puzzled with the negative feedback system

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In the negative feedback system ,why should we consider the feedback
block as an isolate
block to the amplifying block? Should't we consider about the input
resistance of the the
amplifying block? If the input resistance of the amplifying block is
considered, there will
be a complexed problem in the transistor circuit. What should we do
while we are analysising
the analog negative feedback system?
Thanks in advance!

 

[Noway2]

You are hitting on the concept of "loading" and it can effect real
world circuits in ways not accounted for in situations like you
encounter in your school work. In the real world, there are ways to
avoid this issue, such as using amplifiers with high input impedance
and low output impedance to make the loading effect between the blocks
negligble. If you haven't already, you will probably implement
feedback circuits using opamps, which have these properties.

At first, however, these issues are ignored so that the student can
concentrate on grasping the concept of feedback, which can be less than
intuitive. Once these basics are undestood, then you can go back and
start to enrich the model.

 

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Thank you very much!