Auto bias vertical Mosfets in audio power amplifier

As title, I'm searching a simple circuit to biasing vertical mosfets in an audio power amplifier.

The output stage I'm using, is composed by two simple mosfet IRFP240/9240 in push-pull configuration. Before that a buffered VAS. The problem is that the amplifier need to be used at variable power supply voltage and need to adapt to every load at output. With a simple Vbe Multiplier I can't do this. I know, there is already the LT1166, but: I do not want to use third party IC and want a simple circuit made basically of transistor and resistors.



I've already tried this, but the complexity is high and not working well in simulation.



Then I tried this, I don't know the meaning of "dual vas". In my amplifier, the VAS and the driver are the same thing.

northumber82 said:

... I don't know the meaning of "dual vas". In my amplifier, the VAS and the driver are the same thing.

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I think VAS means the same as VCCS, Voltage Controlled Current Source, perhaps Voltage Ampère Source ? At least the first letter G of their device names indicates such sources, whereas F means a CCCS, a Current Controlled Current Source, s. this table:

erikl said:

I think VAS means the same as VCCS, Voltage Controlled Current Source, perhaps Voltage Ampère Source ? At least the first letter G of their device names indicates such sources, whereas F means a CCCS, a Current Controlled Current Source, s. this table:

View attachment 149224​

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No, VAS, in audio amplifier means Voltage Amplifier Stage. In which voltage of the Input Stage is amplified before the Output Stage. The problem is another: my VAS and the gate driver are the same thing.

northumber82 said:

No, VAS, in audio amplifier means Voltage Amplifier Stage. In which voltage of the Input Stage is amplified before the Output Stage. The problem is another: my VAS and the gate driver are the same thing.

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I can't see any voltage amplification in your above circuits (the drivers in the bottom-most circuit show amplification factor = 1). All this additional circuitry is for bias generation, I think.

Rod Elliott's website has several mosfet audio amplifier projects. He addresses a common belief that mosfets need an unchanging power supply.

He asks that his home page be given as a weblink:

http://www.sound-au.com

One of his articles about mosfet audio amps:

**broken link removed**

What was your problem with the Vbe multiplier? I don't understand how other audio amplifiers can work with a Vbe multiplier if those supply voltage varies.
Let's see this: https://en.wikipedia.org/wiki/File:OpAmpTransistorLevel_Colored_Labeled.svg
Also a push-pull output stage, supply can vary from 7V to 36V, and use a Vbe multiplier. Not an audio power amplifier, but with bigger output transistors it could be.
And not MOSFET, but I think it doesn't matter too much. What is vertical MOSFET in your term?

BradtheRad said:

Rod Elliott's website has several mosfet audio amplifier projects. He addresses a common belief that mosfets need an unchanging power supply.

He asks that his home page be given as a weblink:

http://www.sound-au.com

One of his articles about mosfet audio amps:

**broken link removed**

Click to expand...

Yes I already saw that, I found nothing of useful for the scope

frankrose said:

What was your problem with the Vbe multiplier? I don't understand how other audio amplifiers can work with a Vbe multiplier if those supply voltage varies.
Let's see this: http://en.wikipedia.org/wiki/File:OpAmpTransistorLevel_Colored_Labeled.svg
Also a push-pull output stage, supply can vary from 7V to 36V, and use a Vbe multiplier. Not an audio power amplifier, but with bigger output transistors it could be.
And not MOSFET, but I think it doesn't matter too much. What is vertical MOSFET in your term?

Click to expand...

Yes, I already use a Vbe multiplier, but it does not adapt to voltage or load change.

northumber82 said:

Yes, I already use a Vbe multiplier, but it does not adapt to voltage or load change.

Click to expand...

Why it doesn't adapt? Other amplifiers how can adapt it with Vbe multiplier then? And still, what is vertical MOSFET in your term?

frankrose said:

Why it doesn't adapt? Other amplifiers how can adapt it with Vbe multiplier then? And still, what is vertical MOSFET in your term?

Click to expand...

Vertical MOSFET = Hexfet

The Vbe bias current changes at voltage variating. In many amplifiers this isn't a problem because the voltage swing is little (5V).

My problem is that I've to use my amplifier in car, the voltage swing of the battery changes the output voltage of the transformer of about 20-25V. I have to maintain the SAME performances at every voltage.

I see, so you need supply independent bias current rather, then the Vbe multiplier can be good enough, isn't it?
Like this one: https://i.stack.imgur.com/kzZr9.png