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.