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In-Reply-To: <635074543.20161125135600@chriswilson.tv>
References: <635074543.20161125135600@chriswilson.tv>
From: Andy Talbot <andy.g4jnt@gmail.com>
Date: Fri, 25 Nov 2016 14:16:04 +0000
Message-ID: <CAA8k23TbPD7bFBecaD5O1g1c8jgZD-nEFMwBXfh1K-y2UTE2KA@mail.gmail.com>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: H Bridge power MOSFET based amps question
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continuing...

The gate resistor just has to swamp teh Xc of the gate capacitance in a
linear amp, to make it look clean and resistive at the input.  So, for
example, assume 2000pF  Cgs at 475kHz.   Xc = 167 ohms.   SO a shunt R of
say 30 to 50 ohms will dominate.
If you were really daring you might think it possible to resonate out the
Gate C and then get really high gain.   ( Don't do this with an expensive
FET).   But worth trying the experiment to see - power oscillators take on
a completely new meaning if you try it.

drain / source...   emitter / collector.  They're just legacy names.  You
just know what they are without thinking of the actual meaning.   Although,
I suppose, if you think of electron flow...........

'JNT


On 25 November 2016 at 13:56, Chris Wilson <chris@chriswilson.tv> wrote:

> 25 November 2016
>
>
> I have built a 1kW Class D amp for none linear digital transmissions
> around 136kHz.
>
> My Dave Pick designed amp uses four MOSFETS, two in parallel each side
> driven by a dedicated driver chip. I see the odd LF amp design using
> what's described as a H Bridge MOSFET array. The one I most recall
> most, by Andy Talbot used rectified 240 volt single phase mains to run
> it. My question is why use an H Bridge rather than paralleling more
> MOSFETS in what to me is a more conventional format? Does it overcome
> voltage or current limitations? Is it due to adding more capacitance
> with paralleled devices? Or does it allow a linear amp to be built?
>
> I realize a meaningful reply might be quite lengthy, so I assure you I
> have Googled it, but most stuff related to in the articles is
> industrial motor control.
>
> Secondly, in a Class D none linear amp, how is the gate resistor value
> chosen, and is there ever a case for not using one at all?
>
> Thirdly, am I alone in thinking the terms Drain and Source seem back
> to front and the names would intuitively be reversed so a Drain was
> called a Source and vice versa? :)
>
> Thanks. 2E0ILY
>
>
>
> --
>
>
> Best regards,
>  Chris  2E0ILY                           mailto:chris@chriswilson.tv
>
>
>
>

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<div dir=3D"ltr">continuing...<div><br></div><div>The gate resistor just ha=
s to swamp teh Xc of the gate capacitance in a linear amp, to make it look =
clean and resistive at the input.=C2=A0 So, for example, assume 2000pF =C2=
=A0Cgs at 475kHz. =C2=A0 Xc =3D 167 ohms. =C2=A0 SO a shunt R of say 30 to =
50 ohms will dominate. =C2=A0 =C2=A0</div><div>If you were really daring yo=
u might think it possible to resonate out the Gate C and then get really hi=
gh gain. =C2=A0 ( Don&#39;t do this with an expensive FET). =C2=A0 But wort=
h trying the experiment to see - power oscillators take on a completely new=
 meaning if you try it.</div><div><br></div><div>drain / source... =C2=A0 e=
mitter / collector.=C2=A0 They&#39;re just legacy names.=C2=A0 You just kno=
w what they are without thinking of the actual meaning. =C2=A0 Although, I =
suppose, if you think of electron flow...........</div><div><br></div><div>=
&#39;JNT</div><div><br></div><div><br></div><div><br></div><div class=3D"gm=
ail_extra"><br><div class=3D"gmail_quote">On 25 November 2016 at 13:56, Chr=
is Wilson <span dir=3D"ltr">&lt;<a href=3D"mailto:chris@chriswilson.tv" tar=
get=3D"_blank">chris@chriswilson.tv</a>&gt;</span> wrote:<br><blockquote cl=
ass=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;p=
adding-left:1ex">25 November 2016<br>
<br>
<br>
I have built a 1kW Class D amp for none linear digital transmissions<br>
around 136kHz.<br>
<br>
My Dave Pick designed amp uses four MOSFETS, two in parallel each side<br>
driven by a dedicated driver chip. I see the odd LF amp design using<br>
what&#39;s described as a H Bridge MOSFET array. The one I most recall<br>
most, by Andy Talbot used rectified 240 volt single phase mains to run<br>
it. My question is why use an H Bridge rather than paralleling more<br>
MOSFETS in what to me is a more conventional format? Does it overcome<br>
voltage or current limitations? Is it due to adding more capacitance<br>
with paralleled devices? Or does it allow a linear amp to be built?<br>
<br>
I realize a meaningful reply might be quite lengthy, so I assure you I<br>
have Googled it, but most stuff related to in the articles is<br>
industrial motor control.<br>
<br>
Secondly, in a Class D none linear amp, how is the gate resistor value<br>
chosen, and is there ever a case for not using one at all?<br>
<br>
Thirdly, am I alone in thinking the terms Drain and Source seem back<br>
to front and the names would intuitively be reversed so a Drain was<br>
called a Source and vice versa? :)<br>
<br>
Thanks. 2E0ILY<br>
<span class=3D"HOEnZb"><font color=3D"#888888"><br>
<br>
<br>
--<br>
<br>
<br>
Best regards,<br>
=C2=A0Chris=C2=A0 2E0ILY=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0mailto:<a href=3D"mailto:ch=
ris@chriswilson.tv">chris@chriswilson.tv</a><br>
<br>
<br>
<br>
</font></span></blockquote></div><br></div></div>

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