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However, in both cases it is necessary to inject the test signal BETWEEN both nodes of the opening because the DC operating point must not be disturbed (DC feedback must remain unchanged).

The importance of your last point cannot be overstated. It was one of the pieces I was missing.

 

The importance of your last point cannot be overstated. It was one of the pieces I was missing.

Supplementary comment: When such a node (loop opening with neglectable load change ) cannot be found , we must apply the complete procedure as proposed by Middlebrook , which means: In addition to the mentioned ac test voltage source, the loop must be excited also with an ac test current source. Then, both results must be combined using a specific formula.