Offset Voltage and Bias Current Compensation

– Up until now we have mostly considered ideal op-amps in our discussion.

– We must, however, introduce some non-ideal characteristics, since they will have an effect on the op-amp operation.

– Transistors within the op-amp must be biased so that they have the correct values of base and collector currents and collector-to-emitter voltages.

– Ideal op-amp has no input current at its terminals.

– In practice, op-amps have small input bias currents (in the nA range).

– There is also a small offset voltage between the inputs.

Effect of an Input Bias Current

– Consider the inverting amplifier circuit shown below.

– If the input voltage is zero, there should be zero current coming into the inverting input of the op-amp.

– However, there is a small bias current, I₁, that goes through R_f.

– This current creates a voltage at the output equal to I₁R_f.

– This is the error voltage.

– The same voltage will be seen at the output of a noninverting amplifier.

– If we look at the voltage follower circuit shown below, it is easy to see that the output error voltage is –I₁R_s.

Bias current compensation in a voltage-follower

– Somehow we need to compensate for the error voltage due to the bias currents.

– In a voltage follower it is enough to add a resistor, R_f, equal to the source resistance, R_s, in the feedback path.

– The voltage drop created by I₁across the added resistor subtracts from the –I₂R₂ output error voltage.

– If I₁ = I₂, then the output voltage is zero.

– Usually they are not equal, but even then the output voltage error is reduced, since the input offset current, I_OS, is less than I₂:

V­_OUT(error) = |I₁ – I₂|R_s = I_OSR_s

Bias current compensation in other op-amp configurations

– In a noninverting amplifier we add a resistor R_c.

– The compensating resistor value equals the parallel combination of R_i and R_­f.

– The input creates a voltage drop across R_c that offsets the voltage across the combination or R_f and R_i.

– Thus, the output is reduced.

– The same is done for the inverting amplifier.

Input offset voltage compensation

– The output voltage of an op-amp when the differential input is zero should be also zero.

– However, due to unavoidable internal imbalances and due to non-zero bias currents, a small voltage, V_IO, is seen between the terminals.

– ICs provide a means to compensate for this.

– This is generally done by connecting an external potentiometer to pins designated with Offset Null.

– With zero input voltage, the output is set to zero by adjusting the potentiometer.

– The pinout for the 741 op-amp (the most common op-amp IC) is shown next.