Hi every one,
I've just read Ken's book on spice&spetre about ac simulation on feedback circuit. (section 3.4.1)
In his book, he mentioned that the most accurate way to characterize the real feedback circuit is to use two-port parameter(Y,Z,G,H), because this method naturally include all the feedback and feedforward(bi-directional) effect and loading effect into a single two-port presentation even you don't need to open your feedback loop. Once you get parameter decideded, you get all feedback infomation(open loop gain, closed loop gain).
I'm a little curious that what's the practical disadvantage of two-port parameter method, which 2 ports in real circuits should be used to extract the parameter? Which two port parameter should be choosen for a specific circuit?  
Furthermore, Middlebrook method seems to be a little bit more popular than above method, is it right?
Now I'm going to analysis the loop stability of a bandgap (circuit is seem as Khong-Meng Tham's paper, JSSC 1995 May VOL.30 NO.5 "A low Supply Voltage High PSRR Voltage Reference in CMOS Process").
I've to choose one method to characterize the feedback build-in the bandgap. Which method should I choose?
Anyone please give your opinion. thanks very much!