Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

Opamp based MOSFET amplifier design
Opamp based MOSFET amplifier design
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 12th June 2004, 03:27 PM   #11
djk is offline djk
diyAudio Member
djk's Avatar
Join Date: Feb 2001
Location: USA
Your circuit can only drive the output to the rails, less the Vgs of the FET.

With a 15V supply for most opamps that will be on the order of 10V peak-to-peak into the load, or about 6W at 8 ohms.


Then click on 'images' (see the FAQ if you need a TIFF viewer)

This circuit only need provide the Vgs voltage to get the full V from the FETs, with 100V on the FETs you can get 500W/8R, 800W/4R, 1200W/2R, only limited by the current of your supply and how many FETs you use.

There are many fine opamps intended to drive 50R co-axial cable that will drive many pairs of FETs.

Only one 15V supply is needed for the opamps, it could also be shared with a preamp and tone circuits.

Each power amplifier output will require its own transformer winding.

Acoustat/Hafler/Fosgate are built this way.

QSC uses BJT with driver transistors and this same scheme.
  Reply With Quote
Old 12th June 2004, 03:30 PM   #12
millwood is offline millwood  United States
Account Disabled
Join Date: Apr 2003
Location: US
wouldn't it be simplier to use one of those power opamps as a high-voltage driver amp? like lm3875?
  Reply With Quote
Old 12th June 2004, 03:32 PM   #13
PMA is offline PMA  Europe
diyAudio Member
PMA's Avatar
Join Date: Apr 2002
Location: Prague
Opamp based MOSFET amplifier design
FETs can be supplied from power supply with higher voltage than opamp. Most opamps can work up to +/-18V and great many of them up to +/-22V (OPA604 and others).

For most of IRF's, R5 and R6 must be reduced to 510R. The circuit was designed for 2SK413/2SJ118.
  Reply With Quote
Old 12th June 2004, 05:33 PM   #14
jan.didden is offline jan.didden  Europe
diyAudio Member
jan.didden's Avatar
Join Date: May 2002
Location: The great city of Turnhout, BE
Ron (bc),

I seems to me that your T1 is cut off on positive output swings. It takes some time for the opamp to travel back to the point that T1 starts conducting on negative swings. The result would be quite some cross-over distortion.
Some sort of biasing to keep the cross-over region as small as possible would improve this amplifier. For instance, a current source from T1 base to -supply to bias T1 at say .3VDC Vbe

Jan Didden
Music is dither to the brain; lets me think below the usual chaos - me
Get more Linear Audio for less! Check out my Autoranger and SilentSwitcher
  Reply With Quote
Old 12th June 2004, 05:56 PM   #15
fab is offline fab  Canada
diyAudio Member
fab's Avatar
Join Date: Mar 2004
Location: Canada

The circuit attached on this thread:

uses an op-amp with +/- 15V supplies and complementary output mosfet (IRF540/9540) with any higher voltage desired (gain in the output stage). It can be used in class AB since it has bias adjustment. The resistor to polarise the driver bjt of the output mosfet can be replaced by current sources if required. I do not have THD info but I suppose it can be quite low. A "local" feedback could probably be added in the output stage to linearise it more.

The funny thing about this circuit is that it is one of our projetc in EE course back in 1987. I had a weirdy teacher close to retirement and I realised only many years after that he was very knowledgeable.

  Reply With Quote
Old 13th June 2004, 10:58 PM   #16
foo is offline foo  Tibet
diyAudio Member
Join Date: May 2004
Location: ...
here u go
  Reply With Quote
Old 15th June 2004, 10:56 AM   #17
palesha is offline palesha  India
diyAudio Member
Join Date: Nov 2001
Location: Pune India
Kindly give more details like supply, IC type, power etc. If u hv made it then let us know about ur experience.
PMA said : So how about this one? I can guarantee function and high sound quality
  Reply With Quote
Old 15th June 2004, 06:21 PM   #18
PMA is offline PMA  Europe
diyAudio Member
PMA's Avatar
Join Date: Apr 2002
Location: Prague
Opamp based MOSFET amplifier design
Originally posted by palesha
Kindly give more details like supply, IC type, power etc. If u hv made it then let us know about ur experience.
Dear Palesha,

I have only simulated this amp. I have continued in simulation to display the circuit with component types and supply voltages. It is attached now. The output stage is in class A, every pair of the output devices has quiescent current of about 500mA. This can be changed by R5/R6. M1-M4 will need good heatsink. I did not listen to this amp as I did not build it. But according to my experience I assume this could be quite a good amp. Not the absolute top class, of course. Regarding the results of simulation, for simple circuits like this they correspond with real circuits well.
Attached Images
File Type: gif mospower2.gif (10.4 KB, 2204 views)
  Reply With Quote
Old 16th June 2004, 06:51 AM   #19
ronybc is offline ronybc  India
diyAudio Member
ronybc's Avatar
Join Date: Jun 2004
Location: Kerala, INDIA
Hey all...

Thanks for your responses and suggestions. Actually the non-availability of P-channel mosfets (locally) pushed me to attempt N-channels both side. I supposed that most of the problems including the biasing of T1 will be compensated by the opamp by negetive feedback from the output.

(PMA) your circuit is interesting.

(Fab) I tried disconnecting emitter of T1 from output and grounded it through a 100ohms. BUT the result was huge distortions in the output. I don't know what went wrong.. can you help me...

Thank you,
Rony B Chandran
  Reply With Quote
Old 16th June 2004, 10:33 AM   #20
amb is offline amb  United States
diyAudio Member
amb's Avatar
Join Date: Apr 2004
Location: Sunnyvale, CA.
Re: getting more voltage swing (and therefore output power) from opamp-driven power amps

See this article about a way to bootstrap the op amp's power rails to increase the voltage swing:


Alternatively, you can add an additional VAS stage prior to the output MOSFETs to give the circuit voltage gain after the opamp, and supply the VAS and output MOSFETs with higher rail voltages. An example of this is found in Walt Jung's 3rd Edition of the IC Op Amp Cookbook, in the "Amplifier Techniques" chapter.

  Reply With Quote


Opamp based MOSFET amplifier designHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
PCB design for MOSFET based HP amp? zebra100 Solid State 0 1st July 2009 09:01 AM
One of error correction amplifier (opamp based) ANALOG GUY Solid State 3 20th December 2005 09:29 AM
Ultra-High performance OpAmp-based MOSFET Power Amp tlf9999 Solid State 20 7th September 2005 09:41 PM
mosfet and opamp amplifier darkfenriz Solid State 32 15th July 2004 01:17 AM

New To Site? Need Help?

All times are GMT. The time now is 02:14 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.79%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright ©1999-2017 diyAudio