Many times signals can have noise to them. Therefore when the voltage gets close to the threshold voltage, the output may make many transitions. Typically we want only one transition when the output first crosses the threshold. This is very important if the output is going to provide a level for digital logic. To remove the effect of the noise we will add hysteresis to the circuit.
Hysteresis provides a memory of the transition and therefore shifts the threshold voltage.
This is . Determine the amount of hysteresis in terms of R 1 and R 2 and choose R 1 and R 2 to give you 1V of hysteresis. The hystersis is . Measure and V TL , and determine the amount of hysteresis.