Consider what is used as an opamp model (like a spice macromodel for example). There, a voltage dependent voltage source driving an RC (comp section) that drives a buffer output stage. In this model, no 180 inversion exists and so phase shift is at most 90 degrees. Am I correct to say that this model is no good for evaluating stability?
Have you seen what I'm talking about?
Well, any macromodel will have inverting (180º) and noninverting inputs, but if there is only one pole, it should be unconditionally stable unless you add external poles, or if the combination of the feedback resistor and input capacitance is excessive.
Now in a real opamp.. lets say something mundane such as a 741. The 180 degrees of inversion is due to the input stage differential pair with collector output (common emitter config?) is this right? Because the following stage incorporates the comp-cap and output is just something similar to an emitter follower where only Ft phase shift takes place.
I suppose Ft phase shift takes place in _ALL_ transistors but I'm looking to understand where the dominant areas are. Let's not consider the ones that are "way out there."
I think the 741, even though it has 2 gain stages and a follower output stage, has one dominant pole whose corner frequency is set by the product of the output impedance of the 1st stage (many meghoms) and the 30pF feedback capacitor of the 2nd stage. This keeps the phase shift at 90 degrees pretty much throughout the useful bandwidth of the part, making it very stable in the absence of lots of additional phase shift in the feedback loop.
Gain >= 1 and n*360 degrees where n is positive integer >= 1? Intuitively, it seems that this too would satisfy the condition for oscillation (instability). From the feedback point of view, the circuit does not know about time delay really, 360 degrees, 720 degrees, 1440 degrees should all look the same from this standpoint. Can this more general statement be made? I have not seen mention of this in the literature.
Well, time delay=phase/frequency. The higher the phase shift at a given frequency, the higher the delay. I don't know what happens if you still have loop gain at integer multiples of 360º phase shift.
Optikon, I'm not a font of knowledge on this subject. I've just been doing analog design for 40 years, and many years ago (before 741s), I designed and compensated a few discrete transistor op amps. I've forgotten some of the stuff I knew then, and have certainly learned a lot of stuff that I didn't know then.