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: “high” and “low.” The high state is near the comparator’s positive supply voltage, between five and 15 volts, and the low state is near the comparator’s negative supply voltage. As signals at the inputs change, the output state alternates between high and low voltages. The comparator’s duty cycle is the percentage of the time its output is high.
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Comparator Action
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True to its name, a comparator compares two input signals and produces an output as a result of the comparison. One of the comparator’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 comparator’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
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Duty cycle is a measurement of the “on” time of a repeating pulse wave divided by the wave’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.
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Comparators and AC Signals
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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 wave’s duty cycle is 50 percent because it is high for half of the wave’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
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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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