Quiescent Current Control in Class AB..?

[xitronics]

I need a simple but effective quiescent current control circuit for my 10-watt class AB stage. Vbe multiplier is too sensitive to parasitic changes and so I find it inappropriate to make use of.

Regards,
Prasanna
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[lumanauw]

A combination of R and thermistor?

[jaycee]

What about a simple diode string?

[AndrewT]

Originally posted by xitronics
I need a simple but effective quiescent current control circuit for my 10-watt class AB stage. Vbe multiplier is too sensitive to parasitic changes and so I find it inappropriate to make use of.

what circuit is too sensitive for a conventional Vbe multiplier?

[AKSA]

Yes, agree with Andrew, to my knowledge there is no Class AB output stage which cannot be stabilised with a properly configured Vbe multiplier.

Hugh

[Bonsai]

Xitronics, post your circuit and then we can give you some feedback/ideas.

Note that you have to look at the Vbe circuit in conjunction with the physical layout, so you need to indicate (picture would be a good idea) where/how the Vbe sensor is located.

[lumanauw]

Note that you have to look at the Vbe circuit in conjunction with the physical layout, so you need to indicate (picture would be a good idea) where/how the Vbe sensor is located.

If the issue is thermal tracking, then placement is very important.
I've seen a single transistor bias generator (single NPN, not even darlington) can give negative thermal compensation (overcompesation) while the output stage is triple darlington (6 B-E junctions must be compesated). The heatsink base is cooper, though

[xitronics]

Vbe multiplier can provide controlled quiescent current to the class ab stage but cannot deal with variation the bias current source output. The collector current of class AB is a near-exponential function of the bias current. But I do agree with you all tht with a fairly stable biasing source, Vbe multiplier is a very effective solution to the problem.

I have attached the my circuit attached to this post. Kindly suggest modifications wherever necessary.

I had another doubt.. We can use emiiter feedback resistors to set the DC collector current in class AB stage. But by adding bypass capacitors to them to maintain unity AC gain I get sharp crossover distortions in the output. Can anyone explain me why this is taking place?

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Regards,
Prasanna.

[xitronics]

Circuit producing the crossovers is attached here..

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Prasanna

[consort_ee_um]

You need some driver transistors in front of the output transistors. The current demand of the output transistors will overload your current source. When you add the drivers your problem will hopefully disappear