John said:
On Mon, 20 Oct 2008 19:34:52 GMT, "Kevin Aylward"
Well, neither. I was suggesting a fast opamp *per fet*, with feedback
from the source, to make each fet look like an ideal transconductance
device, perfectly linear, no offset or threshold, all exactly matched,
with very low input capacitance.
But how does improving and parallelizing gate drives cost speed? It
I made no criteque on parallelising gate drives. Bigger drive is usually
better.
makes my amps faster and a lot more stable. Your amp (the one you
never built) has a couple of wimpy current sources driving 10 fets in
parallel;
No its doesn't. Its a push/pull class AB current drive to the mosfets, and
secondly, you don't need much. 20ma class drive is way more than enough for
at least 500w at 200Khz power BW.
I'm suggesting a beefy voltage source per fet gate, with
local feedback.
Its the local feedback, within another overall loop that is the problem.
Despite the apparent closed loop nature of the second buffer, it is still an
opamp with an inherent main pole, plus higher order poles within the main
loop. All these poles matter, unless you seriously clobber the whole
response. That is you get better accuracy at LF, but the amplifier will
always be slower than if the amp dd not have that extra op-amp. well not
unless you 100Ghz opamps or such like.
I am addressing the basic concept here. Things are not what they might seem
from an initial examination.
Kevin Aylward
www.kevinaylward.co.uk
