AI Tutor
Send a crisp square wave down a long coaxial cable, and it emerges rounded, attenuated, and covered in high-frequency electrical noise. If you feed that degraded pulse into a standard digital gate, the noisy edges cause false triggering, creating phantom pulses that corrupt your data.
A standard comparator uses a single reference voltage, causing its output to rapidly chatter between high and low whenever noise fluctuates across that single threshold.
How do we modify an operational amplifier circuit to create two separate switching thresholds?

Inverting Schmitt Trigger Design

In 1934, American biophysicist Otto Schmitt invented this circuit while studying how squid nerve fibers fire clean action potentials despite noisy chemical signals. By routing a portion of the output back into the non-inverting input (++), we introduce positive feedback, which forces the circuit into two distinct switching thresholds.
In the inverting configuration, the noisy input signal VinV_{in} connects directly to the inverting input (βˆ’-), while a potential divider formed by R1R_1 and R2R_2 sets the reference voltage at the non-inverting pin (V+V_+).
When the output is saturated at +Vsat+V_{sat}, the potential divider holds the non-inverting terminal at the upper threshold voltage (VUTV_{UT}):
VUT=+Vsat(R1R1+R2)V_{UT} = +V_{sat} \left( \frac{R_1}{R_1 + R_2} \right)
When VinV_{in} rises above VUTV_{UT}, the output snaps to βˆ’Vsat-V_{sat}, pulling the non-inverting terminal down to the lower threshold voltage (VLTV_{LT}):
VLT=βˆ’Vsat(R1R1+R2)V_{LT} = -V_{sat} \left( \frac{R_1}{R_1 + R_2} \right)
The difference between these two points defines the hysteresis width Ξ”VH=VUTβˆ’VLT\Delta V_H = V_{UT} - V_{LT}. Any noise with a peak-to-peak amplitude smaller than Ξ”VH\Delta V_H cannot cause a false transition.