This is my first post here.

I'm currently learning about Negative feedback.

Normally, When we want to know the open loop gain, we rewrite circuit(a) to circuit(b).

My question is why do RE1,RF,RE2 from circuit(a) turn into what we see circuit(b)?

what is a theory behind this?

In my opinion, if we want to find the openloop gain, why don't we just take the RF out.

I may misunderstand something, please help!

Here's the image.

 

Because each side of the resistor sees the resistors on the other side as part of the circuit it is a part of. A really poor analogy would be if you wanted to see what happens to a car if the driver isn't steering it; you don't remove the front wheels, because they are involved in more than just steering the car.