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Dj BASS AMP 27th July 2004 07:18 PM

Irf540 Amplifier

if i will build this it is work good ??! on 35V + - ?!
and it is class A or AB ?

Thanks !
it is importent. i make make a new project.


OliverD 27th July 2004 07:24 PM

Does that work with 22V? ;)

You definitely need more output devices.

richie00boy 27th July 2004 08:33 PM

More output devices? Why do you say that? It looks to me that even a single pair would be perfectly fine with that supply voltage.

Fillout 27th July 2004 08:40 PM

and it is class A or AB
It's AB & it'll workit good on +- 35 if you have a BIG heatsink;)

OliverD 27th July 2004 09:29 PM

This circuit as posted will be good for about 20 - 25 Watts into 8 Ohms... He wants to go 35V rails and probably expects something around 60 - 70 Watts into 8 Ohms.

Having read some of Bass Amp's other threads, I suspect the project will be used in a PA environment (and probably not with 8 Ohm loads), so I recommend a healthy safety margin. The IRF540 comes in a TO-220 package and dissipation should be limited.

But to answer the actual question, by increasing the rail voltage you won't get more power out of this circuit, because the opamp will not be able to swing the output stage to the rails.

Lars Clausen 27th July 2004 10:20 PM

If you want to dissipate more power = better low impedance survivability, better use IRFP140 / IRFP9140 pair instead. And the 0R1 resistors are really not nessescary, unless you use them to measure current for a short circuit protection circuit

I suggest replacing the 510R resistors with a stabilized voltage reference, so you get a stable idle current control. You should have a 4.5 mV/F total decrease in VDS to keep a stable BIAS. Use a Darlington transistor and a TL431 reference in series, it will work really stable.

Another thin is the IRF540 / 9140 is incredibly rugged, you can draw at least 100 Watts of audio power out of this low cost pair. In fact it is just as powery as a pair of TOP3 bipolar transistors, even if they are just in TO220 packages with limited power dissipation capability.

Lars Clausen

Dj BASS AMP 27th July 2004 11:44 PM

2 Attachment(s)

thanks all....

i will do this for 8 Ohm...... active spek'
if i do this to 4 ohm i will double the out put transistors.....OK ?!

i dont understand one thing.......

about the voltage reference, to stable idle current control....

how i do this exactly in the simplest way.......


xplod1236 27th July 2004 11:56 PM

kinda off topic, but why are the gate resistors necessary?

OliverD 28th July 2004 01:27 AM

Still I don't see how the opamp could swing the output stage to the rails. Looking closer at it, the output voltage swing is limited to the opamp's supply minus the Vgs of IRF540. It won't even do 20 Watts into 8 Ohms.

In other words, scaling up the rails to 35V won't give you more power.

Apart from that, I don't like the idea of a weak opamp output fighting against the charge on the gate capacitance of the output devices. Has anybody actually built this and listened to it?

xplod1236, the gate resistors (= stopper resistors) are there to reduce the tendency towards oscillations, together with the gate capacitance of the FET they form a low pass.

Dj BASS AMP 28th July 2004 03:40 AM


Originally posted by amb
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.


soo....... OliverD i will need to do this....... or change the opamp to OPA445......

how do i do the thing in the link in the simple way....

and if i will change to OPA445 it will be ok?


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