REDUCTION OF HYSTERESIS IN FIXED THRESHOLD SCHMITT TRIGGER

Publishing Venue

Motorola

Related People

Russell Marten: AUTHOR

Abstract

The CMOS hex Schmitt trigger can be a very useful tool in the design of audio frequency circuits, such as limiters, frequency and signal amplitude detectors except for one inherent limitation. The magnitude of the hysteresis is a fixed ratio of the supply voltage (typically 113 of supply). Since the supply voltage for a design is usually a given, the application of this device for the circuits mentioned above would be severely limited.

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MOTOROLA Technical Disclosure Bulletin Vol. 1 No. 1 August 1980

REDUCTION OF HYSTERESIS IN FIXED

THRESHOLD SCHMITTTRIGGER

By Russell Marten

   The CMOS hex Schmitt trigger can be a very useful tool in the design of audio frequency circuits, such as limiters, frequency and signal amplitude detectors except for one inherent limitation. The magnitude of the hysteresis is a fixed ratio of the supply voltage (typically 113 of supply). Since the supply voltage for a design is usually a given, the application of this device for the circuits mentioned above would be severely limited.

   The addition of two resistors Fh, Rz as shown~in Figure 1 allows the reduction of hysteresis in these devices from their fixed values down to approximately 5% of the supply voltage while still maintaining symmetry around one-half of the supply voltage. Reduction of the hysteresis below 5% is possible but may require precision components and/or trimming due to part variations in the actual Schmitt trigger thresholds.

Figure 1

  The advantage of this approach ever a more conventional implementation utilizing a standard voltage comparator (339 type for example) is both cost and increased density with comparable performance for most applications. The cost of a CMOS hex Schmitt trigger (74C14 or MC14584) is typically one-fourth of that of a quad voltage comparator (such as a 339 type) while providing 50% more devices per 14 pin DIP. Also, the circuit here described uses only two resistors per device while an implementation utilizing voltage comparators would typically require four resistors per devic...