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bokakob 6th June 2007 12:45 PM

Simple MOSFET amp
2 Attachment(s)
I found this schematic on the net and wonder how good it is for a first project. Any opinion is appreciated thank you.

richie00boy 6th June 2007 12:57 PM

It is reasonable for a first project, but those MOSFETs are expensive and not so easy to find.

Gold_xyz 6th June 2007 07:24 PM

Decoupling supply with R-C can be used in preamplifiers stages
but in drivers it can not provide all voltage for end mosfets.
Also this system do not provide enough supply stability.
(but it needs to try to build it for check this :D )

Arlo 7th June 2007 05:32 PM

i suggest to add emitter degeneration resistors at the input diff. stage
(the value of resister is about 100R ~ 300R)

it improve its linearity and dynamic range and TIM performance.

the position of emitter degeneration resistors is inserting into the emitter of two 2sa872 and the 100r SVR.

unclejed613 8th June 2007 12:35 AM

a current source for the diff amp, and one for the VAS load would be a couple of improvements i can think of.

actually, build the amp as-is, but using some output devices more readily available, such as IRF140 and IRF9140, and leave the 470k resistor in the diff amp, and the 15k resistor in the voltage amp stage, in places where you can easily replace them with other circuitry. then you can build current sources to put in place of the resistors and do some experimenting with the circuit.

FastEddy 8th June 2007 12:38 AM

Interesting resource for audio matched MOSFETs = :smash:

bokakob 8th June 2007 06:56 AM

Thank you all for the feedback. I must admit that I would not be able to change or add anything to this circuit because I am a simple DIY handyman with expertise in some other fields.
I understand just a little in schematics. The reason I picked up this schematic is that it looked simple, looked similar to other amplifier designs, promised sufficient power and used MOSFET as output.

I would ask if those "expensive" MOSFETs in the output stage could be replaced with something more available and less costly.
Perhaps, this could grow in the beginner's Hi-Fi amplifier."

Thanks for all replies.

richie00boy 8th June 2007 07:46 AM


Originally posted by unclejed613
actually, build the amp as-is, but using some output devices more readily available, such as IRF140 and IRF9140
No don't do that. This design is not meant for those vertical MOSFET types and it will go into thermal runaway.

If you want to use cheaper and more easy to find output devices but still use MOSFETs there are plenty of designs out there that are properly thermally designed.

FastEddy 8th June 2007 02:38 PM

bokakob: FYI about those two separate 0.1 uF caps on the power rails across the output FETs = ... make 'em be plastic caps = covers a world of power supply sins ... :smash:

AndrewT 8th June 2007 02:52 PM

alternative FETs are renesas 2sk1056/j160, 2sk1057/j161, 2sk1058/j162, or Exicon ec10n16/p16, ec10n20/p20 or magnatec buz900p/905p, buz900d/905d, buz900/905. These and any other lateral FETs are suitable.

The second stage transistor (VAS) is driving the output FETs directly. I would suggest that you do not parallel output devices to cope with a lower value load. Stick to 8ohm speakers not 6ohm, not 4ohm and not 4 to 8ohm.

The only stability enhancing components I can see are the decoupling caps and the output Zobel. You may have to fit some additional trimming components to ensure stability into reactive loads. You will need an oscilloscope to identify if any additional components are needed and if those fitted do the job adequately.

Arlo's emitter degen are already fitted. The 100r pot achieves the same end and is equivalent to 50r on each emitter. An emitter resistor on the VAS may help.

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