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!
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.
Thank you very much!
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