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Beta Degradation Protection Circuit

IP.com Disclosure Number: IPCOM000059672D
Original Publication Date: 1986-Jan-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 2 page(s) / 42K

Publishing Venue

IBM

Related People

Gaudenzi, GJ: AUTHOR [+4]

Abstract

The circuit described in this article prevents Beta degradation of output devices by shielding the base-emitter junctions of such devices from reverse-biasing in excess of 2.75 volts. In some technologies Beta degradation of a device occurs when the emitter-base junction is reverse-biased by a voltage in excess of 2.75 volts, causing performance and/or functionality problems. I/O circuits are used in environments, however, where the voltage across the emitter-base junction can exceed the maximum reverse-bias voltage, necessitating protective action of the nature disclosed in this article. The Darlington driver circuit shown operates as follows: When the first driver is put into the high impedance mode, nodes A and B are brought low ( 0.2 volt).

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Beta Degradation Protection Circuit

The circuit described in this article prevents Beta degradation of output devices by shielding the base-emitter junctions of such devices from reverse-biasing in excess of 2.75 volts. In some technologies Beta degradation of a device occurs when the emitter-base junction is reverse-biased by a voltage in excess of 2.75 volts, causing performance and/or functionality problems. I/O circuits are used in environments, however, where the voltage across the emitter-base junction can exceed the maximum reverse-bias voltage, necessitating protective action of the nature disclosed in this article. The Darlington driver circuit shown operates as follows: When the first driver is put into the high impedance mode, nodes A and B are brought low ( 0.2 volt). In the configuration shown, the second driver can take control of the output bus (node D). When this occurs, the second driver can put a logical 'one' on the bus, thereby reverse-biasing the base-emitter junction on the output device T4 of the first driver. If the logical levels used on the bus exceed 2.75 volts (overshoots included), a Beta degradation circuit must be used. Such circuit protection includes devices T1, T2, T3 and the Schottky barrier diode S1. When the base-emitter junction of the output device T4 is reverse-biased by 2.4 volts, T1 supplies base drive for T2, thereby "building up" the voltage potential across resistor R1, hence limiting the reverse-bias voltage across th...