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darkfenriz 25th June 2004 01:57 PM

mosfet and opamp amplifier
just take a look at this

what do you think??
to be honest it's my first project of such an amp and I think it;s better than on the 'opamp based mosfet amplifier' forum

any suggestions very welcome
thanks in advance...

kilowattski 25th June 2004 02:03 PM

Your link does not work. I think you need to put the 'http://' in front of it.

uli 25th June 2004 02:19 PM


fab 25th June 2004 03:59 PM

hi darkfenriz

Are you sure that this design is sounding (or even working) fine?

I have some doubts about the auto-bias configuration of the output driver stage (NPN bjt). How does it controls in a stable way the bias current of the mosfets? Driver current is probably a bit low with 22K resistors even if we do not know the VGSoff of the mosfet (type unknown).

What is the purpose of the 22K resistors at collectors of NPN driver bjt (except limit the maximum output swing of the AC voltage thus reducing the maximum possible power of the amp)?

There is no frequency compensation at all in the circuit, I have some doubts on the risk of oscillation...

But, this is only my impressions. Maybe other peoples could comment too.


fab 25th June 2004 04:02 PM

I forgot to mention that the overall closed loop gain seems to be only 2 !?..

xplod1236 25th June 2004 05:09 PM

I just ran that thru a simulator, and it does not work.

Stocker 25th June 2004 05:55 PM

the simulator
the simulation
or the circuit

and why not?

xplod1236 25th June 2004 06:46 PM

2 Attachment(s)
the circuit does not work.
the input is 1v p-p 5khz sine wave. the output is shown in the attached image.

jan.didden 25th June 2004 07:01 PM


Originally posted by Stocker
the simulator
the simulation
or the circuit

and why not?

Well, if you pencil in the DC voltages without input signal, it'll be clear. Like using your brain as the simulator?

Jan Didden

Stocker 25th June 2004 08:27 PM

~some~ of us are at work...

I think it is more constructive to say why/what doesn't work than to just say it doesn't.

Add some toungue in cheek

and you get my commentary / genuine question.

If someone does bother to make the cct work on paper, this is something I just might actually prototype

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