newic wrote on Dec 12th, 2014, 11:16pm:Hi Sheldon,
Does it mean that I cannot rely on the stb phase margin report right?
the STB result a +ve phase margin but when i look at the phase response, it starts at 0degree at 1Hz. it should be unstable right.
In most of the corners, it starts at +180o at 1Hz, but in some cases, it starts in 0 degree.
Let me state the following:
1.) For stable feedback circuits the phase of the loop gain always must start at
-180deg . This is necessary for negative dc feedback (stable operational point).
2.) In this case, the gain margin is to be determined at a frequency where the loop phase reaches -360deg (0 deg).
3.) Sometimes, not the loop gain is analyzed but the product of all units within the loop only (without the minus sign at the summing node).
This product is, of course, identical to the negative of the loop gain. In this case, the phase starts at 0 deg and stability is checked at the -180deg point.