I have some questions regarding Middlebrook's General Feedback Theorem
From
https://sites.google.com/site/frankwiedmann/loopgain/GFT_LTspice_XVII.zip ,
1) May I know the exact purpose of each of the three simulation performed ?
Simulation #1 with vz = 1 (active), ui = 0 (not active) and iz = 0 (not active).
Simulation #2 with ui = 1 (active), vz = 0 (not active) and iz = 0 (not active).
Simulation #3 with iz = 1 (active), ui = 0 (not active) and vz = 0 (not active).
2) Why "It does not matter if ui is a voltage or a current generator." ?