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Re: LF: PA problem

To: [email protected]
Subject: Re: LF: PA problem
From: misan <[email protected]>
Date: Wed, 08 Nov 2017 03:27:09 +0100
Cc: [email protected], "Discussion of the Lowfer (US, European, & UK) and MedFer bands" <[email protected]>, [email protected]
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Hello OM N1BUG,

For class E @85%eff you would need some more components.

- An inductor in the DC between 1u capacitor and the drain / transformer low impedance input, typical >0.5mH at LF! - A 'tuning' capacitor between impedance transformer output and mass (different vallues for both frequencies!)
The drain-source capacity in the fet and the extra cap at the output
plus the inductance of the output transformer have to shift the AC
voltage and the AC current in phase at the drain to meet class E
conditions for improved efficiency. Therfore the tuning is specific for
each freqency (just the same as in a traditional HV valve PA anode
impedance matching unit to meatch 50Ohms for LPF and antenna)
Probably the prototype accidentally met class E/F coditions at the
listed efficiency rating.
To expand lifetime for the fet it could be better to also add a gate
series resistor (10 Ohms).
The LPF filters look ok by design, components might be lossy in power
for standard HV ceramic cap's. You can check if the cap's temperatures
rise. Harmonic power is lost anyway in the LPF, so 80%eff or over is
probably not to be expected.
Kind regards,
Best 73's
Michael, PC4M



N1BUG schreef op 2017-11-07 23:33:
I wonder if anyone can help me with this.

I built one of these:

http://qrp-labs.com/ultimate3/u3mods/lfamp.html

It's not what I want but it's what I can afford. In all cases, I was
driving it with 250 milliwatts at 137 kHz. It was running into a
perfect 50 ohm resistive load (dummy load good through 2 GHz).

At first I had a DMM between the 13.8V power supply and the PA to
measure current. I had preset bias to about 25 mA current with no RF
drive. Output was about 19 watts and efficiency was only 30%. By
changing the bias I was able to get 22 watts out. The bias point for
maximum output was below the point where it draws any idling current
without RF drive. Efficiency increased to 50%.

Next I removed the DMM. Power output dropped off. Re-adjusting the
bias I was able to get 15 watts. Bias was still below where it starts
to draw idling current. Efficiency seemed better as the amp was
running much cooler.

Then I shortened the cable from power supply to PA. Power dropped
again. I re-adjusted bias but this time had to make it quite a lot
higher to peak the output. I didn't have an ammeter in line but from
earlier tests I know it was now well into the region where current is
drawn without RF drive. I was able to get 20 watts. At the instant I
removed RF drive the FET died! I'm guessing oscillation.

In all cases, the output looked clean on an oscilloscope. I was using
a scopematch to measure power.

I am only guessing but it seems like maybe the PA is not properly
decoupled from the power supply? Or perhaps it is just unstable. Any
suggestions? I need to make this work if I am going to be on 2200
meters this winter. I have one more FET and will order a couple more
but I would like to have some idea what to try with the next one.

73,
Paul N1BUG




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