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Original Publication Date: 1982-Jan-01
Included in the Prior Art Database: 2001-Oct-10

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David Reynolds


The low light sensitivity of silicon photo-detectors traditionally has been limited by the high temperature leakage of the photodiode or phototransistor. The monolithic photo-detector of Figure 1 &es the close matching between IC components to compensate for this high temperature leakage. This allows a 20fold increase in sensitivity without sacrificing high temperature performance. The circuit operates as follows: In the dark, identical diodes Di'end D2 supply thermally generated Currents to the bases of matched NPN transistors Ql and Q3, respectively. The amplified leakage current from D2 is mir- rored in the collector current of Q104. Likewise, the leakage current of Dl is mirrored in the collector cur. rent of 04. These two currents cancel, and no current is available to drive the base of Q5. Since IC COm- ponents never perfectly match, Rl and 07 establish a AIC threshold value to protect against a possible 30% cumulative mismatch in component parameters. With the light on, additional photogenerated cur- rent from D2 enters the base of 03. Dl is shielded by metal, and thus, no photogenerated current is added to the base drive of Ql. Therefore, the amplified photogenerated current from D2 appears as AIC and can be used to drive the open collector output stage of 05 and Q6.