current source for photodiodes

elektor july/august 1983 NI~ current source for photodiodes There are many circuits available today which use modulated light signals to transmit information. Generally, the actual receiver consists of one or more photodiodes. In such applications it is important that the dynamic range of the photodiode is sufficient. However, increasing the dynamic range can cause the sensit- tivity of the diode to decrease. Another disadvantage is that photo- diodes are sensitive to changes in ambient light conditions. The cir- cuit described here increases the dynamic range of the photodiode without affecting its amplification. It also filters out the effects of slow variations in light intensity so that the problems with ambient light are greatly reduced. As the diagram shows, the circuit is very simple. When light falls on Dl this diode produces a photocurrent, relative to the intensity of the light. If the current is small, transistor T1 just conducts. When the light inten- sity (and thus th...