how Rout(closed) affects AVopen in V-V feedback

[joyes]

Dear all,

I have this problem for long:

As we all know, Rout(closed) will decrease in Voltage-Voltage feedback.

The question is :?: if Av of amplifier(excluding feedback circuits) decreases too and why.

If you think Av equals to Av(closed) decreased by loop gain, I have the other question
:?:that why I can still use the Av(open) in the closed loop computing.

Thanks.

 

[LvW]

Dear all,
I have this problem for long:
As we all know, Rout(closed) will decrease in Voltage-Voltage feedback.
The question is :?: if Av of amplifier(excluding feedback circuits) decreases too and why.
If you think Av equal to Av(closed) decreased by loop gain, I have the other question
:?:that why I can still use the Av(open) in the closed loop computing.
Thanks.

I am not sure if I got your question, however I try an answer:

1.) The gain of the opamp without any feedback (Av) decreases with rising frequencies because each amplifier gain goes to zero for infinite frequencies (due to capacitive and delay effects)
2.) Therefore, all formulas which assume Av=infinite are valid only in a limited frequency range (in which this presumption holds approximately).

Was this your question ?

 

[joyes]

Hi, LvM

I'm sorry I didn't depict clear enough.

And I have a figure of feedback quoted below.

My question is

:?: How to calculate the dc gain Av(closed) of the OpAmp(excluding the feedback) in the figure?

(1) If Av(closed)=Av(open), it is self-contradictory with that OpAmp's Rout decreases in voltage-voltage feedback.

(2) If Av(closed)=A, in which A is magnification of the whole circuits, why we can use the Av(open) in the calculation of magnification?