where "Z"dif is the op-amp's input impedance to differential signals, and "A"OL is the open-loop voltage gain of the op-amp (which varies with frequency), and "B" is the feedback factor (the fraction of the output signal that returns to the input). In the case of the ideal op-amp, with "A"OL infinite and "Z"dif infinite, the input impedance is also infinite. In this case, though, the circuit will be susceptible to input bias current drift because of the mismatch between the impedances driving the "V"+ and "V"− op-amp inputs.