Unusual Output Stage - Class AB with op-amp

[hbk3]

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]

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]

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]

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]

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]

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

Thanks in any case.