonic frequencies, and they are phase-shifted too. And that's why the 
voltage waveform you see looks more distorted with the antenna than with 
a resistive load, and probably changes with frequency (varying phase 
shifts between fundamental and harmonics).

> I tried inserting a 1:1 balun at the output of the
> output transformer, which had no effect.

No surprise. A balun shouldn't be frequency-selective.

> A LPF cleans up what comes
> out of the filter nicely. However, the input to the filter (amp
> output) still looks distorted in the same way. I am using a T format
> LPF which I believe is the correct configuration for the amp output
> stage which is (correct me if I am wrong) voltage feed b/c of the
> center tapped choke DC connection. 

The amplifier would be a voltage source of RF if it runs saturated, or 
if it has heavy negative feedback. In those cases a T filter would be 
fine. But you aren't saturating it. If you don't have heavy fedback 
there, it's basically a current source, and should be better served by a 
PI-type filter.

> The input drive to the amplifier
> looks like a nice sinewave in all conditions.

Which means that you don't have negative feedback, or at least no strong 
negative feedback. Due to the nonlinear nature of FETs, heavy negative 
feedback results in distorting the gate voltage waveform, in such a way 
that it largely compensates for the device nonlinearity, resulting in a 
pretty clean drain voltage waveform.

> Do you see a problem operating the amp into a T (inductor input and
> output) LPF with the distorted output (voltage) waveform? It seems to
> be running reasonably cool so I don?t see an overheating problem.

It shouldn't cause a big problem, but be sure to measure IMD to 
ascertain this.

> I
> would prefer to see a clean output from the amplifier and would like
> to operate with linear modes occasionally so the distorted waveform
> leads me worry about IMD (I have not run IMD tests).

Many if not most solid state amplifiers work with very dirty internal 
waveforms. That's fine, as long as the output waveform (after the low 
pass filter) is clean enough, both regarding harmonics and IMD. In fact 
the most common amplifiers working inside nearly all our factory-made HF 
radios work essentially with square-wave current waveforms at the drains 
or collectors, and with very "funny" voltage waveforms. After the filter 
they achieve - just barely - adequate IMD and adequate to decent 
harmonics. This could be vastly improved, but hams aren't demanding 
better quality from the manufacturers, and so these go for the cheapest 
circuits that work "well enough".

> Oscilloscope screenshots attached.

Where can I see them?

> BTW, I tried using a center tap on the output transformer instead of
> the separate choke feed. The result was a badly distorted waveform
> into the 50 Ohm dummy load on all bands. 

No surprise...

> I don?t understand why the
> center tap method resulted in such a different output.

That happened because you have 3 turns on that primary. A center tap 
results in what optimists call one and a half turns on each side. The 
problem is that half turns are a physical absurdity. They don't exist! 
What you really have is two separate cores, with independent, uncoupled 
magnetic paths, and one turn through BOTH cores plus one turn through 
ONLY ONE core, on each side. This results in a large uncoupled 
inductance on each side, resulting in the same behavior as if you had 
separate feed chokes for each FET: It forces the total drain current 
(the sum of both drain currents) to be a pure DC, and thus forces the 
load current to be square wave or something close to it. But near the 
zero crossings your sine drive waveform doesn't turn on the FETs enough 
to conduct that fixed total current. As a result the drain voltage on 
both FETs shows high spikes during the signal zero crossings.

99% of commercial ham equipment shows the same problem, attenuated to 
various degrees, depending on band, by strong negative feedback, large 
capacitance from drain to ground, and sometimes by avalanche discharge 
in the FETs!

In your VLF amplifier you can easily avoid this problem by using the 
separate feed choke, or by feeding the amplifier through the 
transformer's center point BUT using an even number of primary turns, 
and building the transformer and the wiring in such a way that the 
leakage inductance is low enough (which means very low). In a 48V 200W 
VLF amplifier this is really easy, instead in 13V 100W amplifiers that 
have to work through 10m or even 6m it's hard to achieve, and with 48V 
1.5kW ones it's nearly impossible. The problem is that the higher the 
voltage, the more ferrite cross section or more turns are needed. This 
results in higher leakage inductance. But the higher the frequency, and 
the lower the drain impedance (given by supply voltage and output 
power), the lower is the maximum acceptable leakage inductance. In your 
VLF amplifier the equation solves easily, but in a big amplifier that 
runs at up to 54MHz, and has an effective drain-to-drain impedance of 3 
ohm, meaning 0.75 ohm drain load impedance on the FET that's conducting, 
you would need Harry Potter's help to get it right.

Manfred

========================
Visit my hobby homepage!
http://ludens.cl
========================