Deficiency of the current models of the CMOS Schmitt trigger

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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Photo by Raychel Sanner on Unsplash

Photo by Raychel Sanner on Unsplash

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.

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