# How do I calculate phase margin for a feedback amplifier

I'm learning analog design and can't seem to get a firm grasp on the concept of phase margin and wanted to ask a few questons:

• First Part: I understand that Phase margin is the difference between the phase at unity gain and 180

Question 1: If I have an inverting amplifier, my output phase starts at 180. When the phase remains 180 up to the 3dB frequency, is my phase margin now the distance from 0deg?

Question 2: Is there an equation to calculate the phase margin? What happens when I have negative phase shift, am I now comparing to -180deg?

• Second Part: I followed this video from Linear.com, and I used the following circuit with the LT1212 op-amp to run an open loop analysis: simulate this circuit – Schematic created using CircuitLab

Running this in LTSpice shows two different 0dB crossing in the bode plot (1MHz [55deg] and downward slope, 7MHz [-86deg] and upward slope). Am I right to assume this circuit would be unstable?