I have to design a Class AB push pull amplifier with VBE multiplier biasing and using a pnp current mirror to bias the VBE multiplier.

I am using OrCAD, with Q2N2222 and Q2N2907A βf=50 and Is=14.34f for both types of transistors.

I attached the schematic for circuit that i have so far, but it's not working, as the output at R6 only shows the bottom half of the wave and it's not really amplifying the input.

Any help is appreciated.

 

Your current mirror is connected between GND and -15volts. In order to turn on Q6, it's base must get higher than GND. The current mirror will not allow it.

 

What is "multiplier biasing"?

 

The VBE multiplier biasing means the biasing current is from the current mirror.

I made the change to the current mirror and this is what i got as for vo vs. vin.

 

Vbe Multiplier, RCA Brute 70 amplifier from somewhere in the 1960's. Resembles a rubber zener. for setting the bias current on a push-pull stage. I'd quote chapter and verse but I can't find my RCA Power Transistor book from 1970.

http://www.diyaudio.com/forums/solid-state/140988-vintage-brute-70-40411-clone.html

 

How about grounding the negative terminal of V5? That should allow the current mirror output to rise above ground.

 

putting that ground helped a lot, as the current mirror does give me the bias current that i need, but my output wave just seems to be offset by a constant value, and is not really amplifying the input.

Should the problems be that my resistor values at R1 and R2 are off?

 

It isn't a voltage amplifier. It's a current amplifier.

 

There is no bias on the collector of Q4.

 

It isn't a voltage amplifier. It's a current amplifier.

ahah, you are right, i misread the question that i have to answer. It only asked me to show vo with a peak swing greater than 5V, not a gain of greater than 5.

 

When Q3 is "on", ie. conducting, it's Vbe is about 0.7 volts. That is the only thing between Vin and Vout. A little more complicated for Q6 conducting, but still can be easily checked.

 

Been working on the same project, how did you come up with the values for R1 and R2?

 

It's kind of set R2 and adjust R1 as you go. The point is to get the right idle current for Q3 and Q6 and it depends on the output transistors you use. Q1 is also supposed to be thermally coupled to the output transistors.