Negative feedback - Open loop gain question

[okami11408]

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.

http://forum.allaboutcircuits.com/cache.php?url=http://img821.imageshack.us/img821/1626/60542118.jpg

 

[Jony130]

We need to include this resistors into account for one simple reason.
Those resistor load our amplifier. So the Open loop gain will change.

And to change circuit(a) into circuit(b) we simply need to find the resistance seen from emitter Q1 terminal into the ground. And the resistance seen from Q3 emitter into the GND.

 

[okami11408]

We need to include this resistors into account for one simple reason.
Those resistor load our amplifier. So the Open loop gain will change.

And to change circuit(a) into circuit(b) we simply need to find the resistance seen from emitter Q1 terminal into the ground. And the resistance seen from Q3 emitter into the GND.

Now I got it. Those resistor load our amplifier too.

The reason why I confuse is I think "open loop" maybe something like we take the feedback circuit out of the circuit.

Thank you so much