Opamp based MOSFET amplifier design

ronybc · 11th June 2004, 01:38 PM

See dynamp schematics at http://www.ronybc.8k.com

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, 03:35 PM

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

ashok · 11th June 2004, 04:14 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.
Cheers.

PMA · 11th June 2004, 04:21 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, 08:39 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, 09:15 PM

Hi!

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

mandat · 11th June 2004, 10:24 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, 12:18 PM

So how about this one? I can guarantee function and high sound quality

mandat · 12th June 2004, 02:44 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, 03:02 PM

mandat,

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

11th June 2004, 01:38 PM	#1
ronybc diyAudio Member Join Date: Jun 2004 Location: Kerala, INDIA	Opamp based MOSFET amplifier design See dynamp schematics at http://www.ronybc.8k.com 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..?

11th June 2004, 04:14 PM	#3
ashok diyAudio Member Join Date: Jun 2002 Location: 3RS	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. Cheers. __________________ AM

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11th June 2004, 03:35 PM	#2
PMA diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	Did not you have a chance to use power mosfets of the opposite polarity (N-MOS and P-MOS).

11th June 2004, 04:21 PM	#4
PMA diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	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.

11th June 2004, 08:39 PM	#5
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	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.

11th June 2004, 09:15 PM	#6
Pafi Sin Bin Join Date: May 2004 Location: Budapest	Hi! The values in component list are definitely wrong, or they were swapped.

11th June 2004, 10:24 PM	#7
mandat diyAudio Member Join Date: Oct 2003 Location: Gliwice	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.

12th June 2004, 02:44 PM	#9
mandat diyAudio Member Join Date: Oct 2003 Location: Gliwice	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?

12th June 2004, 03:02 PM	#10
PMA diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	mandat, 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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