how to measure phase margin

[So.nia]

hi all;

i need some help form the definition of phase margin . it is the difference between the 180° and the phase at -3 dB or some thing else????


regards

 

[Hyao89]

It should be the phase+180deg @cross freq (unity gain frequency, UGF). Say if you have got -120deg at the UGF, then your PM is 180+(-120)=60. This is a spec of open loop condition. Thank you!

 

[LvW]

The phase margin (PM) is detrmined based on the loop gain of a circuit with feedback.
The loop gain LG is the gain that can be measured/simulated by opening the loop and injecting a test signal into the loop.
Now the PM is detrmined at the point where the LG is 0 dB. If the phase at this frequency is -360 deg. the PM is zero.
Thus the PM is the distance to -360 deg. (Example: -320 deg gives a PM=40 deg).

The above mentioned threshold of -180 deg (post of so.nia) is used for theoretical calculations only and does NOT include the phase shift of 180 deg caused by the negative feedback action of the opamp.
That means: Only the phase contributions of the various components are considered (amplifier and feedback elements) WITHOUT the negative sign of feedback.
(Hint: Special care is to be taken by opening the loop because the dc operating point may be lost and/or the load condition are altered; therefore, some specific methods for loop gain determination exist).

 

[So.nia]

LvW do you say , the system is stable enven if > -360° !!!???

 

[LvW]

LvW do you say , the system is stable enven if > -360° !!!???

.......enven if > -360°

What does this mean?
The loop phase will certainly go through -360 deg.
And it is important if the loop gain at the corresponding frequency will be above or below 0 dB

 

[diarmuid]

see Razavi's book for a good explanation.