PhotoDarlingtons

Photodarlingtons are light-sensitive devices that use a Darlington transistor pair to amplify photocurrent. In a photodarlington arrangement, a photodetector element such as a photodiode or photosensitive transistor feeds the base of a first transistor, whose amplified output drives a second transistor in a Darlington configuration. This yields a very high current gain, allowing small amounts of incident light to produce relatively large output signals. Photodarlingtons are encountered as discrete components and as the output stage in certain optocouplers and photodetector modules.

Construction and operation are centered on converting light into an electrical signal with gain. The photodetector portion generates a small current in response to illumination, which is then amplified by the Darlington pair. The overall gain can be orders of magnitude higher than a single transistor photodetector, but the trade-off is slower response and sometimes higher input bias requirements due to base-emitter voltage of the Darlington pair. They are commonly used in analog sensing circuits where high sensitivity is more important than speed.

Key characteristics include high current gain, relatively simple interfacing to logic or microcontroller inputs, and sensitivity to low light levels. Bandwidth and response time are limited by the device’s junction capacitances and the Darlington configuration, often resulting in slower operation compared with faster photodiodes or phototransistors. Noise and offset can also be considerations at very low light levels.

Applications for photodarlingtons include optocouplers with high-gain outputs, low-light sensing in automation and instrumentation, and detectors in systems where a simple, direct current output is acceptable. Related concepts are photodiodes, phototransistors, and Darlington transistor pairs.