I am trying to figure out the stability of an amplifier, for which I have to calculate the loop gain and make sure its not negative for instability. In Cadence one can use 'stb' analysis to calculate loop gain. The loop gain and phase looks as follows
With respect to the phase of the loop gain starting at -180 degrees, this has to do with a sign convention adopted by cadence. Please check these links if you are more interested https://sites.google.com/site/frankwiedmann/loopgain https://community.cadence.com/cadence_technology_forums/f/custom-ic-design/30938/stb-analysis-phase-margin-gain-margin-functions/1338980#1338980
In this case the 0 dB gain is at roughly 9.2 MHz and the phase margin is negative 5 degrees, indicating unstable behavior. But if I try to calculate phase margin from direct plot form, I do not get a value of phase margin as shown below:
Can anyone throw light on this problem?