Output Offset Voltage(Voos) in Op-amp

Output Offset Voltage(V_oos)

The effect of input bias current, input offset current and input offset voltage is to add the error to the expected d.c. output voltage. These parameters produce the output voltage when both the input terminals are grounded.

Thus the d.c. voltage present at the output terminals when both the input terminals are grounded is called output offset voltage denoted as V_oos.

The output voltage caused by input offset voltage Vios can be either positive or negative with respect to ground. Similarly, the output voltage due to the input bias current Ib2 can be also positive or negative with respect to ground. If these output voltages are of different polarity then the total output offset voltage is very small. But if these voltages are of same polarity then output offset voltage is large. Thus the output offset voltage is given by,

V_oos = V_o due to V_ios + V_o due to I_b

To compensate the output offset voltage due to the input bias current a compensating resistance is used as shown in the Fig. 2.12.1. It is denoted as R_comp. This is called offset minimizing resistor.

While the effect of offset voltage is nullified using voltage compensating network. 

For many op-amps, the offset compensation pins are provided to nullify the offset voltage. The manufacturer's specifications must be referred to use these compensation pins. For example, in case of IC 741 op-amp, the pins 1 and 5 are provided as compensation pins. The manufacturer recommends that a 10 kΩ potentiometer must be connected between the pins 1 and 5.

A wiper must be connected to the negative supply pin which is pin number 4. By adjusting this potentiometer wiper, the output can be nullified.

The arrangement for IC 741 op-amp is shown in the Fig. 2.12.2.

If the op-amp does not have the offset compensation pins, then the separate external compensating network is required to be designed.

Review Question

1. Define output offset voltage. Explain a method to nullify offset voltage.

Dec.-05, May-11, Marks 6