Question 12.13: Effect of Input Offset Voltage on an Amplifier Determine the...
Effect of Input Offset Voltage on an Amplifier
Determine the effect of the input offset voltage V_{\text {os }} on the output of the amplifier of Figure 12.48.
Known Quantities: Nominal closed-loop voltage gain; input offset voltage.
Find: The offset voltage component in the output voltage, v_{\text {out os }}.
Schematics, Diagrams, Circuits, and Given Data: A_{\mathrm{nom}}=100 ; V_{\mathrm{os}}=1.5 \mathrm{mV}.
Assumptions: Assume input-offset-voltage-limited (otherwise ideal) op-amp.
Analysis: The amplifier is connected in a noninverting configuration; thus its gain is:
A_{V \text { nom }}=100=1+\frac{R_{F}}{R_{S}}
The DC offset voltage, represented by an ideal voltage source, is represented as being directly applied to the noninverting input; thus:
V_{\text {out,os }}=A_{V \text { nom }} V_{\text {os }}=100 V_{\text {os }}=150 \mathrm{mV}
Thus, we should expect the output of the amplifier to be shifted upward by 150 \mathrm{mV}.
Comments: The input offset voltage is not, of course, an external source, but it represents a voltage offset between the inputs of the op-amp. Figure 12.51 depicts how such an offset can be nulled.
The worst-case offset voltage is usually 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). Typical values are 2 \mathrm{mV} for the 741c general-purpose op-amp and 5 \mathrm{mV} for the FET-input TLO81.
