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Controlled Hysteresis Current Switch

IP.com Disclosure Number: IPCOM000084585D
Original Publication Date: 1975-Dec-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 2 page(s) / 24K

Publishing Venue

IBM

Related People

Gersbach, JE: AUTHOR

Abstract

Shown is an improved Schmitt trigger type of current switching circuit. The threshold levels (a and b) are determined by the transistor characteristics and the speed and fan-out capability is severely limited. Levels a and b are those levels at which the loop gain from input to output exceeds unity. These levels are near the final level of the input transistor T1 and are highly variable in time, since the input wave form is exponential.

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Controlled Hysteresis Current Switch

Shown is an improved Schmitt trigger type of current switching circuit. The threshold levels (a and b) are determined by the transistor characteristics and the speed and fan-out capability is severely limited. Levels a and b are those levels at which the loop gain from input to output exceeds unity. These levels are near the final level of the input transistor T1 and are highly variable in time, since the input wave form is exponential.

The circuit switches when the available input current is low and tends to slow the input voltage even further. The addition of resistor RB reduces the input threshold level a to level c and reduces the loop gain of the circuit, such that the input wave form is distorted less. The addition of resistor RA increases the input threshold for a negative input from b to d.

The thresholds are symmetrical about the center of the input voltage swing when RA = RB, assuming the driving source is a similar circuit. The following resistor value R1 = 300 ohms, R2 = 400 ohms. RA = 100 ohms, and RB = 100 ohms, result in a threshold near the center of the input swing.

A known alternative is to provide a reference voltage at the base of transistor T2, but this has the disadvantages of requiring another line to be routed to all circuits on chip, more power to generate the voltage and there is no hysteresis at all. Hysteresis is desirable because noise tolerance is greater for a given voltage swing.

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