Page 384 - Op Amps Design, Applications, and Troubleshooting
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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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