Thanks Harold,

> I have not been following this thread closely but saw the comment on
> Miller effect. To me, Miller effect is an apparent multiplication of
> capacity due to voltage amplification. Imagine we have a capacitor to
> ground that has a reactance such that 1V at 1kHz yields a current into th=
e
> capacitor of 1mA. Now add an amplifier with a gain of -10 from the top of
> the capacitor to the bottom (bottom of capacitor driven by amplifier
> output instead of ground). Now, when we drive the top of the capacitor
> with 1V, we are driving the bottom of the capacitor with -10V (10V with
> phase reversed), and we have 11V across the capacitor. We now see 11mA
> through the capacitor. To the voltage source, it looks like the capacitor
> value has been multiplied by 11 since the current has gone up by 11 times=
..
>
> Looking at Vgs, assuming an N channel FET, as Vgs increases, Vds
> decreases. Capacitive coupling from the drain to the gate would decrease
> Vgs, not increase it. One possibility for gate failure is inductance in
> the source lead. When the FET is turned off, the gate voltage goes to zer=
o
> and the source voltage could go substantially negative due to inductance
> between the source and ground. I've dealt with this by adding resistance
> in series with the gate so that gate to source capacitance pulls the gate
> down with the source. This does slow the circuit down, though, especially
> due to the above discussed Miller capacity. You can really see this by
> watching the drain voltage. When the FET is turned on by driving the gate
> positive, the drain voltage will come down, but Cdg will then drive the
> gate voltage down tending to turn off the FET. As I recall (it's been
> years since I worked on this), the voltage drops, tends to plateau, then
> drops to ground.
>
> Good luck! I blew out a lot of FETs!
>
> Harold

Really appreciate the clear explanation of some of the mechanisms behind=20
what is going on inside the FET and the Miller effect. Hopefully it will=20
help me in blowing less FETs :)
Jim

--=20
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