# Clipping Class-AB Output

This assignment we have been given has been causing me major headaches. I feel like I'm finally close to getting this amp to work after hours of fiddling around, but now I've found the output is clipping at a very low voltage. It must have a differential input and a class AB output. Minimum of 1500 voltage gain. Stability and wasted power aren't a concern, as long as I can write up where power is lost.

I'm sure there's enough base current, and don't think there's any loading, but there is no voltage swing available at the output.

What is causing this clipping?

I'll buy a pint for anyone who can get this thing to work. I want to understand where I'm going wrong, I just can't work it out for the life of me.

Thanks for looking!

EDIT:

Scope with DC coupling shows the following waveforms. The left hand one is the base of Q11, and the right hand side one is the load output (before cap). It seems like together they make a full sine wave.. What is going on???

Edit 2: Here are both waveforms on the same scope: red output, blue input to Q11.

• What you need to learn is a systematic approach to solve issues like this. Follow the signal through the amplifier to find the point / part of the circuit where the distortion occurs. I'm suspicious of the stage around Q9 as you have a large emitter cap to ground there so the gain is not well defined. That makes this stage very sensitive to almost anything. Dec 27, 2017 at 16:42
• Probe the collector current of Q3. Is this what you expect?
– τεκ
Dec 27, 2017 at 16:48
• I've tried a systematic approach, but the problem is that I simply don't know how the stages affect each other. We were just told to try to combine these elements, and it's ending up being basically trial and error, and very unprofessional for everyone. It's due in a couple of weeks and I'm losing patience with it. The value of C3 was supposed to be 7.9u as calculated for 20Hz shoulder frequency, that's my mistake. The collector current at Q3 is about 1.5mA, which I set with R1, I think this is OK? The output of the common emitter is fine, so it's the output stage that must be wrong.. Dec 27, 2017 at 16:55
• Your design may actually meet the design specifications. It seems linear, at least over a very small range before clipping sets in. From here, you must only justify why the linear output swing is so small. When you probe with the 'scope, try DC-coupling rather than AC-coupling. Dec 27, 2017 at 17:06
• @Yossarian Glad to hear things are working now and I won't add any of my own thoughts to that. But I probably would have approached this with Darlington-arranged long-tailed pair, used a current mirror in the collector legs, and skipped the $Q_9$ stage entirely, instead driving $Q_{11}$ directly from the difference current that results. Might have added NFB, if there was some gain to spare for it. (Assuming this was for simulation and not for fabrication on a board with discrete parts.)
– jonk
Dec 27, 2017 at 20:34