Anyway, back to the technical stuff. Why do you say Vf/Vin does not represent the loop gain?
What is V
f? No, seriously; it's just been introduced out of thin air.
If V
f is, as a I said a naïve assumption would be, the voltage at the negative input, then V
f/V
in can't be the loop gain.
Let's take the classic feedback equation
V
out = V
in A / (1 + Aβ)
Where V
out and V
in are obvious, A is the open loop gain and β is the feedback fraction.
The loop gain (by definition) is Aβ, if V
f is as assumed above then V
f = βV
out. So (substituting)
V
f / V
in = βV
out / V
in ~= 1 and is not = Aβ.
The bloody SMF editor stole the keystrokes that should have produced a β and instead decided that I wanted to save a 1/4 written draft message - bastard!