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.
>
>