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Comparator With Hysteresis

IP.com Disclosure Number: IPCOM000041858D
Original Publication Date: 1984-Mar-01
Included in the Prior Art Database: 2005-Feb-03
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Murfet, P: AUTHOR

Abstract

Comparators on integrated circuits often require small amounts of hysteresis, for example, from 10 to 100 millivolts. The circuit described here provides a standard comparator with small, accurate and symmetrical hysteresis with very few additional components. In order to reduce offset, comparators often are provided with long-tail pair input stages. Such an arrangement is shown in the figure where the voltage input VIN across terminals 1 and 2 is applied to the inputs of comparator 3 through a long-tail pair input stage comprising transistors T1 and T2 and current source 4 drawing constant current I. The difference output voltage VOUT appears at the output terminal 5 from the comparator.

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Comparator With Hysteresis

Comparators on integrated circuits often require small amounts of hysteresis, for example, from 10 to 100 millivolts. The circuit described here provides a standard comparator with small, accurate and symmetrical hysteresis with very few additional components. In order to reduce offset, comparators often are provided with long-tail pair input stages. Such an arrangement is shown in the figure where the voltage input VIN across terminals 1 and 2 is applied to the inputs of comparator 3 through a long-tail pair input stage comprising transistors T1 and T2 and current source 4 drawing constant current I. The difference output voltage VOUT appears at the output terminal 5 from the comparator. Hysteresis is provided by the addition of a further long-tail pair comprising transistors T3 and T4 and current source 6 drawing constant current I/n driven from the output stage of the circuit. Calculations show that the hysteresis voltage VH depends upon the ratio of the two current sources, which can be accurately defined, and the thermal voltage VTH (about 25 mV). The expression for hysteresis voltage is as follows: VH = ÅVTH log ((I + n)/(I - n)) The overall action of this modified circuit is such that the output voltage state will not change until the input voltage has changed polarity and exceeded the hysteresis value.

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