Ken Kundert wrote on Nov 8th, 2012, 9:23pm:Buddypoor,
I don't know what you are referring to when you say "the correct one". Does it involve 1F capacitors?
-Ken
OK agreed, I should have been more specific.
As mentioned somewhat later in my former posting - I consider a loop gain simulation as "correct" under the following pre-conditions:
*restauration of the correct dc bias point
*proper loading at the breakpoint (assuming uni-directional signal propagation).
I think, these conditions are fulfilled by Middlebrook´s double-injection technique.
However, for many opamp applications I have learned that an opening at the opamp output and injection of a series ac signal (without correction using a current source in an second step) gives results, which are exact enough.
Let me define "exact enough":
Knowing that
* each amplifier simulation model never can reflect real conditions
* in reality each passive part has tolerances
* each hardware realization suffers from parasitic influences
I think, the procedure to find the loop gain for a circuit with feedback may allow for a small systematic error - if this error remains within the limits set by the aforementioned uncertainties (other error sources).
As an engineer I follow the rule: A design as well as a measurement procedure must not be as exact as possible but as exact as necessary
(but I know that in some cases it is not easy to specify what really is "necessary").
I hope now you understand why I ask for justification of your statement "ac analysis approach ...should never be used".
Thank you and regards.
B.