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Re: LF: PA problem

To: [email protected]
Subject: Re: LF: PA problem
From: N1BUG <[email protected]>
Date: Fri, 10 Nov 2017 13:44:24 -0500
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Hi Stefan,

Your answer is not late at all. After working very hard on antennas the past days I am just now getting back to the little PA.
About RF choke in the supply line: This is something I don't
understand about most of the LF/MF PA designs I have looked at. I
don't see such a choke in any of them! If I do see one, it is just a
few uH. Why don't all LF/MA PAs need a big choke in the +DC lead?
So today I put in a new FET. I set the bias so it draws 10 mA at
idle. Several people suggested I start there. With my Fluke DMM
between the power supply and the PA I got 20 watts RF output but
efficiency was very poor. The FET was generating a lot of heat!
I removed the DMM but left everything else exactly as it was. Now I
had only 17 watts RF output but the FET was running cold. This is
very similar to results I obtained some days ago.
So I started to probe around with the scope. I see a somewhat
distorted sine wave at the gate, a much greater amplitude distorted
sine wave at the drain, same at the PA RF output. I see a nice clean
sine wave at the other side of the LPF. But I see a lot of RF (36
volts peak to peak) at the +13.8V power connection to the PA! Surely
it is not supposed to put 36V peak to peak RF on the power supply!?
That seems crazy to me.
So either it does need a big choke there, or something is wrong with it.

I didn't do any experiments with different bypass capacitors yet. I do not see any signs of oscillation with my 100 MHz scope but who knows.
73,
Paul N1BUG



On 11/08/2017 08:37 AM, DK7FC wrote:
Hi Paul,

My answer may be a bit late.

It sounds to me like a EMC issue. A RF choke is needed in the supply
line. 1 mH may be a good choice. And i would add a 1000 uF electrolytic
capacitor with a low internal resistance (the good ones :-) ) near the
Drain, in parallel to the 1 uF capacitor.

A few more arguments: It is not ideal to drive a class E with a linear
signal. It is better (regarding efficiency) to use a hard keying by
using a MOSFET driver such as ICL7667, TC4422 or so. A class E is not a
design for linear modes, it is for a constant output power, good for
QRSS/DFCW, CW, WSPR, OPERA, EbNaut...

I would recommend to build a class D PA. You can use the existing
components. A class D is much less critical regarding matching (SWR) and
the efficiency can be in a range of > 90% at LF as well...

73, Stefan




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