Ambient Light Compensation for Photodetector
Original Publication Date: 1986-Jan-01
Included in the Prior Art Database: 2005-Mar-08
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.