[moved] What is the input resistance of Opamp

[Bjtpower]

What is the Input resistance of Opamp?

i am not asking input Impedance i am looking for the Input Resistance of opamp.

What is role of that?

 

[wdmsystem]

The input resistance is defined as the ratio of the input voltage to the input current. The input voltage is Vin and the input current is simply the differential input current Id. Thus Rin is equal to Vin/Id and is very large because of the vanishingly small value of Id.

 

[betwixt]

There is no real answer because it depends on the internal circuit of the op-amp but wdmsystem's formula is correct.

On most op-amps, the signal input pins are connected directly to the base or gate of a transistor so there is likely to be a non-linear realtionship between input voltage and current, this means the 'resistance' could be different depending on how you measure it. On FET input op-amps, the resistance will typically be very high, in the tens or hundreds of million Ohms region but on some bipolar input amplifiers it can be quite low, only a few K Ohms.

Brian.

 

[Audioguru]

The datasheet for a National Semi LM741 opamp says its typical input resistance (not impedance) is 2M ohms.
Philips says the input resistance of a uA741 is also typically 2M ohms.

 

[Vbase]

The input doesn't behave like a resistor at all. It is accurate enough to assume that input resistance is infinite. Input resistance is irrelevant in using op amp, the input bias current is used instead.

 

[Audioguru]

The input resistance of an opamp is important if you use a very high impedance piezo vibration sensor that has its low frequency spec's with loads of 5M, 10M and 1G ohms.
At the very low frequencies of vibrations then the resistance is the same as the impedance.

 

[FvM]

The input doesn't behave like a resistor at all. It is accurate enough to assume that input resistance is infinite. Input resistance is irrelevant in using op amp, the input bias current is used instead.

A complete OP equivalent circuit involves both, input current sources and input impedance (resistance || capacitance). You'll find it in many data sheet specifications.

At least for bipolar input stages, you'll have separate common mode and differential impedance elements because differential input resistance of a differential pair is considerably lower than common mode.