MOSFET output stage
Don Pearce wrote:
Eeyore wrote:
Vladimir Vassilevsky wrote:
4) FETs do not require high base currents of BJTs; that simplifies the
driver stage.
So what charges and discharges the damn gate capacitance you dozy bugger ?
The classic Class A voltage gain driver stage probably needs as much current
as if you were driving darlington output devices with a simple design..
In my ultra-low THD design I had a bipolar complementary Class A emitter
follower driving the mosfet gates ! It also removes the capacitive loading
from the voltage gain stage, increasing phase margin and loop HF response.
Where did you put the dominant pole cap? - still around the voltage
amplifier I'm guessing. It is vital that the dominant pole cap is the
only one that shows up in the phase response up to unity O/L gain, so
buffering of the fet gate caps is pretty much a given.
It wasn't dominant pole compensated. But you got the location right after a
fashion.
The big problem you need to overcome with mosfets is that they have an
essentially log response, resulting in (or from, depending how you look
at it) huge - maybe 10/1 Gm changes over the operating range of
currents.
Not on laterals.
Graham
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