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how does the resistor work in the OPAMP

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zhengchao201105

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I found something strange in this circuit below
the sine with DC will become "pure",how does this happen?
resistor_AC_circuit.pngresistor_AC_sim.png

if I remove this R4,the output will "up"
resistor_no_AC_circuit.pngresistor_no_AC_sim.png
so what is the function of R4?
 
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R4 provides a DC path.The small bias current of the opamp will charge the cap otherwise.
 

R4 is use to discharge the capacitor forcefully so that u will get voltage starts from zero otherwise u will get voltage level to be shifted because capacitor will be charge it will not be discharge fully.
 

For correct biasing both opamp inputs need a dc path to ground.
 

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