Output-Offset Voltage and Input Offset Current - Operational Amplifiers Types Tutorials Series

Output-Offset Voltage and Nulling Out Offset:-

•A parameter called the output-offset voltage may be used to represent the internal imbalance of an op-amp, rather than the input-offset voltage.
–The output-offset voltage is defined as the measured output voltage when the input terminals are shorted together, as shown at the left-top fig.

–The output-offset voltage may be modeled by placing a voltage source AoVIO  in series with the output voltage source –Ao(v+ - v-)

•Consequently, the output-offset voltage is essentially the input-offset voltage multiplied by the open loop gain.
•How can we correct for offset voltage?

–Some op-amps provide two terminals (offset-null terminals) for adjusting out the offset voltage

•A potentiometer is connected across the offset null terminals with the VNEG supply voltage connected to the adjustable center tap

–If the op-amp does not have an internal null adjustment provision, an external adjustment can also be provided.


Input Offset Current Definition:-

•Non-zero input bias currents I+ and I- may not always be equal (some opamps) 

–Variation in bipolar transistor beta may cause base currents to non-track, or perhaps there are circuit design issues causing non equal offset I

•We define a parameter “input offset current”

IIO = I+ - I-

–Typical values of IIO are 5-10% (of I-) although it can be as high as 50%

•Example based on figure above-

–R1 = 1K, R2 = 20K ohms

–Assuming Ibias = 1 uA and IIO = 100 nA, find I+, I-, and the effect of IIO on vout

–Since (I+ + I-)/2 = 1 uA and I+ - I- = 0.1uA, we can solve for I+ = 1.05 uA and I- = 0.95 uA

–Using the expression for Vout from slide 2-26 with Vin = 0 and Rx = R1 || R2 gives us

–vOUT = R2 (I- - I+) = -IIO R2 = -2 mV