How to improve nasty form clipping behavior in differential input stage
My theory (although I wasn't able to get the simulation running properly to verify) is that when the differential input voltage rises too high, the current mirror load on the differential pair (Q4 and Q5) cuts off altogether, and it won't restart until the input voltage drops below the feedback voltage.

It would be interesting to see whether putting a resistor between the collectors of Q1 and Q2 would provide a path to keep the mirror running. This would also have the effect of reducing the gain of that stage somewhat.



[Kaz] Dave Tweed's hypothesis is right. However, instead of using a resistor between the collectors of Q1, I put in diodes, which only become active when they have to and do not affect the circuit very much when the closed loop is functional.

The modification to the circuit looks like this:







simulate this circuit &ndash; Schematic created using CircuitLab

The choice of three diodes is empirical, based on observing the voltage difference between those collectors, which is about a little over a volt. Two diodes are too close to the margin and "leak", causing a gaping DC offset in the output of about five volts. Three diodes have no noticeable effect on the non-clipped test case.

However, here are simulation results from the clipped test case, including a new plot showing difference between Q2 and Q1 collector voltages:





Looks great! A a little assymetric, which is fine.



Out of curiosity, I tried replacing the diodes with a BJT-based servo:







simulate this circuit

The diode D10 prevents reverse base-emitter breakdown. There seems to be no real advantage to this. The part count is greater and the clipping seems to show a more pronounced "meniscus", though some of that perception could be the difference in vertical scales between the graphs. Perhaps that can be fine-tuned with the resistor values, though.