Browse Prior Art Database

Ambient Light Compensation for Photodetector

IP.com Disclosure Number: IPCOM000059675D
Original Publication Date: 1986-Jan-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Langer, P: AUTHOR

Abstract

This article describes a circuit arrangement which provides ambient (background) compensation for a phototransistor, thereby preventing the phototransistor from saturating due to excess light, or not switching on due to insufficient light. Phototransistors which are biased for high sensitivity to light saturate (go blind) when the ambient light level is excessive. The circuit arrangement disclosed herein is illustrated in Fig. 1 and shows a phototransistor Q1 connected to a pull-up resistor R2 and operational amplifier. The circuit provides ambient compensation for the phototransistor Q1 by providing negative feedback to the base of Q1, so that for ambient light, the output voltage (VOUT) is always the same value as the voltage at the inverting terminal of the operational amplifier.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 97% of the total text.

Page 1 of 2

Ambient Light Compensation for Photodetector

This article describes a circuit arrangement which provides ambient (background) compensation for a phototransistor, thereby preventing the phototransistor from saturating due to excess light, or not switching on due to insufficient light. Phototransistors which are biased for high sensitivity to light saturate (go blind) when the ambient light level is excessive. The circuit arrangement disclosed herein is illustrated in Fig. 1 and shows a phototransistor Q1 connected to a pull-up resistor R2 and operational amplifier. The circuit provides ambient compensation for the phototransistor Q1 by providing negative feedback to the base of Q1, so that for ambient light, the output voltage (VOUT) is always the same value as the voltage at the inverting terminal of the operational amplifier. This voltage reference is generated by the R3/R4 resistor pair. When a light pulse signal strikes Q1, VOUT goes low to VCE SAT before the amplifier can pass the signal through low-pass filter formed by R1 and C1. Fig. 2 illustrates a typical waveform at VOUT which results when pulsed light strikes the photodetector Q1. This bias technique permits high sensitivity to signals and a wide dynamic range of ambient background. In summary, the concept allows a slowly changing light level or fixed light level to be compensated for as ambient light and biases the phototransistor Q1 to be responsive to pulsed light signals.

1

Page 2 of 2

2

[This page...