Op-Amp Stability From Closed Loop Bode Plot

[flamingo2010]

Hi,

I'm using an op-amp, analog device AD8506 as a unity gain buffer. It will have to drive some capacitance load. From the AC analysis, how do I tell if the system is stable base on the bode plot of the closed loop system? I understand that if you look at the open loop plot and measure the PM at unity and if it's below -180, it's unstable. However, how do you tell stability from closed loop bode plot?

Thanks.

 

[FvM]

I'm not clear about your terminology of open versus closed loop. To determine the stability, we look at the loop gain. An unity gain phase of -180 degree means zero phase margin, or in other words "guaranteed" instability. A practical PM requirement would be 60 degree (no gain peaking) down to 45 degree.

Closed loop gain is the overall gain of the feedback amplifier. An instable amplifier would show an infinite gain magnitude peaking. Practically, we require no up to e.g. 3 dB gain peaking for a suitable amplifier.

 

[LvW]

Hi flamingo,
I like to add something to FvM's response.
He has mentioned an "infinite gain magnitude peaking" for the closed-loop gain of an amplifier that is unstable.
However, this is true only in case the phase margin is zero (case of oscillation).
For negative phase margins the magnitude plot "returns" to quasi-normal behaviour.
That means, from the ac magnitude plot you cannot see if the circuit is stable or not.
However, if you look at the phase response, you will observe a rising phase (positive slope).
This is always a typical indication for instability.