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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?

enter image description here

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???

enter image description here

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

enter image description here

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    \$\begingroup\$ 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. \$\endgroup\$ – Bimpelrekkie Dec 27 '17 at 16:42
  • \$\begingroup\$ Probe the collector current of Q3. Is this what you expect? \$\endgroup\$ – τεκ Dec 27 '17 at 16:48
  • \$\begingroup\$ 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.. \$\endgroup\$ – Yossarian Dec 27 '17 at 16:55
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    \$\begingroup\$ 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. \$\endgroup\$ – glen_geek Dec 27 '17 at 17:06
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    \$\begingroup\$ @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.) \$\endgroup\$ – jonk Dec 27 '17 at 20:34
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Q11 biasing

AC input is a sine wave, which has a positive swing and a negative swing, and goes through zero twice each cycle. For this zero point, biasing of Q11 seems nearly acceptable.
Your problem arises when Q11's base voltage rises toward its positive peak. Q11's collector current increases: voltage drop across R8 increases. This would cause Q11's collector voltage to become lower.
So you have the situation where Q11 base voltage is rising, and Q11 collector voltage is falling. They meet, and Q11's collector cannot pull any lower.

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    \$\begingroup\$ Thanks Glen, I've biased Q11 with a lower DC voltage at its base and the clipping has gone. Problem solved! \$\endgroup\$ – Yossarian Dec 27 '17 at 18:58
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DC coupling between Q9 and Q11 means that the Q9 DC parameters and bias point afect the operating range of Q11. As the scope shots show, the output stage is doing what it is told by Q9's collector. Q9 is an inverting stage, so when the base gets a large negative signal peak, Q9 turns off. At this point, only R3 can pull the collector high, and the scope shows that it is not.

With Q9 off, R3, R10, and Q1's base-emitter junction form a series string. Using Ohm;s Law, the max base current is less than 2 mA. Seems a bit light to me. If you pull Q9 from the circuit, does the output stage saturate high?

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    \$\begingroup\$ That's exactly what was happening. I put a cap between Q9 and Q11 and biased Q11 a bit lower. \$\endgroup\$ – Yossarian Dec 27 '17 at 19:00

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