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What Is a Duty Cycle of a Comparator?

A comparator is an electronic circuit having two inputs and an output. The output has only two states: ̶0;high̶1; and ̶0;low.̶1; The high state is near the comparator̵7;s positive supply voltage, between five and 15 volts, and the low state is near the comparator̵7;s negative supply voltage. As signals at the inputs change, the output state alternates between high and low voltages. The comparator̵7;s duty cycle is the percentage of the time its output is high.

Comparator Action
- True to its name, a comparator compares two input signals and produces an output as a result of the comparison. One of the comparator̵7;s inputs is inverting, that is, it turns a negative voltage into a positive one and vice-versa; the other input is non-inverting. With no signal applied to either input, the comparator̵7;s output is low. A positive voltage at the non-inverting input sends the output high, as will a negative voltage at the inverting input. The output goes high if the positive voltage at the non-inverting input is greater than the negative voltage at the inverting input. Otherwise, the output stays low.
Duty Cycle
- Duty cycle is a measurement of the ̶0;on̶1; time of a repeating pulse wave divided by the wave̵7;s total cycle time. For example, a wave with a frequency of 100 Hz has a total cycle time of .01 seconds. If the pulse wave is on for .001 seconds and off for .009 seconds, the duty cycle equals .001 / .01 or .1, which is 10 percent. A pulse wave which is on for .009 seconds and off for .001 seconds has the same frequency as the earlier example, but its duty cycle is .009 / .01 or .9, which is 90 percent.
Comparators and AC Signals
- Because its output is either high or low and has no in-between values, you can think of a comparator as producing a rectangular pulse output. If you connect a sine wave oscillator at either input, the comparator produces a square wave output with the same frequency as the sine wave. The square wave̵7;s duty cycle is 50 percent because it is high for half of the wave̵7;s cycle. If you connect the sine wave oscillator to the inverting input and an adjustable voltage to the non-inverting input, an increasingly negative voltage steadily reduces the duty cycle of the output to zero, or fully off. A positive voltage increases the duty cycle to 100 percent, or fully on.
Uses for Variable Duty Cycle Signals
- Designers use comparators to convert analog signals into rectangular pulse equivalents. A class D amplifier circuit, for example, uses a comparator with a high-frequency signal generator at one input to turn audio signals into variable duty cycle pulses. The circuit amplifies the pulses with excellent efficiency, then filters the high-frequency component out, leaving an audio signal at an increased level of power.

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