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# Differential Input Schmitt Trigger

IP.com Disclosure Number: IPCOM000080559D
Original Publication Date: 1974-Jan-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 2 page(s) / 27K

IBM

## Related People

Scotto, VA: AUTHOR [+2]

## Abstract

The present circuit provides a variable upper threshold voltage with near zero crossover detection.

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Differential Input Schmitt Trigger

The present circuit provides a variable upper threshold voltage with near zero crossover detection.

A voltage V is set by the divider combination of R1 in series with the parallel combination of R2 with R3+R4. This assumes some current I flowing through R3. Therefore, the voltage at A sets at IR3 below V. If the input impedance is large enough, no current flows into node B and, therefore, its voltage is identical to node A. This means that the FDA output is in a down level.

With the above condition set up, the transducer must produce a voltage equal to IR3 before the FDA output switches to an up level. Therefore, IR3 sets the upper threshold level for the trigger. When the input switches to an up level, D1 becomes forward biased and clamps the output voltage to a diode drop above V. This places point B at a point IR3 + (VD)R3 Over R3+R4 above V. This means that the transducer must move IR3 in a direction negative to where it sets at present to arrive at its zero crossing. It must traverse further by (VD)R3 Over R3+R4 to switch the FDA to a down level. This means that the lower threshold is negative with respect to the zero crossing of the transducer. The smaller VD and R3 are with respect to R3+R4, the closer the lower threshold is to the zero crossing of the transducer.

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