What is it about?

The CMOS Schmitt trigger (ST ) is one of the most basic building blocks for electronic circuits. It can be used for applications such as noise reduction, pulse-width preserving, pulse stretching, conversion of continuous wave to square wave and relaxation oscillators. The CMOS ST was patented in 1975, but accurate approximations for the ST high and low threshold voltages for all ratios of feedback are still lacking.

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Why is it important?

Several authors presented expressions for calculating the threshold voltages of the CMOS ST decades ago. Although the expressions for the threshold voltages appear to be good approximations for strong positive feedback (wide hysteresis window), they totally fail for weak feedback (narrow hysteresis window). Sometimes it is important to include a narrow hysteresis window to a comparator in order to avoid inappropriate output changes of the comparator due to interference signals of very-high frequency. Another important application of comparators of narrow hysteresis is that of ripple-based control of DC-DC converters. We have developed a model based on a simple approximation for both the output voltages at the trip points that allows the calculation of the high and low threshold voltages of the classical CMOS Schmitt trigger in strong inversion, for any feedback factor.


After more than 40 years the CMOS SCHMITT trigger is finally correctly mised en equations.

Carlos Galup-Montoro
Universidade Federal de Santa Catarina

Read the Original

This page is a summary of: Inadequacy of the classical formulation of the CMOS Schmitt trigger, International Journal of Circuit Theory and Applications, March 2021, Wiley, DOI: 10.1002/cta.2992.
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