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need help! unit resistor change the close loop frequence response

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b12345

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In this circuit, IN is common voltage, and the differential signal input through IP
the res fix the coefficient of feedback, now, if i change the value of the res (but not change the proportion), the frequence response will change.
if the value increases, the BW becomes narrower and phase margin reduces, BUT THE FEEDBACK COEFFICIENT DO NOT CHANGE, how could the res impact the response??
THX for your reply :)
1.jpg
 

Probably because the input impedance of the IC is capacitive. In other words, there is some capacitance inside the chip between the inverting and non-inverting inputs.
 
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    FvM

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Okay, I have checked this, it is not the problem, anyway, thanks very much
 

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