Book Analog Interfacing to Embedded Microprocessors
V1 ¥ R2 R2 ˆ
Vo = V1 + or Vo = V1 ÊË 1 +
R1 R1 ¯
This is the equation normally used for the transfer function of a nonin-
verting amplifier. However, it was derived by assuming that Av is large enough
to make anything divided by Av in the previous equation effectively zero. What
happens in a practical opamp? The LM318, a low-power opamp, has a gain
that ranges from 50,000 to over 200,000. Suppose we have a noninverting
amplifier with R1 = 10K and R2 = 50k, and V1 is 1.2v. Using the ideal equa-
tions we get an output voltage of:
R2 ˆ 50K ˆ
Vo = V1 ÊË 1 + = 1.2 v ÊË 1 + = 7.2 v
R1 ¯ 10K ¯
Now, if the opamp is an LM318 with a gain of 100,000, and using the non-
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