Quote from: MileHigh on May 14, 2012, 04:54:49 PM
Poynt:

Thanks for the sims.  You can see how for the positive bias mode that the zener diode in Q2 is doing a very decent job of clamping the Vgs of Q1.

Are you sure? It appears to me that Q2's diode is outside the VGS loop for Q1.

Quote from: TinselKoala on May 14, 2012, 05:02:27 PM
Thanks.. I understand the second case  after yesterday's experimentation I think.
But why weren't there oscillations present in your first case?  Isn't that, except for the missing Moses mosfet, the same as the "negative gate bias" oscillation mode?

Two reasons it doesn't oscillate in my simulation:

1) 50 Ohm RGEN is too high.
2) 24V supply is too low.

Poynt:

QuoteAre you sure? It appears to me that Q2's diode is outside the VGS loop for Q1.

Talking positive bias mode:

The 152 mA current flow will cause cause a big drop across the 50-ohm Rgen.  Q1 switches on, pulls the Q1 drain low.  That causes the Q2 zener to switch on, the 152 mA of current starts to flow, and that pulls Vgs down to 4.37 volts.

Does that make sense?  That looks like potential for another negative feedback mechanism.

Interestingly enough, TK pointed out a DSO capture today where we see the the positive bias at the gate start high and then undergo what resembles an exponential decay.  So that could be the effect in action.  It takes about 10-15 uSec to do this though, which my gut feel is telling me is too slow.  Another interesting test.  Do a setup where you try to test for this effect.  Adding capacitance here and there should slow it down, which would be interesting.

MileHigh

So getting back to GL's original problem... Poynt99's second case drawing indicates that if we move the "bias" current measuring point to the gate leg of Q1 (Moses) only, we will see only the 1pA leakage current (once the gate is charged of course, in a tiny fraction of a second). This will confirm that all that 90-150 mA measured current is going through the Zener in Festus. Sorry, Q2. Right?

I've moved the meter in the drawing below.

(Obviously (I hope) I'm just trying to cover all the bases. Here we hope to show that no DC passes the gate of the Q1 mosfet other than the 1pA leakage which is undetectable by our present instruments and methods.)

Quote from: MileHigh on May 14, 2012, 05:22:57 PM
Poynt:

Talking positive bias mode:

The 152 mA current flow will cause cause a big drop across the 50-ohm Rgen.  Q1 switches on, pulls the Q1 drain low.  That causes the Q2 zener to switch on, the 152 mA of current starts to flow, and that pulls Vgs down to 4.37 volts.

Does that make sense?  That looks like potential for another negative feedback mechanism.

MileHigh

This was what I thought you meant, the first time you said "negative feedback" . That's why I listed it separately from the "oscillation feedback" which happens in the negative bias mode. And I think maybe you are right.