On Fri, 19 Nov 2004 13:16:44 GMT, "Kevin Aylward"
wrote:

Again, potential issue of stability. I do have an interesting, somwhat
original output circuit that does this with a first order loop enclosing
the outputs. It gets me wonderfully low distortion at 20Khz. In one
sense topologically its essentially the same as other compound pairs,
but this does behave a little different. The feedback diodes are zeners
at 10V. The drive circuit on its own 0.0001% THD 20Khz, according to
spice that is. Note the feedback transisters can be low voltage ones,
hence fast ones. DC loop gain is 135db, open loop more!

http://www.anasoft.co.uk/Mospoweramp.jpg



One problem here is current sharing. Linear-mode paralleled mosfets
don't do it very well. One of my earlier gradient amps was done with
paralleled fets (with source resistors, at least) and we wound up
making a fixture so we could match sets of fets for production... a
real pain in the sternparts. The advantage of an opamp per fet is that
the gate drive becomes very simple and sharing is forced to be
perfect.

John