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Date: Wed, 08 Nov 2017 03:27:09 +0100
From: misan <misan@xs4all.nl>
To: rsgb_lf_group@blacksheep.org
Cc: rsgb_lf_group@yahoogroups.co.uk,
        "Discussion of the Lowfer (US,
 European, & UK) and MedFer bands" <lowfer@mailman.qth.net>,
        owner-rsgb_lf_group@blacksheep.org
In-Reply-To: <22d6c712-43af-3a5d-099e-adb285f086a1@n1bug.com>
References: <22d6c712-43af-3a5d-099e-adb285f086a1@n1bug.com>
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Subject: Re: LF: PA problem
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Reply-To: rsgb_lf_group@blacksheep.org

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