Opamp Stability confusion clarification wanted

[bababui]

Opamp Stability

Hello,
When looking at the frequency response of an opamp, I have learned that opamp is considered to be stable if at the frequency where it has unity gain, 180-phase=positive number. In my textbook, bode plot of an opamp has phase starting off at 0 degrees at low frequencies and shifting down. However, in the opamp that I built, phase starts at 180 degrees and starts going down, and it's total phase shift by frequency where it has unity gain is around 240degrees.
My question is, should the phase start off at 0? Am I correct to measure stability in terms of total phase shift?

Thanks

 

[ankitgarg0312]

output phase depends on which node u are applying the ac source..
if u specify (ac 1) at in+ node u will get a phase start from 0 at out put node, if u have ac 1 at IN- node u will get 180 at out node..

way i checks for stability is that output node should not get more than 120 phase shift from dc value..
e.g. if ur phase starts from 180 so it should not go beyond 300 so that i get a phase margin of 60 degrees..

 

[youstu]

yeah, it depends on which port of opamp you connected with the input signal.