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phase margin in negative feedback

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wafi_zuhdi

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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?

thanks in advance
 

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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.

its the loop gain response. sorry, i dont quite understand what you mean by -180 deg at lower freq scale. could you explain a bit more? thanks btw
 
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its the loop gain response. sorry, i dont quite understand what you mean by -180 deg at lower freq scale. could you explain a bit more? thanks btw

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.
 

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