About phase margin of inverting amplifiers
Quote:
Phase margin is usually defined as how far is the phase of the loop gain from -180:
PM=∠(loop gain)+180∘
I would not agree to this definition. The LOOP GAIN is the gain of the complete loop when it is opened at a suitable point. Hence, it contains also the summing junction and the corresponding sign (negative or positive feedback).
Therefore, the definition does not make a difference between positive and negative feedback and the phase margin is simply the &quot;distance&quot; to the point of instability (oscillation condition):
When the magnitude of the loop gain is unity, the phase shift of the loop gain  must not yet have reached the critical value of -360 deg (and the difference to 360 deg is the margin PM).
Comment: The problem with the quoted (simplified) definition is the following:
There are many circuits with negative feedback where the minus sign is NOT introduced at the summing junction but anywhere within the loop. Some other circuits have negative and positive feedback pathes at the same time. In all these cases, it is necessary to use the 360deg criterion.