Bias Current and Offset Voltage 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 Rf.
– This current creates a voltage at the output equal to I1Rf.
– 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 –I1Rs.
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, Rf, equal to the source resistance, Rs, 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, IOS, is less than I2:
VOUT(error) = |I1 – I2|Rs = IOSRs
Bias current compensation in other op-amp configurations
– In a noninverting amplifier we add a resistor Rc.
– The compensating resistor value equals the parallel combination of Ri and Rf.
– The input creates a voltage drop across Rc that offsets the voltage across the combination or Rf and Ri.
– 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, VIO, 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.
