John Popelish wrote:
(I just realized that I was talking about one configuration,
common source output but was thinking of the more usual
common drain output.)
(snip)
But the feedback network (that can include signals from both the opamp
output and the MOSFET output) can have other gains and phase shifts at
other frequencies.
A very simple, though, perhaps sub optimal (not the highest frequency
response) version would be a resistor from MOSFET output back to + and a
capacitor from the opamp output back to +. This pair has a perfect gain
of 1 at DC from the MOSFET output (since the capacitor has infinite
impedance at DC). But above some frequency, where the capacitor
impedance approaches the resistor resistance, the capacitor takes over
as the effective feedback, and the MOSFET output no longer follows the
input, but the opamp is kept in a stable feedback situation.
Of course, that won't work as stated, because the opamp
output is inverted (at least at low frequencies), compared
to the MOSFET output. So capacitive feedback around the
opamp to the + input is positive feedback, not negative
feedback. Duh! So with a common source output stage, an
additional inversion is needed to make use of the opamp
output as negative feedback, or that feedback has to be
connected to the - input, in some way.