How it works
In the non-inverting Schmitt trigger with supply ±15 V, the upper threshold VUT = +Vsat·R1/(R1+R2) and the lower threshold VLT = −Vsat·R1/(R1+R2). For R1 = 10 kΩ and R2 = 100 kΩ with Vsat ≈ 13.5 V, VUT ≈ +1.23 V and VLT ≈ −1.23 V, giving a hysteresis width of 2.46 V. The output switches HIGH only when input rises above VUT, and switches LOW only when input falls below VLT — any signal variation within the hysteresis band is completely ignored. The inverting Schmitt trigger swaps the input and reference connections; its threshold polarity reverses accordingly.
Key points to remember
Hysteresis width = VUT − VLT = 2·Vsat·R1/(R1+R2) for the symmetric supply case; reducing R1 narrows the hysteresis, making the trigger faster but more susceptible to noise. The Schmitt trigger is a bistable circuit — it has two stable output states and remembers which threshold it last crossed, which is exactly what distinguishes it from a simple comparator. IC 555 timer in astable mode uses internal Schmitt-trigger action at VCC/3 and 2VCC/3 thresholds to generate square waves. The term "regenerative comparator" is synonymous with Schmitt trigger and appears in some textbook question stems — recognise it.
Exam tip
Every university analog exam has a question where you are given R1, R2, and supply voltage and must calculate VUT, VLT, and hysteresis — always show the formula for each before substituting, and confirm VUT > VLT.