If the input signal is very slow or very noisy, the comparator output can trip several times as the input signal passes through the threshold. Using positive feedback to add hysteresis to the switching can reduce or eliminate this problem. The positive feedback can be added by a high value resistor (R_F). This results in two switching thresholds, one for increasing signals and one for decreasing signals. A capacitor can be added across R_F to increase the switching speed and provide more short term hysteresis. This can result in greater noise immunity for the circuit.

See Figure 5-2, Figure 5-3 and Figure 5-4.

Note that very heavy loading of the comparator output, such as LED drive or bipolar logic gates, changes the output voltage and shift the voltage thresholds.

R_F ≫ R₁ and R_F ≫ R₂

Figure 5-2 Positive Feedback for Hysteresis

Figure 5-3 Without Positive Feedback (No Hysteresis)

Figure 5-4 With Positive Feedback (Hysteresis or Memory)