Re: LF: Why 34nb20 blocking cap?

[brian hodgson <g3ykb@yahoo.co.uk>]

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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