First comment, as mentioned by frankrose in post #6, signal polarity is wrongly sketched in the drawing.
My question is, does the ac simulated gain loop indicate the gain (G) of the amplifier or the closed loop gain Gx=G/(1+G*A)?
" A" is the gain of the feedback.
There are some discrepancies in the terms and formulas used in the discussion.
In the above given formula, G is called open loop gain, A is feedback factor and Gx is closed loop gain. But the previously given expression A = Rx/Ry is not correct. Instead, Rx/Ry is the approximate Gx value for G*A >> 1. Feedback factor calculates correctly as
A = Ry/(Rx + Ry)
To derive the exact transfer function, it's useful to refer to an extended feedback scheme with an additional forward factor.
The extended scheme has been used in several previous threads, e.g.
https://www.edaboard.com/showthread.php?t=273725
For this scheme, you can derive
Acl=Aol*α/(1+Aol*β)
The used formula signs are
Acl closed loop gain
Aol open loop gain
α forward factor
β feedback factor
Aol*β is also designated loop gain (actually -Aol*β, because the negative sign constituting a negative feedback loop is not included in Aol or β in the schematic.
Referring to the differential amplifier circuit, you get
α = -Rx/(Rx+Ry)
β = Ry/(Rx + Ry)
You can derive the factors for a simpler single ended amplifier circuit
The approximate closed loop gain for sufficient large loop gain is
Acl ≈ -Rx/Ry (respectively -Rf/Rin)