mosfet and opamp amplifier

darkfenriz · 25th June 2004, 01:57 PM

just take a look at this
www.darkfenriz.webpark.pl/mosfet.jpg

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

link

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

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

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

Quote:

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

25th June 2004, 01:57 PM	#1
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	mosfet and opamp amplifier just take a look at this www.darkfenriz.webpark.pl/mosfet.jpg 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...

25th June 2004, 02:03 PM	#2
kilowattski diyAudio Member Join Date: Aug 2003 Location: Long Island, New York	Your link does not work. I think you need to put the 'http://' in front of it. __________________ ----------------------------------------------- Kilowattski

25th June 2004, 02:19 PM	#3
uli diyAudio Member Join Date: Dec 2002 Location: Vienna, Austria	link link __________________ 'Jazz is not dead, it just smells funny' F.Zappa

25th June 2004, 05:55 PM	#7
Stocker diyAudio Member Join Date: Dec 2003 Location: Austin	the simulator the simulation or the circuit and why not? __________________ Jesus loves you.

25th June 2004, 08:27 PM	#10
Stocker diyAudio Member Join Date: Dec 2003 Location: Austin	~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 __________________ Jesus loves you.


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25th June 2004, 03:59 PM	#4
fab diyAudio Member Join Date: Mar 2004 Location: Canada	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	#5
fab diyAudio Member Join Date: Mar 2004 Location: Canada	I forgot to mention that the overall closed loop gain seems to be only 2 !?..

25th June 2004, 05:09 PM	#6
xplod1236 diyAudio Member Join Date: Dec 2003 Location: On the couch	I just ran that thru a simulator, and it does not work.

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