7-92. Normally, difference amplifier circuits have the ratio of the inverting input resistor

to the feedback resistor equal to the ratio of the noninverting input resistors. In other

words, for Figure 7-24:

R1 R2

=

R3 R4

and by inverting both sides:

R3 R4

=

R1 R2

7-93. For an easy explanation for the circuit shown in Figure 7-24, all the resistors have

a value of 1 kilohm. However, any value could be used as long as the above ratio is true.

For a subtractor circuit, the values of R1 and R3 must also be equal, and therefore, the

values of R2 and R4 must be equal. It is not necessary that the value of R1 equal the value

of R2. Using Figure 7-24, assume that the input signals are:

E1 = *+*3 V

E2 = +12 V

The output signal should be:

EOUT = E2 - E1

EOUT = (+12 V) - (+3 V)

EOUT = +9 V

To check this output, first compute the value of R2 plus R4:

R2 + R4 = 1 kΩ + 1 kΩ

R2+ R4 = 2 kΩ