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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 16:53:53 -0500
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OK. I understand the problems with having RF reach the meter and power supply.
To be honest I don't fully understand what you are suggesting with a
very small coil (few uH at best) and capacitor...
On my own lack of knowledge I put a 1.2 mH choke in the DC supply
lead at the amplifier. This made very little reduction in RF I was
seeing on the DC side of the choke!
So I put a big (1000 uF) capacitor to ground from the DC side of the
choke. That removed all the RF there but make the amplifier output
dirty. Seemed maybe it went into some lower frequency oscillation. I
tried 330 uF, which seemed to make it dirty for a couple of seconds,
then it cleaned up. With 100 uF it is always clean and there is very
little RF component on the DC supply lead.
It seems almost as good to have the 100 uF capacitor but remove the
big choke. Maybe this is what you were saying, that a big choke is
not needed. I will try replacing it with just a few uH and see what
happens.
It's supposed to be a class AB2 amplifier I guess, but when set for
any amount of idling current, efficiency is extremely poor. About
30%. If I set the bias lower, well below the point where it draws
any idling current, efficiency becomes about 80%. As I understand
it, the circuit is not correct for class E but that seems to be how
it wants to run.
73,
Paul


On 11/10/2017 02:35 PM, DK7FC wrote:

Am 10.11.2017 19:44, schrieb N1BUG:
I see a nice clean sine wave at the other side of the LPF.
With a good LPF, the output often looks quite good on a scope, even with
massively distorted input signals.
But I see a lot of RF (36 volts peak to peak) at the +13.8V power
connection to the PA!
Yes and your power supply doesn't like it, it can be damaged from the
high RF peak currents. Or the voltage regulator can do crazy things. And
BTW the current measurement is likely to be totally wrong with such an
AC component. With the coil you can decouple the AC component from the
supply, at least most of it.
Just take a piece of enameled cu wire, 1mm diameter, and wind a coul
with 15 turns on something with 15mm diameter. Or use a T106-2 core for
example. Of course the big C is needed as well. For the 1 uF cap i would
use WIMA MKP-10 or so. Without calculating i would say that this
improves things significantly.

73 and have fun,
Stefan



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