It is the difference between the current flowing from inverting and non-inverting currents of an op-amp.

             Ios = IB+ – IB 
With bias current compensation the offset current will produce an output voltage when the input voltage is zero

We have,

          V1 =IB+ Rcomp 

           And   I1 = V1/R1

KCL at node ‘a’ gives

           I2 = (IB – I1)

              = IB[IB+ (Rcomp/R1)]

Also,

                V0 = I2 Rf – V1

          V0 = I2 Rf – IB+ Rcomp 

            = [IB– IB+Rcomp]Rf – IB+Rcomp

We know, Rcomp = R1Rf / R1+Rf

Substituting Rcomp, we have,

        V0 = Rf [IB– IB+]

The effect of offset current can be minimized by keeping feedback resistance value be small

 

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