Influence of the Input Resistance of Operational Amplifier

The influence of the input resistance can be determined with Kirchhoff’s law. By applying Kirchhoff’s law to the circuit in Figure 2-5, we can use the following equations:

Figure 2-5. Influence of the Input Resistance

If the open-loop gain is infinite, the differential input voltage will be zero and the value of input resistance (if it is not zero) will have no influence. Since VDI = VO/AVD, the following equations apply:

The previously listed equations indicate that the input resistance (unless it is small relative to R I) will have little or no effect on the ratio of output voltage to input voltage. Therefore, the closed-loop gain for typical applications is independent of the input resistance.