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CHAPTER NINE
Arithmetic Function Circuits
This chapter presents several circuits designed to perform mathematical opera-
tions, including adding, subtracting, averaging, absolute value, and sign chang-
ing. Several other common but more complex circuits that perform mathematical
functions are presented in Chapter 11.
9.1 ADDER
An adder circuit has two or more signal inputs, either AC or DC, and a single out-
put. The magnitude and polarity of the output at any given time is the algebraic
sum of the various inputs. In Chapter 2, we discussed an inverting adder circuit,
called an inverting summing amplifier. If we make the feedback resistor and all
input resistors the same size, the circuit provides a mathematically correct sum
(i.e., no voltage gain). The following discussion will introduce the noninvertmg
adder circuit.
9.1.1 Operation
Figure 9.1 shows the schematic diagram for a noninverting adder circuit. The
input signals may be AC, DC, or some combination. The op amp, in conjunction
with RI and R& is a simple noninverting amplifier whose gain is determined by
the ratio of R 2 to JR*. Whatever voltage appears on the (+) input will be amplified.
The voltage that appears on the (+) input is the output of a resistive network
composed of R 3 through JR N and the associated input voltages. Since all input
resistors are equal in value and connect together at the (+) input, we can infer that
the relative effects of the inputs are identical. The absolute effect, of course, is
determined by the gain of the amplifier. If R 2 is set to the correct value, then the
gain of the op amp will be such that the output voltage corresponds to the sum of
the input voltages.
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