Page 395 - Op Amps Design, Applications, and Troubleshooting

Absolute Value Circuit 371

Negligible current flows in or out of the (-) input pin, so Kirchhoff's Current Law
will show us that the sum of the input currents must be flowing through R 3, That is,

The voltage across K 3 will be equal to the output voltage, since one end of K 3
is connected to a virtual ground point and the other end is connected directly to
the output terminal. Therefore, we can conclude that

Now, for proper operation of the averaging circuit, all of the input resistors must
be the same value; let us call this value JR. Further, the feedback resistor (R 3) is cho-
sen to be equal to R/N where N is the number of inputs to be averaged. Making
these substitutions in our previous equations gives us the following:

This final expression for the output voltage should be recognized as the equation
for computing arithmetic averages. That is, add the numbers (input voltages)
together and divide by the number of inputs. The minus sign simply shows that
the signal is inverted in the process of passing through the op amp circuit.

9.4 ABSOLUTE VALUE CIRCUIT

An op amp circuit can be configured to provide the absolute value (I VALUE 1) of
a given number. You will recall from basic mathematics that an absolute value
function produces the magnitude of a number without regard to sign. In the case
of an op amp circuit designed to generate the absolute value of its input, we can
expect the output voltage to be equal to the input voltage without regard to polar-
ity. So, for example, a +6.2-volt input and a -6.2-volt input both produce the same
(typically +6.2-volt) output.

9.4.1 Operation

There are several ways to obtain the absolute value of a signal. The schematic dia-
gram in Figure 9.9 shows one possible way. The first stage of this circuit is a dual