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Design >> Analog Design >> Middlebrook's General Feedback Theorem
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Message started by blue111 on Jul 26^th, 2020, 4:24am

Title: Middlebrook's General Feedback Theorem
Post by blue111 on Jul 26^th, 2020, 4:24am

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." ?

Title: Re: Middlebrook's General Feedback Theorem
Post by Frank Wiedmann on Jul 27^th, 2020, 11:45am

The three simulations simulate the effects of each one of the three sources. As explained in https://zenodo.org/record/160231/files/GFT_SPICE.doc, by using the superposition principle, the relative amplitudes of these sources required in order to satisfy the nulling conditions of the GFT can be calculated. From these results, all the quantities of the GFT can then be determined.

The type of the input source does not have any effect on the behavior of the circuit. As you probably know, you can transform a current source into an equivalent voltage source and vice versa (see https://en.wikipedia.org/wiki/Norton%27s_theorem and https://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem).