What is the best phase margin for a loop?

Discussion in 'General Electronics Chat' started by anhnha, Jun 12, 2014.

1. anhnha Thread Starter Active Member

Apr 19, 2012
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What is the best phase margin for a loop?
I read somewhere that 90 degree is the ideal phase margin and 60 degree is practical. Could you explain why?
Can phase margin be larger than 90 degrees?

2. ErnieM AAC Fanatic!

Apr 24, 2011
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It involves negative feedback and also depends on what system you are dealing with.

Any feedback system where the feedback is in phase (zero or 360 degrees) will oscillate. The closer you get to 360 the closer you get to outright oscillation or a tendency to oscillate but then damp out.

With negative feedback you've already used us 1/2 of that limit, or 180 degrees. If there are no reactive components (i.e., just resistors and no capacitances or inductances) you should achieve very close to 180 degrees (as there are always some C or L about).

One example is most switching power supplies: a basic supply is inherantly unstable due to the LC filter on the output. Thus one shapes the feedback to get zero gain with some phase margin at a lower frequency the that LC breaks (and other methods may also be applicable).

Anything more definiative will depend on someone else who knows the math better then I've forgotten.

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3. ramancini8 Member

Jul 18, 2012
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Phase margin and speed are inversely related in op amps. A phase margin of 90 degrees is unconditionally stable, but the op amp circuit has very poor dynamic characteristics. Smaller phase margins (like 45 degrees) yield faster circuits, but the output will overshoot with a step input. Lower phase margin equals more overshoot. The circuit requirements dictate the phase margin required. Oscillation starts at 0 degrees PM while overshoot starts at less than 90 degrees PM.

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4. anhnha Thread Starter Active Member

Apr 19, 2012
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Thank you, ErnieM and ramancini.

So, if I build two LDOs (Low Dropout Linear Regulator), one for analog and the other for digital uses.
The one used in digital circuit usually have less phase margin (faster) than the one used for analog circuit?