The input resistance of the operational amplifier is low and the input voltage seems to drop. How much ohms is the value of the input resistance of it?

Operational Amplifier

Nisshinbo Micro Devices Inc. does not describe the input impedance in datasheets of almost operational amplifiers because a general purpose operational amplifier without concern the input impedance on your design.

Generally, when the operational amplifier is operating in the negative feedback state, the input resistance is boot-strapped by the input voltage applied to the negative input terminal, it will be much higher when the input resistance of the operational amplifier is compared with the input resistance of the discrete transistor.

The input resistance is a value that can be ignored in practice except in special cases such as voltage hold circuits.
The drop in the hold voltage of the sample holding circuit and the error in the DC high-precision circuit must be greatly affected by the input bias current, then consider following. (e.g., it should generate an offset voltage for 1mV when 100nA bias current from input terminal flows to 10k ohm).

To reduce the effect of input bias current, we recommend to use J-FET input type Op Amp. If the input terminal of Op Amp stotructure has a bipolar technology, it can be calculated the input resistance with the bias current(Base current).
The details of the way to calculate describes to the way of calculate to the differential input resistance as refer.