Someone asked me this question:

When we have a negative-feedback system:

We know; The system will be stable as long as the overall gain stays below 0dB (with some margin)when we reach -180° phase shift point.

a) What happens if the phase shift goes to +180°, for some reason, and the gain is more than 0dB?

b) What happens if the phase goes to more than +180° (say +200°, 250°) with more than 0dB gain?

I looked through my old books but can't find any example of a negative-feedback system having large positive phase shifts.

All the examples only deal with stability issues approaching the -180° point.

I hope some control expert can clarify this.

You just have to consider the whole phase shift around the loop of the feedback system. It answers all your questions about stability.

a) If the overall loop achieves a total of 360 degrees or more with > 0 dB gain, you will have instability

b) See "a"