Browse Prior Art Database

Integrated Circuit Photodetector Amplifier and Logic Driver

IP.com Disclosure Number: IPCOM000078776D
Original Publication Date: 1973-Mar-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 32K

Publishing Venue

IBM

Related People

Crauwels, GL: AUTHOR

Abstract

This circuit detects and translates changes in light levels into logic levels. It may be readily fabricated in integrated circuit form and does not thereafter require calibration. It is also adaptable for transistor - transistor logic (T/2/L).

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Integrated Circuit Photodetector Amplifier and Logic Driver

This circuit detects and translates changes in light levels into logic levels. It may be readily fabricated in integrated circuit form and does not thereafter require calibration. It is also adaptable for transistor - transistor logic (T/2/L).

In Fig. 1, T1, T4 and T6 are matching PNP transistors. T1 and T6 are covered by an opaque material to render them insensitive to light. T4, however, is light sensitive. T2 and T3 match the beta of T1 to T4, provided T2 and T3 have a beta greater than 3D. T7, T8 and T9 are essentially constant-current supplies for T1 and T4. In the absence of light T4 is set in a low or nonconducting condition. T5 directs current from Vcc and T6 to T8 and T9. T6 is placed in a conducting condition which results in the output voltage across R3 approaching Vcc.

When light illuminates T4, additional hole-electron pairs appear in the base region. Many of the holes are swept into the isolation. However to maintain charge neutrality in the base region, more holes must be injected at the emitter. This increases the current through T4 which saturates. The voltage across T8 and 9 increases which cuts off current flow through T5 and T6 turns off. The output voltage across R3 approaches ground.

Fig. 2 shows the circuit of Fig. 1 adapted for use with T/2/L. In Fig. 2, T10 is substituted for R3 and the circuit operates in a manner similar to that shown in Fig. 1.

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