Tlaloc wrote on Sep 18^th, 2008, 12:58pm:

Regarding Eldo, I found http://www.edaboard.com/ftopic265486.html. It seems to use Middlebrook's old method from 1975 that was developed without taking into account backward transmission.

imd1 wrote on Sep 19^th, 2008, 5:43am:

I concluded this from one of the messages in http://www.edaboard.com/ftopic265486.html :
Code:

* .lstb This command improves the analysis of circuit stability.
* The .LSTB command measures the loop gain by successive injection (Middlebrook
* Technique). A zero voltage source is placed in series in the loop: the first pin of the voltage loop
* must be connected to the loop input, the other pin to the loop output. 

It mentions Middlebrook but not his General Feedback Theorem.

Quote:

The only simulator I know which has Tian's method directly built in is Spectre. In LTspice, I have implemented it by using some ideal sources together with a parameter sweep and an expression that calculates the loop gain from the simulation results. It should be possible to implement Tian's method in a similar way in most other Spice simulators.

The question regarding the correct loop gain is a bit more difficult to answer. From a usability standpoint, I like Tian's method best because of its symmetry. With respect to formal correctness, Middlebrook's General Feedback Theorem probably has the better arguments on its side. This question is somewhat academic, however, because for practical circuits, the results of all three methods discussed here are usually very close to each other. They also will all tell you correctly if the circuit is stable or not (see my webpage http://www.geocities.com/frank_wiedmann/loopgain.html for details).

I have no idea where the difference comes from. Spectre's stb analysis uses Tian's method and you should get exactly the same result if your circuit and your device models are the same. Maybe there is another error in your setup?

Hello, Frank

I think I kind of understand what you talked in your website and just try to draw a photo for verification. If the method in the photo is right, I can use the same way to simulate the common mode loop. All I have to do is put a probe in the common mode path.
Thanks.