Rising Threshold Switches High - Electronic Circuits V & VI: Front-End Conditioning and Op-Amp Feedback Step by Step

A Schmitt input with LOW history switches HIGH only after the rising input crosses the upper threshold, so the previous state matters until that boundary is exceeded. The comparison is an ideal threshold ledger; real circuits have input offset, resistor tolerance, supply drift, noise, finite propagation delay, output rail limits, and voltage-measurement uncertainty.

A Schmitt input uses the upper threshold to leave a prior LOW state. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

The prior state is LOW before crossing

The input starts from a LOW history. The upper threshold is 3 volts.

V_{\text{upper}} = 3\ \text{V}

Crossing the upper threshold switches HIGH

The input is 4 volts, above the upper threshold, so the output becomes HIGH.

4\ \text{V} \ge 3\ \text{V}\Rightarrow \text{HIGH}