Where does an op amp's input bias current come from if capacitor is used?
1) In the case that you do not have R1 in place you would expect the + input node to be floating. But that is not the whole story. All the pins of opamps and in fact every chip have ESD protection diodes between the pins and the supplies. So if you apply a signal at the left of C1 there is a way to charge and discharge C1 if the input signal goes below or above the supply voltage. Also leakage currents can raise or lower the voltage of the input.


The resistor should not allow the DC bias current through* Actually it should and it must. You found that R1 is needed, even though the DC input bias current can be extremely small, the resistor is still needed to provide a path for it.


2) The - input's input bias current flows from the output of the opamp through R3. If C2 was not there that current could also flow through R2, or it could flow partly through R2 with the rest through R3, it depends on the DC voltages at each point how the current flows.

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