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Date: Thu, 29 Mar 2007 19:25:31 +0100 (BST)
From: brian hodgson <g3ykb@yahoo.co.uk>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Why 34nb20 blocking cap?
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Hi Guys - I also like to use the blocking capacitor circuit. I've had a few FETs go gate-drain short (probably from drain voltage spikes) - without the capacitor the driver chip gets the full HT voltage.  - Brian (G3YKB)

Scott Tilley <sthed475@telus.net> wrote:
  J.

This is to ensure DC doesn't drive the FETs and blow them up in the 
event that for some reason the exciter stops and the driver provides a 
constant DC signal to the FETs gate. This clamping type arrangement is 
to ensure only + going pulses drive the FETS and this is a critical FET 
safety feature. I have seen designs without it but I'm sure the users 
must have a box of spare FETs handy and a warm soldering iron...

Jump into your way back machine and do a circuit analyst of it and 
you'll see what I mean. The cap AC couples the drive, the diode ensures 
that it is clamped as a + DC pulse train... Therefore, a constant DC 
bias on the gate can't happen...

G0MRF was the first I noticed using this technique in amateur design of 
2200m class D amps.

73 Scott

PS - check your diode polarity!!!


J. Allen wrote:
> Hello Tech types.
>
> While I am working on the amp, I noticed that some of the FET drive 
> circuits are fed directly from a driver device (G3YXM circuit p66 LF 
> Today) and some use a blocking capacitor followed by a diode (G0MRF 
> circuit, p72).
>
> Why are the capacitors and diodes required in one FET circuit and not 
> in the other?
>
> Thanks,
>
> J.
>
>



 		
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<div>Hi Guys - I also like to use the blocking capacitor circuit. I've had a few FETs go gate-drain short (probably from drain voltage spikes) - without the capacitor the driver chip gets the full HT voltage.&nbsp; - Brian (G3YKB)<BR><BR><B><I>Scott Tilley &lt;sthed475@telus.net&gt;</I></B> wrote:</div>  <BLOCKQUOTE class=replbq style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #1010ff 2px solid">J.<BR><BR>This is to ensure DC doesn't drive the FETs and blow them up in the <BR>event that for some reason the exciter stops and the driver provides a <BR>constant DC signal to the FETs gate. This clamping type arrangement is <BR>to ensure only + going pulses drive the FETS and this is a critical FET <BR>safety feature. I have seen designs without it but I'm sure the users <BR>must have a box of spare FETs handy and a warm soldering iron...<BR><BR>Jump into your way back machine and do a circuit analyst of it and <BR>you'll see what I mean. The cap AC couples the drive,
 the diode ensures <BR>that it is clamped as a + DC pulse train... Therefore, a constant DC <BR>bias on the gate can't happen...<BR><BR>G0MRF was the first I noticed using this technique in amateur design of <BR>2200m class D amps.<BR><BR>73 Scott<BR><BR>PS - check your diode polarity!!!<BR><BR><BR>J. Allen wrote:<BR>&gt; Hello Tech types.<BR>&gt;<BR>&gt; While I am working on the amp, I noticed that some of the FET drive <BR>&gt; circuits are fed directly from a driver device (G3YXM circuit p66 LF <BR>&gt; Today) and some use a blocking capacitor followed by a diode (G0MRF <BR>&gt; circuit, p72).<BR>&gt;<BR>&gt; Why are the capacitors and diodes required in one FET circuit and not <BR>&gt; in the other?<BR>&gt;<BR>&gt; Thanks,<BR>&gt;<BR>&gt; J.<BR>&gt;<BR>&gt;<BR><BR></BLOCKQUOTE><BR><p>&#32;
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