Stability analysis Phase margin and loop gain

Dear Rakesh,

RakeshPRG said:

However it not getting added to my results list.

What could be a problem? Is there a way to add phase margin to the results using the calculator? 

Does your circuit have a valid phase margin? If not, the Direct Plot form will not show an "Add to outputs" radio button as shown below.

Is the phase margin already in your list of outputs (perhaps is it listed under a different output name?) If so, it will not be added a second time.

You might try to add it manually by adding the following output expression and give it a name such as "Phase Margin".

getData("phaseMargin" ?result "stb_margin")

RakeshPRG said:

I also find that the gain from the stability analysis(the closed loop gain) is different from that of the gain obtained for the closed loop simulation in AC analysis. Why is the difference, how is it computed in stability analysis?

The stability analysis does not determine the closed loop gain directly. It provides the open loop gain as that is the important parameter as a function of frequency to determine stability. You can certainly compute the closed loop gain - but the results displayed are the open loop gain. I believe the following link also addresses your question RakeshPRG:

https://support.cadence.com/apex/ArticleAttachmentPortal?id=a1Od0000000nSkpEAE&pageName=ArticleContent

Shawn

Unknown said:

It provides the open loop gain

Being pedantic, it's really giving the loop gain when the loop is closed. That is not the same as the gain from the input to output when the loop is closed though (and neither is it strictly the same as the open loop gain, because the loop is actually closed). So what do I mean by that? Well, it measures the gain from a point in the loop that you ask to probe at, all the way around the loop to the same point. This is what matters for stability.

There's no reason why this should or would be the same as the gain of the amplifier from input to output, and that's not important for stability anyway.

Shawn, sorry to be pedantic here. It's just that open loop gain implies that the loop is open when this is measured, which is not the case, and not what you want anyway, because it would mean that the output of the amplifier is not loaded by the feedback path. Other than my picky point, I completely agree with what you've said!

Andrew.

HI Andrew,

Andrew Beckett said:

Being pedantic, it's really giving the loop gain when the loop is closed.

I stand corrected - your comment is absolutely correct. I think my choice of wording was poor. I believe I fully understand the concept of the "loop gain" and often refer to it as "open loop gain". However, you are correct, my use of the "open loop gain" term is a misnomer as it expressed the gain around the loop when the loop is closed.

I hope I did not confuse RakeshPRG and appreciate your correction!

Shawn

HI Andrew,

Andrew Beckett said:

Being pedantic, it's really giving the loop gain when the loop is closed.

I stand corrected - your comment is absolutely correct. I think my choice of wording was poor. I believe I fully understand the concept of the "loop gain" and often refer to it as "open loop gain". However, you are correct, my use of the "open loop gain" term is a misnomer as it expressed the gain around the loop when the loop is closed.

I hope I did not confuse RakeshPRG and appreciate your correction!

Shawn

Thank you Shawn and Andrew for those insights. I am able to plot the loop gain and phase together. Besides your suggestion on the function using getdata has become helpful in plotting Phase margin Vs feedback capacitance.

-Rakesh.