I would like to add two remarks here:
What is generally known as Middlebrook's method for measuring loop gain is not the method described in his GFT manual but the method presented in his much earlier article "Measurement of Loop Gain in Feedback Systems", Intl. J. of Electronics, 38, 485—512, Apr. 1975. This method neglects backward transmission.
The definition for loop gain given by Middlebrook in his GFT manual is slightly different from the definition given in the article mentioned by Ken (
http://www.thekunderts.net/ken/docs/c%26d2001-01.pdf) which is used for Spectre's stb analysis. In terms of equation (20) of this article, the expression for Middlebrook's definition of loop gain would be T=
k1/(
k3+
Ye+
Yf). The advantage of Middlebrook's definition is that it is more closely related to the closed-loop gain of the circuit. Its disadvantage is that it is not symmetrical, the orientation of the probe
does matter in his case. In most practical cases, the difference between both definitions will be negligible (if the probe is oriented correctly for Middlebrook's definition). Both definitions of loop gain will always tell you correctly if a circuit is stable or not, and there will generally be only very minor differences in calculated phase margin and gain margin.