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Date: Sat, 9 Jan 2016 22:13:29 +0000
Message-ID: <CAA8k23QcwN5SkdzeTsWg1EmQBQBdHaosK7g97_fQUHx19GMRNA@mail.gmail.com>
From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@yahoogroups.co.uk, rsgb_lf_group@blacksheep.org
Subject: LF: Re: [rsgb_lf_group] Linear Amp for LF
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Shortly after my original post on this topic I received two PMs, both
pointing at a design by W7IUV for 400 Watts output, with a hint more is
possible, and that its robustness is proven in practice..    This was for
use at 475kHz but its design is broadband, so by choice of ferrite could
easily span 137 and 475kHz. I won't repeat the URL here as both responders
chose to reply individually with it, but if you Google (*) W7IUV something
will show.

One reason this design appeals to me  is the use of degenerative feedback
formed from multiple source resistors.   In any amplifier design, using
devices substantially below their maximum frequency - which is the case
with switching FETs at LF - some sort of feedback should, really,  always
be used.  It will help with bias stabilisation, as well as keeping the
overall design more stable.   MOSFET amps for HF nearly always apply NFB,
often via a dedicated feedback transformer.

The G7IUV design shown doesn't give details of bias circuitry or PSU but
these are pretty standard for MOSFETs, although th euse of multiple
parallel FETs does ideally require that FETs be all from teh same batch to
ensure moderate matching.


Andy  G4JNT

(*) Other search engines are available (allegedly)


> On 9 Jan 2016, at 09:14, "David G0MRF g0mrf@aol.com [rsgb_lf_group]" <
> rsgb_lf_group@yahoogroups.co.uk> wrote:
>
>
>
> Hi Terry.
>
> Ask away...You have almost read my mind.......
>
> I've been looking at the class D circuit in detail over the past few
> weeks....FETs obsolete....SWR detector not quite right etc.
> I've even used it on its limits to make a nice QSO with SV on 630m last
> weekend.  - Got so hot the snubber resistor on one of the drains fell off
>
> I plan on producing around 50 boards if my Chinese manufacturer hasn't
> gone bust in the stock market crash....
> However, I'm not sure the linear board has much sales potential as CW /
> WSPR / JT9 can all be done with non linear amplifiers.
>
> Also, the recently introduced power FETs are all optimised for rapid
> switching in power supply applications. That's very different from 15yrs
> ago when the Vg and Id transfer characteristics were much more 'linear' in
> nature.
>
> What do you think??
>
> David
>
>
>
>

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<div dir=3D"ltr">Shortly after my original post on this topic I received tw=
o PMs, both pointing at a design by W7IUV for 400 Watts output, with a hint=
 more is possible, and that its robustness is proven in practice.. =C2=A0 =
=C2=A0This was for use at 475kHz but its design is broadband, so by choice =
of ferrite could easily span 137 and 475kHz. I won&#39;t repeat the URL her=
e as both responders chose to reply individually with it, but if you Google=
 (*) W7IUV something will show.<div><br></div><div>One reason this design a=
ppeals to me =C2=A0is the use of degenerative feedback formed from multiple=
 source resistors. =C2=A0 In any amplifier design, using devices substantia=
lly below their maximum frequency - which is the case with switching FETs a=
t LF - some sort of feedback should, really, =C2=A0always be used.=C2=A0 It=
 will help with bias stabilisation, as well as keeping the overall design m=
ore stable. =C2=A0 MOSFET amps for HF nearly always apply NFB, often via a =
dedicated feedback transformer.</div><div><br></div><div>The G7IUV design s=
hown doesn&#39;t give details of bias circuitry or PSU but these are pretty=
 standard for MOSFETs, although th euse of multiple parallel FETs does idea=
lly require that FETs be all from teh same batch to ensure moderate matchin=
g.</div><div><br><div><br></div><div>Andy =C2=A0G4JNT</div><div><br></div><=
div>(*) Other search engines are available (allegedly)=C2=A0<br><div class=
=3D"gmail_extra"><br><div class=3D"gmail_quote"><blockquote class=3D"gmail_=
quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1=
ex"><div style=3D"background-color:#fff"><div><div><div><div><br>On 9 Jan 2=
016, at 09:14, &quot;David G0MRF <a href=3D"mailto:g0mrf@aol.com" target=3D=
"_blank">g0mrf@aol.com</a> [rsgb_lf_group]&quot; &lt;<a href=3D"mailto:rsgb=
_lf_group@yahoogroups.co.uk" target=3D"_blank">rsgb_lf_group@yahoogroups.co=
.uk</a>&gt; wrote:<br><br></div><blockquote type=3D"cite"><div>


=20

<span>=C2=A0</span>


    <div>
     =20
     =20
      <p><font color=3D"black" size=3D"2" face=3D"Arial, Helvetica, sans-se=
rif">
</font></p><div><font color=3D"black" size=3D"2" face=3D"Arial, Helvetica, =
sans-serif">Hi Terry.</font></div><font color=3D"black" size=3D"2" face=3D"=
Arial, Helvetica, sans-serif">

<div>=C2=A0</div>

<div>Ask away...You have almost read my mind.......</div>

<div>=C2=A0</div>

<div>I&#39;ve been looking at=C2=A0the class D=C2=A0circuit in detail over =
the past few weeks....FETs obsolete....SWR detector not quite right etc.</d=
iv>

<div>I&#39;ve even used it on its limits to make a nice QSO with SV on 630m=
=C2=A0last weekend.=C2=A0 - Got so hot the snubber resistor on one of the=
=C2=A0drains fell off</div>

<div>=C2=A0</div>

<div>I plan on producing around 50 boards=C2=A0if my Chinese manufacturer h=
asn&#39;t gone bust in the stock market crash....=C2=A0</div>

<div>However, I&#39;m not sure the linear board has much sales potential as=
 CW / WSPR / JT9 can all be done with non linear amplifiers.</div>

<div>=C2=A0</div>

<div>Also, the recently introduced power FETs are all optimised for rapid s=
witching in power supply applications. That&#39;s very different from 15yrs=
 ago when the Vg and Id transfer characteristics were much more &#39;linear=
&#39; in nature.</div>

<div>=C2=A0</div>

<div>What do you think??</div>

<div>=C2=A0</div>

<div>David</div>

<div>=C2=A0</div>

<div>=C2=A0</div></font></div></div></blockquote></div></div></div></div></=
blockquote></div></div></div></div></div>

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