MOSFET amp what d you think?

[ttako]

Hi,

I was drawing a little experimental MOSFET amp.
Could you check it? What do you think about it?

Thanks Tamas

[hbc]

Bound to oscillate in many different ways and then probably explode, slightly less likely if class A. But I could be wrong, you don’t say much about application.

[ttako]

Originally Posted by hbc

Bound to oscillate in many different ways and then probably explode, slightly less likely if class A. But I could be wrong, you don’t say much about application.

Application - hm... Audio :-)
I had a goal: to get a stabilized Bias, and an easy structure.
I also would like to use VERY BIG MOSFETs that is why you see the 22nF capacitors at the gates, as these are just there to mimic the BIG gate capacitance...

also the stages are relatively easy to modell (and also measure) separately,
so compensation could be easily calculated/ tryed.

cheers,

Tamas

[hbc]

Hmm your picture says 200v rails, that is serious power- audio – or PA- or mains power regeneration....., I personal would not want to hook anything like that up to my domestic speakers for fear of a “back to the future” moment!

[ttako]

Originally Posted by hbc

Hmm your picture says 200v rails, that is serious power- audio – or PA- or mains power regeneration....., I personal would not want to hook anything like that up to my domestic speakers for fear of a “back to the future” moment!

Yes, you are right! I just wanted to write there some big numbers for the theoretically max possible MOSFETs available in N and P version (200V P MOS is the max I have found)

If I ever build that I will start with some +/-30...50 V rails

cheers,

Tamas

[hbc]

If you like prototyping things first, I would try some thing like this first, separate the gain from the current buffer, get the output working first, then the gain, then together, and then with loop feedback, and then add more mosfets and stuff like your original circuit. You will have probs with your buffer stage opamps when they go open loop every time the mosfets turn off, causing worse crossover distortion. Can fix a bit with diode in feedback loops probably, look at precision opamp circuit with 2 diodes iirc, has sorted same problem already.

modded (ehem) your circuit to something a bit easier to get going (hopefuly)

h

[Bigun]

If this is your first amp, something even simpler might be a better place to start. Simple doesn't mean it won't be good, in fact it might be better and you might want to go in a different direction than what you have shown above.

I'm thinking single NE5532 driving a pair of output devices with simple bias into Class AB with diode string. I'm sure somebody has a circuit for this already.

[djk]

William Chatter has some FET bias servo designs in Audio Amateur and Audio Express, and a KT88 servo design as well.

Yours is similar, but he has all the details worked out.

The easy way to do it is to drive ground with the FETs, and take the output from the transformer center tap (QSC and Hafler style). Chatter uses a suspended supply.

[ontoaba]

Those opamps also need input protection, diode or something.

[ttako]

Originally Posted by djk

William Chatter has some FET bias servo designs in Audio Amateur and Audio Express, and a KT88 servo design as well.

Yours is similar, but he has all the details worked out.

The easy way to do it is to drive ground with the FETs, and take the output from the transformer center tap (QSC and Hafler style). Chatter uses a suspended supply.

Thanks DJK,

Any link to the William Chatter servo FET amplifier schematics?
I become very interrestd :-)

Thx,

Tamas