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Tristable Threshold Circuit

IP.com Disclosure Number: IPCOM000094406D
Original Publication Date: 1966-Oct-01
Included in the Prior Art Database: 2005-Mar-06
Document File: 2 page(s) / 31K

Publishing Venue

IBM

Related People

Gardner, PA: AUTHOR

Abstract

This tristable circuit discriminates between bipolar inputs of type A and B which are produced by some memory devices, e.g., thin-film magnetic stores. The circuit is basically bistable with a third or reset state due to nonlinear feedback coupling. The circuit is initially set to the reset state. Input A triggers the circuit to state 1. Input B will trigger the circuit to state 2. In either state 1 or state 2 the circuit is insensitive to subsequent reverse polarity pulses. The circuit also rejects noise below the threshold level.

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Tristable Threshold Circuit

This tristable circuit discriminates between bipolar inputs of type A and B which are produced by some memory devices, e.g., thin-film magnetic stores. The circuit is basically bistable with a third or reset state due to nonlinear feedback coupling. The circuit is initially set to the reset state. Input A triggers the circuit to state 1. Input B will trigger the circuit to state 2. In either state 1 or state 2 the circuit is insensitive to subsequent reverse polarity pulses. The circuit also rejects noise below the threshold level.

Transistors T1, T2, and T3 together with feedback path via transistors T4 and T5 and silicon diodes D1, D2 form a latch. In the reset state, the emitters of T1 and T3 are at the same potential. The circuit is stable as the feedback path is open due to the high impedance of diodes D1 and D2.

Consider the effect of input signal A when the circuit is in the reset state. The emitter potential of T3 falls until the conduction threshold of D1 is reached. Regeneration then occurs until T1 saturates. The circuit is now stable in state 1. Values are chosen so that in this condition the emitter current of T1 is sufficiently large to prevent the reverse polarity of input A from resetting the latch.

Consider the effect of input signal B when the circuit is in the reset state. The emitter potential of T3 increases until D2 conducts. generation then occurs until T1 is cut off. The circuit is now stable in state 2. Values...