Comparator Hysteresis and Schmitt Triggers

by Lewis Loflin



Hysteresis is defined as:

For slow time changing input signal, an output oscillation can appear while the input signal remains close to the reference voltage. Also low amplitude signal on high impedance can cause oscillations due to noise background. Such unwelcome behavior can be solved by hysteresis. The principle of hysteresis consists of two different input threshold voltages depending on actual output state.

Here I'll concentrate on examples not followed on my Comparator Circuits Examples Tutorial.

Also see Voltage Comparator Circuits

See my page Looking at Window Comparator Circuits

Comparator based Schmitt Trigger
Fig. 2 Comparator based Schmitt Trigger.

In Fig. 2 we have a comparator based Schmitt trigger which are used to assure clean switching with noisy or erratic signals. When the input voltage on TP2 is less than TP1 the comparator is in the OFF condition. TP3 is pulled up to nearly 12-volts by R4 a 3K resistor.

Schmitt triiger resistor network off.
Fig. 3

Fig. 3 illustrates how when the comparator is OFF as R4 and R1 form a series 30K which is in parallel with R2 shifting TP1 (Vref) to 6.56-volts.

Without R1 Vref would be 6-volts.

Schmitt triiger resistor network on.
Fig. 4

When the comparator is ON TP3 is switched to ground through the internal open collector transistor where 47K resistor R1 is now in parallel with 10K resistor R3 forming a total resistance of 8245 ohms. This drops Vref on TP1 to 5.36-volts.

It takes 6.56-volts on Vin to turn ON the comparator but the voltage will have to drop to 5.36-volts to turn off. This produces a switching gap or hysteresis value of ~1.2-volts helping to assure stable operation.