Mosfet amplifier design

hi Guys, I found this design somewhere, and I know it works because I have seen it. But I'm curious how it works. How is the bias controlled here? I guess the overall gain of the amp can be controller with R59/R52?
Can some of the more experienced designers here post some thaughts?

Thanks!
Peter
 

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mikek

Disabled Account
2002-05-15 1:12 pm
Hi petervg..

looks like one of vallemans designs....the op-amps linearize the mosfets, while simultaneously controlling standing current in the output stage......:up:

The design is a combination of series applied-shunt derived voltage feedback to the inverting input of ic7, and current feedback applied to the output of said op-amp.

Note that there appears to be an error in the schematic, as T10, and T11 sources should be connected to the positive supply rail.......

Michael, Chigwell, Essex, UK.
:)
 

mikek

Disabled Account
2002-05-15 1:12 pm
....notice also, that op amps A1 to A4 require non-zero referenced supply rails......:)

Vallemans DC offset protection circuit, which you've ommited, is one of the better designed schemes....wouldnt say the same for their SOA protection, (a single slope, linear foldback affair), which i find rather rudimentary...
 
T11, T13 drains should connect to LS+ too.

You could easily extend this amp with more mosfet / opamp sections because the current sharing will be virtually perfect, limited only by the matching accuracy of the source resistors. Variations in threshold voltage and Rdson don't matter a toot. Each fet's opamp forces it to conduct the amount of current it is told. As a result though, the output impedance is sky high because the output stage is a current source, relying on the the overall negative feedback to make it a low impedance voltage source, which is what we really want.

GP.
 

mikek

Disabled Account
2002-05-15 1:12 pm
Hi circlotron...

I think you'll find that the output impedance is near zero...the output fets are inclosed in a local feeedback loop defined by ic7 r28, r60....

Think of the whole as one large fet-bjt complimentary feedback pair, (if we neglect A1-A4),....with global feedback implimented by r52, r59.