From: niels.rudberg@post.tele.dk
To: rsgb_lf_group@blacksheep.org
Date: Tue, 20 Jul 2004 21:19:12 +0200
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Message-Id: <20040720191912.JNGM23564.fepX.post.tele.dk@post.tele.dk>
Subject: Re: LF: Re: TX output circuit
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James Moritz <james.moritz@btopenworld.com> wrote:
> Dear Scott, LF Group,
  > 
> To save long-winded explanations, I have drawn a circuit
> diagram of my modified version of the G0MRF PA output
> circuit, with some details of the output transformer. It is
> too big to send via the reflector, but if anyone wants it,
> let me know and I will e-mail it direct.
> 
> My unit gives 400W out with 37V supply - for 300W out with a
> 48V supply, I reckon the primary windings of the output
> transformer should be changed from 4 to 6 turns. The value
> of the choke in the DC supply is not critical - the iron
> dust cores can often be found in old PC SMPSUs. The most
> significant change to the original circuit was to remove the
> decoupling capacitor from the centre tap of the transformer
> primary, and increase the inductance of the choke,
> converting the circuit into a "current feed" class D
> configuration. This means the DC supply has a high impedance
> at 137kHz, which prevents high transient currents into the
> low pass filter capacitors when the MOSFETs switch. The
> ideal drain voltage waveform would be a half sine wave,
> allthough in practice there is still considerable ringing at
> the switching transitions, allthough much reduced. The value
> of the damping resistors and capacitors is a compromise;
> reducing R and increasing C reduces the amount of HF
> ringing, but increases dissipation in the resistors.
> 
> Cheers, Jim Moritz
> 73 de M0BMU 
> 
> 

Please sende me a circuit diagram.
Thanks in  advance!
73  OZ8NJ  Niels+