Op - Amp with power output stage

[andy257]

Hi Guys

The circuit below shows an inverting op amp with a gain of 10 and an input of 0.5V ac at 1KHz.

Therefore at point X we should be getting 5V and at point Y, 4.3V becasue of the volt drop across the emitter base.

The problem with this circuit is that cross over distortion is formed when the output is les than 0.7V.

If i connect point X to point Y which will result the output being part of the feedback loop what is the likely output voltage going to be? How do i figure out what amount of voltage is fed back into the op amp input?.

I cant seem to work out the math to calculte this.

Yes this is homework but i dont understand how the feeback bit can be predicted with maths like i have with no feedback connection.

any help is more than welcome

thank you

andy

**broken link removed**

 

[Nigel Goodwin]

If you connect the 10K to point Y instead of point X, the transistors effectively become part of the opamp - so just calculate as you normally would for an opamp.

This is a common use of an opamp, the high gain of the opamp overcomes the Vbe drop in the transistors - and although not perfect (for hifi use) is good enough for many purposes.

 

[andy257]

Thanks for the reply


Is there no way to calculate what the new output will be then, or what fraction of the output is fed back into the op amp?

Does that mean the output will allways get the full 5V?

 

[Oznog]

The gain remains unchanged, it's R2/R1. In this case, 10.