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Is there anyone can help me understand the negative feedback

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zhenyuanwu

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In the following pictures,what is the value of the negative feedback voltage of the output voltage?
It says that "Vref=Vo*Re1/(Re1+Rf)" in the text book,why is that?
Should I consider about the output resistance of the emitter pole of the first transistor?I think the Re1 should be replaced by Re1//(rbe/(β+1)) in the small signal model,and there also should be an equivalent negative voltage that equals to be vi*rbe/(Re1*(β+1)).
I think there must be something wrong with my comprehension, who can point it for me?
Thank you very much in advance!
 

Re: who can help me think about the negative feedback ?

This is a series-feedback shunt-sensing feedback network. To find the feedback factor (Vref/Vout), because of the series-feedback, consider the Vref node is isolated from the amplifer input. As a result, it becomes a simple voltage divider.
 

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    zhenyuanwu

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But I still don't understand, they will affect each other . The Vf will be changed as soon as connected with the negative feedback node. Why do you "consider the Vref node is isolated from the amplifer input".I am so puzzled! Who can help me!
Thanks in advance!
 

Check the book you are reading. The aurthor must have included the feedback resistance loading into the forward gain calculations, so since Acl is different from Aol, the feedback factor B is Re1/(Re1+Rf)
 

assume the gain of amp is large enough, you can get it
 

I have found the answer in another book written by Paul. Thank you very much!
 

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