_____________________________________________________________________ Special Amplifiers
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
Figure 7-24. Subtractor Circuit
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
23 June 2005
TC 9-62
7-31
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