Phase margin only makes sense with respect to the loop gain of the circuit (and so indeed is only one parameter). For simple circuits like your example, the loop gain can be simulated by physically opening the loop. In general, however, it will be difficult to apply the correct operating points and impedances at the opening, so that the methods developed by Middlebrook or Tian are recommended. If the voltage and current sources inserted by these methods correspond to an "opening" of the loop is probably more of a philosophical question.

Part II

Hi Sheldon,

thank you for the simulation results you have sent.
However, they contain not really some new information for me.
Therefore, I have problems to consider these graphs as an answer to my extensively described opinion.  
Moreover, I am a bit confused cause I don't know what to do or how to comment the graphs.
For my opinion, the best way to discuss a technical matter is to ask and to answer questions.
Therefore, what is your comment to my last posting and the attachement? Anything wrong?

By the way, may I direct your attention to Tian`s & Kundert's paper (that, more or less is an extension of Middlebrook's method)?
It contains several times the phrase "broken loop".
Examples:
page 31, right columne, line 5
page 33, right columne, line 5 to 8 in "Null Double-Injection Technique"
page 34, right columne, line 7 below eq. (13) and line 8 above eq. (14)
_______________
Kind regards
Lutz

Buddypoor,

  Guess I need to invoke Occam's Razor

  Your "open loop" testbench is basically the closed loop testbench with
the loop broken. It implicitly assumes that the loop is closed since the
input voltage source is tuned to force the amplifier output to the
appropriate voltage. Both testbenches give the same results, see the
attachment. As implemented this testbench can not be used for corner
analysis, parametric sweeps, Monte Carlo analysis, ... because the
input voltage needs to be manually tuned. The good news is that both
testbenches produce the same results.


                                                          Best Regards,

                                                            Sheldon

Hi Sheldon,

Thanks for the reply, however I didn't get everything, as for example

Your "open loop" testbench is basically the closed loop testbench with
the loop broken.


Maybe, the reason is my limited knowledge of your language.

It implicitly assumes that the loop is closed since the
input voltage source is tuned to force the amplifier output to the
appropriate voltage., ...
.................
because the
input voltage needs to be manually tuned.

No, there is no need to tune any voltage manually. Both schemes ( I didn't invite them!) do not require any "tuning". But - I am aware that the second "test bench" in my former pdf attachement does not work properly for real opamps. It was used only to demonstrate that the first scheme (simplified Middlebrook) produces exact the same input/output voltages resp. the same ratio as the "classical and robust" open-loop method.  
Anyway, thanks for the discussion.

Remember the original question if there are two phase margins (open resp. closed loop): I think, at least this question has been clarified.  
It would be interesting to hear from the forum member who has posted the question if he is satisfied now.
Thank you and regards
Lutz