Gain and Phase margin in loop stability.

Thread Starter

mishra87

Joined Jan 17, 2016
536
Could anybody explain me what is the meaning of these terminology in loop stability. Why it does require in feedback loop satabilty.

I want to know pratical defintion of it . Why that Margin needed in loop stability .

I don't want this explaination like where gain curve crosses 0db and phase curve 180deg line that margin is phase and gain margin respectively. I know this.

Thank you .
 

danadak

Joined Mar 10, 2018
3,573
The practical issue is a gain stage where part of its output is
fed back to its input, either intentionally, such as in common
opamp circuits, and unintentiually due to parasitics. If we feed
back to input a signal that is in phase with output, then because of
gain, the signal will grow without bounds until supply rails for the
amplifying component allow no further excursions. Often result
will be an oscillator which unwanted if what you wanted was to
amplify the signal.

Phase margin is when there is no gain left, eg. G = 1, what is the phase thru
the loop at that point. If 0 then oscillator, no longer linear amplifier. Typical
designs try to shoot for 45 degrees or more margin to accommodate component
tolerances.

Google "phase margin opamp", tons of reference material out there.

Regards, Dana.
 

crutschow

Joined Mar 14, 2008
23,312
In summary, you need gain and phase margin in a negative feedback loop to avoid oscillations, and minimize overshoot and ringing in the step function response.
 
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