DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
  s=s1024; d=btinternet.com;
  h=Received:X-Yahoo-SMTP:X-YMail-OSG:X-Yahoo-Newman-Property:Message-ID:From:To:References:Subject:Date:MIME-Version:Content-Type:Content-Transfer-Encoding:X-Priority:X-MSMail-Priority:X-Mailer:X-MimeOLE;
  b=ENPNam0+F8djAJslCAZQgIWMoAhuOTTq5TeqzpW6/9Jp+cbD1G5quSDQhvDGGug/clKZ7Ivkk5gNPue8ShT7lKsoaZx5fXaFjMDYtuAN92w+YRMeIOAD1igmNhZfQ/6eOuPBlcc9I5fvzzACGFTT2kDRh1C7U/0lzwUUeyEATk4=  ;
Message-ID: <001901ca6de6$a34b30c0$0900a8c0@lark>
From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <BLU146-DS158D4B7A1E9F536E4A2D0CF9C0@phx.gbl> <C1D951F883084B1C9F845B4EDA782028@df2py>
Date: Wed, 25 Nov 2009 15:45:39 -0000
MIME-Version: 1.0
DomainKey-Status: good (testing) 
Subject: LF: Re: instability on MosFet amps
Content-Type: text/plain;
	charset="iso-8859-1"
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


Hi Wolf this may not be the case with the properly tuned Class-E stage which
has relatively low distortion.  It does not output a square-wave as some
think. See The QST article by Nat Sokal.

Bear in mind filters complicate things further since they can transform
mis-matches and only really work if resistively terminated. Even more reason
to get your antenna right first. Loaded diplexor filters are a little more
forgiving if you have harmonic problems but should not be necessary on
Class-E a simple low-pass half-wave filter works well but use air cored
coils, not toroids.

Alan G3NYK

----- Original Message -----
From: "Wolf Ostwald" <df2py@t-online.de>
To: <rsgb_lf_group@blacksheep.org>
Sent: Wednesday, November 25, 2009 3:25 PM
Subject: LF: instability on MosFet amps


> Hello group !
>
> I found that almost all amp configurations generate harmonics. In case of
a
> homebrewed HF amp, with a push -pull design, I had to build hi-pass
filters
> with a resistive termination, so called split filters, for each band.
> Without dissipating the harmonic energy the stage will have to cope with
> excess voltages and thus bury the MosFets .
>
> My last amp stage for the DI2BJ-beacon is followed by a hi-pass that opens
> at 750 kcs . This way the first harmonic and all the higher ones go into
> thermal dissipation.
>
> A T-type adjustable low-pass is used to match and reduce harmonics
further.
>
> Have fun                     de wolf  df2py
>
>