#1
My current undergrad lab project is intended to build a simple class AB audio power amp, but being a uni lab project, we're being led through it step by step so we actually learn something from it and have some vague electronics knowledge that we can actually use in future. Right now i'm trying to get a working push-pull stage, but something very weird is going on, so i thought i'd see if there were any braniacs here who might help me. Please see the attached diagram for details of the actual amp stage.

For some reason, the voltage divider isn't working properly - the DVM shows 16.7V across the top resistor, and 7.3V across the other. I'm guessing this is some kind of series-parallel weirdness, but frankly after a whole day in labs i'm not able to figure this sort of thing out. Anyway, the end result of the circuit seems to be a steady -5 volt output on our oscilliscope, instead of reproducing the input voltage with from the signal generator with some cross-over distortion.

I know this sort of circuit isn't the best, in the end we're going to be using diodes to bias it into class AB, probably using either Darlington or Sziklai pairs to up the gain, and we haven't really gotten into the details of biasing the transistors properly.
Attachments:
Push-Pull.jpg
#4
As I said, we're going to be diode biasing it later, but our lab demonstrater 'advised' (read: told) us to start with a design this basic. I'm more worried about balancing the circuit first, as while this circuit should be pretty crap, it should still work.
#5
Quote by Mad_BOB
For some reason, the voltage divider isn't working properly - the DVM shows 16.7V across the top resistor, and 7.3V across the other. I'm guessing this is some kind of series-parallel weirdness, but frankly after a whole day in labs i'm not able to figure this sort of thing out. Anyway, the end result of the circuit seems to be a steady -5 volt output on our oscilliscope, instead of reproducing the input voltage with from the signal generator with some cross-over distortion.
go back to the basics.

Ohms law tells you that you'll have 8.3 mA flowing in the top bias resistor and 3.65 mA in the bottom. so you have 4.65 mA going somewhere.

With respect to ground, the voltage at the bases of the transistors is -4.7 V
If the voltage at the output is -5 volts, the bases of the transistors are more positive than the emitters. So the only transistor that should be turned on at all is the top one. But this makes no sense if the circuit is as drawn. if the top transistor is conducting, the voltage at the emitter would be positive with respect to ground, not negative. Things are not adding up here.

you apparently have a serious problem with leakage in the bottom transistor. or perhaps you have the two transistors in the wrong places. or something is shored from base to emitter in the lower transistor. but something doesn't fit the drawing.

that's about all i can tell from the given. you'll want to measure voltage drop across the emitter resistors and from B to E of each transistor.
Meadows
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