The simple comparator circuit in the previous post has two disadvantages. For a very slowly varying input, the output swing can be rather slow. Worse still, if the input is noisy, the output may make several transitions as the input passes through the trigger point (Figure 1). Both these problems can be remedied by use of positive feedback (Figure 2).
The effect of R3 is to make the circuit have two thresholds, depending on the output state. In the example shown, the threshold when the output is at ground (input high) is 4.76 volts, whereas the threshold with the output at +5 volts is 5.0 volts. A noisy input is less likely to produce multiple triggering (Figure 3).
Furthermore, the positive feedback ensures a rapid output transition, regardless of the speed of the input waveform. (A small “speed-up” capacitor of 10–100 pF is often connected across R3 to enhance switching speed still further.) This configuration is known as a Schmitt trigger. The output depends both on the input voltage and on its recent history, an effect called hysteresis. This can be illustrated with a diagram of output versus input, as in Figure 4.
The design procedure is easy for Schmitt triggers that have a small amount of hysteresis. Use the circuit of Figure 2B. First choose a resistive divider (R1, R2) to put the threshold at approximately the right voltage; if you want the threshold near ground, just use a single resistor from non-inverting input to ground. Next, choose the (positive) feedback resistor R3 to produce the required hysteresis, noting that the hysteresis equals the output swing, attenuated by a resistive divider formed by
R3 and R1 || R2.
Finally, if you are using a comparator with “open-collector” output, you must add an output pull-up resistor small enough to ensure a nearly full supply swing, taking account of the loading by R3. For the case in which you want thresholds symmetrical about ground, connect an offsetting resistor of appropriate value from the non-inverting input to the negative supply. You may wish to scale all resistor values to keep the output current and impedance levels within a reasonable range.
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