Posted on Mar 23, 2013

This self-biasing configuration is useful any time srnall changes in light level must be detected, for example, when monitoring very low flow rates by counting drops of fluid. In this bias method, the photodarlington is de bias stabilized by feedback from the collector, compensating for different photodarlington gains and LED outputs. The 10-I`F capacitor integrates the collector voltage feedback, and the 10-MO resistor provides a high base-source impedance to minimize effects on optical performance.

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The detector drop causes a momentary decrease in light reaching the chip, which causes collector voltage to momentarily rise, generating an output signal. The initial light bias is small because of output power constraints on the LED and mechanical spacing system constraints. The change in light level is a fraction of this initial bias because of stray light paths and drop translucence. The high sensitivity of the photodarlington allows acceptable output signal levels when biased in this manner. This compares with unacceptable signal levels and bias point stability when biased conventionally, i.e., base open and signal output across the collector bias resistor.

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