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Feedback Control Of Photo-transistors

IP.com Disclosure Number: IPCOM000097045D
Original Publication Date: 1962-Apr-01
Included in the Prior Art Database: 2005-Mar-07
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Jackson, RT: AUTHOR

Abstract

The output of a photo-transistor, used as a sensing device in a reader, is a certain current for 1 and another, much smaller, current for 0. The currents should be largely unaffected by changes in either the ambient temperature or the brightness of the source of illumination. A way of producing this result is to generate a feedback voltage from a sample 1 current and to use this to control the operating point of the photo-transistor.

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Feedback Control Of Photo-transistors

The output of a photo-transistor, used as a sensing device in a reader, is a certain current for 1 and another, much smaller, current for 0. The currents should be largely unaffected by changes in either the ambient temperature or the brightness of the source of illumination. A way of producing this result is to generate a feedback voltage from a sample 1 current and to use this to control the operating point of the photo-transistor.

As an alternative to employing each phototransistor to provide its own feedback voltage, the circuitry uses a sample photo-transistor to supply a feedback voltage to all other transistors. Reliance is made upon the similarity of transistor performance to produce the required result. The sample transistor is illuminated identically as is a reader transistor at the 1 level. A voltage proportional to its output current is fed back to all the transistor bases.

The variation in output current from transistor to transistor must then be taken into account in the design of the photo-amplifier.

This variation is normally very small compared to the difference between 1 and 0 currents. The problem is further eased by the fact that the 0 current decreases with temperature when feedback applied. A typical distribution of output 1 currents about a 2ma nominal is from 1.5 to 2.3ma with corresponding 0 currents between 10 and 16 ua.

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