microe wrote on Nov 7th, 2014, 6:39pm:Thanks for the reply.
It is the same reason as we connect amplifier in feedback when we do stb analysis: 1) we want it DC biased correctly 2) we want include the feedback loading
OK - that`s what I have expected. However, for analyzing under open-loop conditioins you must open the loop (ac wise!) - otherwise, you have no open-loop conditions. On the other hand - I know what you means: You want to restore the dc operational point as well as the load properties.
To achive this, there are several methods - depending on the particular conditions.
For example, if you can neglect load changes (opamp output) the simplest method is to place an ac test source BETWEEN the low-resistive output and the feedback path.
As another simple method, you can use a very large inductor for ac opening the loop - and inject the test sighnal using a very large capacitor.
For a 100% correct method you must use the method as described by Middlebrook (open-loop analysis with double injection).
As far as I know, the STB analysis also makes use of Middlebrooks method.