What is a Photo Darlington?

Table of Contents:

  1. Photodarlington transistor basics
  2. Symbol
  3. SDP8105  Silicon Photodarlington
  4. References
  5. See also

1 Photodarlington transistor basics

Photo Darlington is a phototransistor that is related to Darling transistor family  also called Darlington phototransistor or photodarlington.
The photodarlington transistor provides a much higher degree of sensitivity when compared to other phototransistors, but this is at the expense of response time and frequency response.
In the photodarlington transistor configuration, the first transistor acts as the photodetector, and its emitter is coupled into the base of the second transistor. This gives a very much higher level of gain, but it is very much slower than the ordinary phototransistor, having a maximum frequency of around 20 kHz.

SDP8105  Silicon Photodarlington

2 Photdarlington symbol

The photodarlington symbol is a combination of the standard phototransistor symbol and the Darlington transistor symbol.
Photdarlington symbolIts also can be considered as phototransistor, the photodarlington symbol indicates that often the base connection is not available or it is left open circuit.
Photo Darlingtons are only used where low frequencies are required. The combination of the lower frequency response of the phototransistor as opposed to the photodiode and the darlington configuration as opposed to a single transistor means that the photodarlington typically has a low frequency response. Some may only have a bandwidth of a few tens of kHz. It also has a much higher ON voltage between the collector and emitter than a single transistor.

3 SDP8105  Silicon Photodarlington


  • T-1 plastic package
  • 20¡ (nominal) acceptance angle
  • Consistent optical properties
  • Mechanically and spectrally matched to SEP8505 and SEP8705 infrared emitting diodes

Tolerance 3 plc decimals ±0.005(0.12)
2 plc decimals ±0.020(0.51)
3.1 Descriptions
The SDP8105 is an NPN silicon photodarlington transfer molded in a T-1 black plastic package to minimize effect of visible ambient light. Transfer molding of this device assures superior optical centerline performance compared to other molding processes. Lead lengths are staggered to provide a simple method of polarity identification.
Download datasheet of SDP8105  Silicon Photodarlington


  • Courtesy of Steven Engineering, Inc.

5 See also

Revision date 13/9/2018

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.