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ronybc 11th June 2004 02:38 PM

Opamp based MOSFET amplifier design
See dynamp schematics at

I build this amplifier and it sounds not bad. I likes to know... is there anything fundamentally wrong with an opamp+mosfet design..? or this particular design...? Why such designs are not so often used arround..?

If someone have an oscilloscope and dare to build this amp then please tell me whether there is any imperfections with the output waveform.

Any suggestions..?

PMA 11th June 2004 04:35 PM

Did not you have a chance to use power mosfets of the opposite polarity (N-MOS and P-MOS).

ashok 11th June 2004 05:14 PM

There have been many amps using opamps and bipolar outputs. As time went by the MOSFET came along and so did cool Power Amp IC's . I guess from the early 80's ( Hitachi HA1397 etc).
So today you could make a power amp with higher rail voltages
( higher output) and very low parts count . Maybe that's why opamp driven output stages are not around. They do have a supply voltage limitation which sometimes could be worked around. It would however make things more complex.
You must have come across the few dedicated high voltage 'driver' IC's meant for use with power output stages.
Possibly makes the standard opamp plus power output stage less attractive.

PMA 11th June 2004 05:21 PM

OPA445 is the opamp for supply rails as high as +/-40V. But I am afraid of driving large input mosfet capacitance only by opamp output.

richie00boy 11th June 2004 09:39 PM

The 'Dynamp' doesn't appear to have any method of biasing the output stage. It looks to me like a textbook beginners power amp. During an early lab on my EE course we certainly made something much like this, only with bipolars.

Pafi 11th June 2004 10:15 PM


The values in component list are definitely wrong, or they were swapped.

mandat 11th June 2004 11:24 PM

At the Polish forum is shown a simple power amp, based on OPA551/2 opamp at the input stage and complementary MOSFET pair IRF540/9540 at the output. The voltage of gates is setting with TL431. The sonic quality is quite impressive.

PMA 12th June 2004 01:18 PM

2 Attachment(s)
So how about this one? I can guarantee function and high sound quality ;)

mandat 12th June 2004 03:44 PM

semiconductors: OPA552AP/IRFP240 (Farchild]/IRFP9240 (I.R.)/TL431
R+C: 7 + 10

PSU: +/- 28V,
gain: +41V/V
load: 4 Ohm
quiscent current:120mA
THD+N: 0.008% (22Hz to 22kHz, P =1 to 10W)
THD+N: 0.014% (22Hz to 22kHz, P = 11 to 40 W)
THD+N: 0.6% (22Hz to 22kHz, P = 41 to 75W)
SR: +22/-18V/us

Can your amp offer more?

PMA 12th June 2004 04:02 PM


whom do you address your post to? ;)
If it's me, then:
Regarding power - not, as the amp shown is limited by supply voltage of opamp. In case you use OPA445 as the opamp, then yes, as supply voltage of OPA445 is up to +/-45V.
Regardind distortion - this would be more or less the same.
The amp I displayed is a class A amp and needs pretty good heatsinking. And it was shown only as an example of workable amp, contrary to the first post of this thread.

Keep cool,
Pavel ;)

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