Question 12.14: Effect of Input Offset Current on an Amplifier Determine the...

Known Quantities: Resistor values; input offset current.

Find: The offset voltage component in the output voltage, v_{\text {out os }}.

Schematics, Diagrams, Circuits, and Given Data: I_{0 \mathrm{~s}}=1  \mu \mathrm{A} ; R_{2}=10  \mathrm{k} \Omega.

Assumptions: Assume input-offset-current-limited (otherwise ideal) op-amp.

Analysis: We calculate the inverting and noninverting terminal voltages caused by the offset current in the absence of an external input:

v^{+}=R_{3} I_{B^{+}} \quad v^{-}=v^{+}=R_{3} I_{B^{+}}

With these values we can apply KCL at the inverting node and write:

\begin{aligned}&\frac{v_{\text {out }}-v^{-}}{R_{2}}-\frac{v^{+}}{R_{1}}=I_{B^{-}} \\&\frac{v_{\text {out }}}{R_{2}}-\frac{-R_{3} I_{B^{+}}}{R_{2}}-\frac{-R_{3} I_{B^{+}}}{R_{1}}=I_{B^{-}} \\&v_{\text {out }}=R_{2}\left[-I_{B^{+}} R_{3}\left(\frac{1}{R_{2}}+\frac{1}{R_{1}}\right)+I_{B^{-}}\right]=-R_{2} I_{0 \mathrm{~s}}\end{aligned}

Thus, we should expect the output of the amplifier to be shifted downward by R_{2} I_{0 s}, or 10^{4} \times 10^{-6}=10 \mathrm{mV} for the data given in this example.

Comments: Usually, the worst-case input offset current (or input bias currents) are listed in the device data sheets (see the data sheets in the accompanying CD-ROM, or in the device templates in the Electronics Workbench { }^{\mathrm{TM}} libraries, for an illustration). Values can range from 100 \mathrm{pA} (for CMOS op-amps, e.g., LMC6061) to around 200 \mathrm{nA} for a low-cost general-purpose amplifier (e.g., \mu \mathrm{A} 741c).