EFFECT OF BIAS CURRENT
Transistor’s within the OP-AMP must be biased so that have the current values of base and collector current and collector to emitter voltages. The ideal OP-AMP has no input current, but infact, the practical OP-AMP has small input bias current typically in the nano ampere range.
Mostly the bias current problem is produced in inverting and non-inverting amplifier and in voltage follower. An inverting amplifier with zero input voltage is shown in below Figure (a).
Ideally the current through Rin is zero because the input voltage is zero and the voltage at the inverting terminal (-)is zero. The small input current I1 through Rf from the output terminal. This current creates a voltage drop across Rf is the output terminal and hence the output error voltage is I1Rf.
Voltage follower with zero input voltage and a source resistance Rs is shown in Figure (b). in this case an input current I1 produces a drop (I1Rs) across Rs. Hence the voltage at the inverting input terminal decreases to –I1Rs because the negative feedback tends to maintain a differential voltage of zero. Since the inverting terminal is connected directly to the output terminal the output error voltage is –I1Rs.
A non-inverting amplifier with zero input voltage is shown in Figure(c). Ideally the voltage at the inverting terminal is also zero, as indicated but the input current I1 produces a voltage drop across Rf and creates an input error voltage of I1Rf same as with the inverting amplifier.
BIAS CURRENT COMPENSATION
In Voltage Follower
The error voltage at the output of voltage follower due to bias current can be reduced by adding a resistor equal to Rs in the feed back path. This is shown in Figure (d).
The voltage drop created by I1 across the added resistor subtracts from the –I2Rs output error voltage. If I1 = I2 then the output voltage will be zero. Usually I1 does not quite equal I2 but in this case the output error voltage is reduced as follows because input offset current is less than I2
Vout(error) = | I1 – I2 | Rs
As I1 – I2 = Ios
and Vout(errors) = Ios Ros .
In Inverting and Non-inverting Amplifiers
Figure (e) shows the bias current compensation in the inverting and non-inverting amplifiers. To compensate the effect of bias current in the non-inverting amplifier, a resistor Rs is connected as shown in Figure (e). The compensating resistor value equals the parallel combination of Ri and Rf. The input current creates a voltage drop across Rs that offsets the voltage across the combination of Ri and Rf. Hence sufficiently reducing the output error voltage.
In the case of inverting amplifier the compensation of bias currents is similar, as shown in Figure (e).
Note that in both circuits the compensating resistor Rs is connected with not inverting terminal of the OP-AMP.