Positive/negative bias current effect and reduction

Input bias currents are the dc base (or gate) currents of the input transistors. The current into the non-inverting input is called the positive input bias current IB⁺ (or IB2), and the current into the inverting input the negative input bias current IB⁻ (or IB1). (The currents flow into the op-amp for NPN input transistors and out of it for PNP inputs; only the sign of the error changes.)

                 Rf
          +----/\/\/----+
          |             |
 Vi --R1--+--> IB1 -(-) |
                     >--+---- Vo
     +-----> IB2 ---(+)
     |
   Rcomp
     |
    GND

Effect of IB⁻ (inverting input)

With Vi = 0 and Rcomp = 0, the inverting input is a virtual ground, so no current flows in R1 and IB⁻ must flow from the output through Rf:

Vo1 = +IB⁻ · Rf

Effect of IB⁺ (non-inverting input)

IB⁺ flows through Rcomp and makes V+ = −IB⁺·Rcomp. The circuit amplifies this as a non-inverting amplifier:

Vo2 = -IB⁺ · Rcomp · (1 + Rf/R1)

Total effect

By superposition:

Vo = IB⁻·Rf - IB⁺·Rcomp(1 + Rf/R1)

The two currents produce outputs of opposite polarity, so they can be made to cancel.

Methods of reduction

1. Compensating resistor: choose Rcomp(1 + Rf/R1) = Rf, i.e. Rcomp = R1‖Rf. Then

Vo = Rf(IB⁻ - IB⁺) = Rf·Ios

which is much smaller since Ios << IB. 2. Use FET-input or super-β op-amps with very small bias currents (pA). 3. Use smaller resistor values (or a T-feedback network for high gain), since the error is proportional to Rf. 4. Null the remaining error with the offset-null pot (741: 10 kΩ between pins 1 and 5). 5. In ac amplifiers, use coupling capacitors so the dc gain is unity.