Help me about opamp amplifier ?

[Nhan95]

In this inverting amplifier opamp, Why do they add the resistor R3 = R1||R2 to the non-inverting input although It doesn't affect to the result of Vout = R2/R1 * Vin. Thanks 

[rs20]

Explained here:



TLDR: Because an actual, real-life op-amp has a small amount of current flowing at the inputs, which tends to be balanced between them. Having equal impedances at the input means these input currents cause an equal voltage drop, and so the effect on the output is nil.

[w2aew]

In this inverting amplifier opamp, Why do they add the resistor R3 = R1||R2 to the non-inverting input although It doesn't affect to the result of Vout = R2/R1 * Vin. Thanks 

To provide fundamental cancellation of the input offset caused by the op amp's small input bias currents.