Page 151 - Op Amps Design, Applications, and Troubleshooting

CHAPTER THREE

Voltage Comparators

3.1 VOLTAGE COMPARATOR FUNDAMENTALS

A voltage comparator circuit compares the values of two voltages and produces
an output to indicate the results. The output is always one of two values (i.e., the
output is digital). Suppose, for example, we have two voltage comparator inputs
labeled A and B. The circuit can be designed so that if input A is a more positive
voltage than input B, the output will go to + V SAT. Similarly, if input A is less posi-
tive than input B, the output will go to -V SAT. In general, the voltage comparator
circuit accepts two voltages as inputs and produces one of two distinct output
voltages depending on the relative values of the two inputs.
During the preceding discussion, we were careful not to consider what hap-
pens when the two input voltages are equal. In a simple voltage comparator, this
condition can produce indeterminate operation. That is, the output may be at
either of the two normal output voltage levels or, more probably, oscillating
between the two output levels. This erratic behavior is easily overcome by adding
positive feedback to the comparator. With positive feedback, the circuit has hystere-
sis. In the simple comparator circuit, output switching occurs when the two input
voltages are equal. Hysteresis causes the circuit to have two different switching
points. This important concept will be explained in greater detail in Section 3.3.
Voltage comparator circuits are widely used in analog-to-digital converter
applications and for various types of alarm circuits. In the alarm application, one
input to the comparator is controlled by the monitored signal (e.g., the voltage
produced by a pressure transducer). The second input is connected to a reference
voltage representing the safe level. If the pressure in the device being monitored
exceeds the safe limit, the comparator output will change states and sound an
alarm. Figure 3.1 illustrates a voltage comparator circuit used in conjunction with
a pressure sensor and a potentiometer. If the pressure being monitored exceeds a
certain prescribed value, the voltage generated by the pressure sensor exceeds the
preset voltage on the potentiometer. This causes the output voltage to change
states and to sound the alarm.

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