How to analyze the stability of an opamp using return ratio?

[lhlbluesky]

how to analysis the stability of opamp using return ratio?
or other methods?

what's the relationship of GBW for open-looped and closed-looped?

how to decide the PM for large closed gain of opamp(for example,4000)?

please help mw,thanks !

 

[naalald]

Re: retunn ratio

Hi,
For stability analysis you should examine the amplitude and phase of the Aβ, where A is the open loop transfer function and β is the feedback factor. According to control theory, for a feedback system to be stable, the gain and phase margin should be greater than zero. But in electronic circuits, generally and for good settling behavior, the phase margin should be greater than 45°.
A single pole opamp is inherently stable (pm > 90°). For the opamps with two or more poles which are modeled with two poles, the UGF (Unity Gain Frequency) of Aβ should be less than the second pole to insure the stability.
In practice a pm of 60° - 70° generally has the best behavior and still not over-designed.
More information could be achieved on Gray book and Johns book.

 

[lhlbluesky]

retunn ratio

thanks a lot!

but any other solutions?

 

[zyyang]

retunn ratio

In fact, the raturn ratio method is adaptive for the feedback circuits which are very difficult to identify the feedforward and the feedback network. By the raturn ratio method you only use the formular shown in Gray's book. So the method may be not suit to understand the feedback circuit in physical.
regards!

 

[steve_guo]

retunn ratio

Return Ration and Two-port network are so different in basic theory although they look same sometime.
You can check a series paper written by Paul J. Hurst. where you can find sufficient material about them