hi, i'm working on a negative feedback system and getting confused on phase margin and gain margin. i've attached the AC response of the system.
Now i know that phase margin can be found by looking at when gain crosses 0db, gain margin is when phase is at 180. what happen if you have a negative gain overall, how do you look for the phase margin? does the system ever stable in this condition?
Your gain is not "negative". Rather, it is far below unity (app. -70 dB).
I suppose, something is wrong in your simulation arrangemnent.
Question: Does your graph show the loop gain response? In this case, the phase - in case of negative feedback - must be -180 deg at the lower frequency scale.
Question: Does your graph show the loop gain response? In this case, the phase - in case of negative feedback - must be -180 deg at the lower frequency scale.
In the (normal) case of negative feedback the loop gain phase is -180 deg. for low frequencies. Otherwise, it is positive feedback. Perhaps you did not show the loop gain, but the transfer function of all the loop components - without the negative sign at the input summing point ? In this case, the phase response is OK.