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Unusual Output Stage - Class AB with op-amp

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hbk3

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Hi everybody.

I'm looking for information on the attached output stage - it's a class AB and it uses op amp as well. Image is attached.

I couldn't understand how it works - to my understanding there should be a connection between the emitter of Q6 and the output, but I was told explicitly that the stage should work without it as well.

I can't see how. When I try to simulate it in PSPICE I get almost always (almost) zero output voltage.

Can you help me?
I searched in many books after this circuit, with no success.
 

keith1200rs

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It can never work with those values (assuming it works at all). Q6&7 can never turn on with R19/23 being so low.

Keith
 

FvM

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to my understanding there should be a connection between the emitter of Q6 and the output
You're right. Did you try to connect it. It should work then.
but I was told explicitly that the stage should work without it as well.
No chance, I fear.
P.S.:
It can never work with those values (assuming it works at all). Q6&7 can never turn on with R19/23 being so low.
It can work in class B, with considerable cross-over distortions.
 

    hbk3

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hbk3

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This circuit is part of a board in a lab in a university.

I complained to the chief instructor, and he said he had asked the professor and he was conclusive that the stage should work.
The board isn't new, so I guess it should work...

Any chance?
 

FvM

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You already found, that it doesn't work in simulation. You may want to measure the real board. But be aware of hidden wires!

Simply, you have to find a way to control the base-emitter voltage of the driver transistors from the OP output. Obviously
it can't work without connecting the emitters. It's my last word in this regard. (Unless someone proved I'm wrong)
 

hbk3

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Simulations are bound to mistakes, and if the professor was conclusive, I'd check myself again.

Thanks in any case.
 

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