Offset Voltage Compensation and Bias Current explanation



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:
OUT(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.
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