Analog Interfacing to Embedded Microprocessor Systems, 2nd Edition

Dividing the right fraction by Av/Av and allowing Av to be very large:

Vo ¼

V1R2Rf þ V1R2R3 ðV2Rf RL þ V2Rf R2Þ

R1R3 þ R2R3

¼

V1  R2

R1 þ R2

1 þ

Rf

R3



 V2

Rf

R3

If R2 ¼ RF and R1 ¼ R3, then we get

Vo ¼ðV1  V2Þ

Rf

R3

So the differential amplifier multiplies the difference between the inputs by the gain,

Rf/R3. If a voltage divider was not used on the noninverting input, and V

þ

was connected

to the V1 input, the output would be

Vo ¼ V1 1 þ

Rf

R3



 V2

Rf

R3

Without the voltage divider on the noninverting input, the gain for V1 is greater than

the gain for V2. With the voltage divider, if V1 ¼ V2, Vo will be 0. Without the divider, this

is not the case.

General Opamp ...