Question on Phase Margin

Hi,

I have some question here regarding phase margin calculation in feedback loop. Before this in my circuit design, i always see that my phase start either at 180 or 0 degree, so that at loop gain at unity i always get phase margin from 60 to 100 degree. But for this time, one of my circuit in feedback showing the phase start at -180 degree and at loop gain at unity the phase margin showing 304 degree. So, in this case what is the phase margin actually? can the phase start at -180 degree? Please explain.

Thanks,
Anachip.

if phase margin greate 180, then you can use 360-phase margin.

-180 deg is same as 180 deg. It's 360 deg difference.

Anachip said:

Hi, I have some question here regarding phase margin calculation in feedback loop. Before this in my circuit design, i always see that my phase start either at 180 or 0 degree, so that at loop gain at unity i always get phase margin from 60 to 100 degree. But for this time, one of my circuit in feedback showing the phase start at -180 degree and at loop gain at unity the phase margin showing 304 degree. So, in this case what is the phase margin actually? can the phase start at -180 degree? Please explain.

Click to expand...

isaacnewton said:

-180 deg is same as 180 deg. It's 360 deg difference.

Anachip said:

Hi, I have some question here regarding phase margin calculation in feedback loop. Before this in my circuit design, i always see that my phase start either at 180 or 0 degree, so that at loop gain at unity i always get phase margin from 60 to 100 degree. But for this time, one of my circuit in feedback showing the phase start at -180 degree and at loop gain at unity the phase margin showing 304 degree. So, in this case what is the phase margin actually? can the phase start at -180 degree? Please explain.

Click to expand...

Click to expand...

So, in this case, is the phase margin is 304-180= 124 degree. Please explain

yes, 304-180.

try to draw a circle and analyze your previous case then apply on this case.