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FAQ-OPAM-0008
- Q:
- Is it possible to calculate differential input resistance?
- A:
- The op. amp (input element = bipolar transistor)'s value of differential input resistance [Rin] can be written by using the formulas below, based on physical characteristics of the PN junction.
Formula1-1- The formula below can be used at room temperature.
Formula1-2 - The formula above can be applied to for op. amp situations where all input bias currents go though one base of a bipolar transistor, such as NJM4558, NJM2904, NJM2100, NJM5532etc.
Even though the input element is a bipolar transistor, it's not possible to apply the formula above for NJM2137 series and NJM2730 series that have PNP/NPN parallel-connected transistors, or for NJMOP-07 that is equipped with bias compensation circuit.
NJM06x, 07x, 08x, NJM216x, and NJM2082 all feature junction FET as an input element, so the formula above is not applicable.The same not applies to NJU70xx series, which are C-MOS type. - The formula above can be different from the actual value since the formula is the theoretical value. This formula is especially designed for small signal operation, so sometimes it is not suitable for large amplitude operation and comparator operation with ON/OFF operation by input element.
The differential input resistance is equivalent to DC resistance between + input terminal ? input terminal in op. amp. It is different from input impedance of linear amp using an op. amp. Generally, in linear amp operation, input resistance appears to be much higher than that of Rin, because the Rin becomes bootstrapped by negative feedback.(Fig8_3.)
Fig8_3.Example of input resistance of voltage follower.
