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Biased Photocell Amplifier

IP.com Disclosure Number: IPCOM000091484D
Original Publication Date: 1968-Feb-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 23K

Publishing Venue

IBM

Related People

Coburn, RL: AUTHOR

Abstract

Variations in photocell output due to variations in ambient light, illuminating light, and photocell impedance are compensated by fixing the DC photocell current with a current source and by sensing current changes. These variations result in poor signal-to-noise ratios in conventional detectors. In the quiescent state, all current drawn by source 10 is produced through photocell 11. No current is passed through diode 12 into capacitor 15 and transistor 16 is off. When photocell 11 changes from dark-to-light resistance, a higher current flows through 11. The current change, dI, is coupled through 12, 15, and diode 18 into current amplifier Ai. When 11 returns to its dark resistance, 12 is reverse biased and develops a voltage which turns 16 on so that 15 is discharged through 16 and diode 20.

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Biased Photocell Amplifier

Variations in photocell output due to variations in ambient light, illuminating light, and photocell impedance are compensated by fixing the DC photocell current with a current source and by sensing current changes. These variations result in poor signal-to-noise ratios in conventional detectors. In the quiescent state, all current drawn by source 10 is produced through photocell 11. No current is passed through diode 12 into capacitor 15 and transistor 16 is off. When photocell 11 changes from dark-to-light resistance, a higher current flows through 11. The current change, dI, is coupled through 12, 15, and diode 18 into current amplifier Ai. When 11 returns to its dark resistance, 12 is reverse biased and develops a voltage which turns 16 on so that 15 is discharged through 16 and diode 20. This negative transition is blocked from Ai by 18. The current change, dI, is defined by dI = Iq(Rd/R1) - Iq where Rd is the dark resistance of 11, R1 is the light resistance of 11, and Iq is the fixed quiescent current drawn by
10. By reversing the positions of 10 and 11, and appropriate attention to polarities, light-to-dark transitions can be detected.

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