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Date: Wed, 28 Mar 2007 21:50:13 -0700
From: Scott Tilley <sthed475@telus.net>
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Subject: Re: LF: Why 34nb20 blocking cap?
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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.
>
>