Answer 1—The resistor makes the impedance of the two circuits feeding the two inputs of the op-amp equal. This is important because of the input bias current required by the op-amp. If the current were zero, the negative feedback of the overall circuit would make sure that the two inputs of the op-amp are extremely close to the same voltage. A nonzero input bias current introduces a voltage drop in the resistance connected to that input. As long as both inputs have the same current and impedance, this drop is the same for both inputs and cancels out. However, if the impedances are not equal, a voltage offset is introduced that contributes to the total error of the circuit.

Contributor: David Tweed

Published December 2003