What is a Photodarlington Transistor?

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- GoPhotonics

Sep 29, 2023

A Photodarlington Transistor is a specialized type of phototransistor that incorporates a phototransistor into a Darlington pair configuration with another Bipolar Junction Transistor (BJT). This configuration is designed to amplify low-level light signals. In the Darlington configuration, which is also known as a Darlington pair, two BJTs (NPN/PNP transistor) are connected in a way that provides high current gain. Specifically in Photodarlington pair, the emitter of the first phototransistor is connected to the base of the second BJT, and the collector of first transistor is connected to the collector of second BJT. This arrangement allows for the efficient amplification of the small current produced by the phototransistor in response to incident light on the base.

Figure1: Photodarlington Transistor

While most phototransistors are made of a single material, some may be composed of multiple materials depending on their specific application requirements and performance criteria.

The materials used in a Photodarlington Transistor can vary depending on the specific design and application requirements. However, the common materials used in the construction of Photodarlington Transistors include:

  • Semiconductor materials such as silicon (Si) or germanium (Ge). These materials provide the necessary electronic properties for the transistors to function effectively.
  • The photodetector component in the Photodarlington Transistor, such as a photodiode or phototransistor, can be made from a variety of materials. These may include silicon, gallium arsenide (GaAs), indium gallium arsenide (InGaAs), or other III-V compound semiconductors.

Working Principle of Photodarlington Transistor

First Phototransistor: This is the light-sensitive component. The base of the first phototransistor is exposed to incoming light. When light falls on its base region, it generates electron-hole pairs, which control the emitter current of this phototransistor. The current amplification is a characteristic of bipolar transistors, and it occurs due to the transistor's inherent properties and the biasing conditions. It act as a photodetector.

Second Bipolar Transistor: The emitter of the first phototransistor is connected to the base of the second bipolar transistor. The amplified current from the first transistor is then fed into the base of the second BJT. This second BJT also amplifies the current further. Since the output of the first phototransistor becomes the input to the second BJT, the amplification process is repeated, resulting in an even larger current being drawn from the second BJT’s collector terminal.

The second BJT acts as the output transistor of the photodarlington, where the output current of the Photodarlington transistor is the collector current of the second transistor. This output current is significantly amplified compared to the initial base current generated by incident light on the first transistor's base.

Figure 2: Photodarlington Pair

Unlike a standalone phototransistor or photodiode, this setup allows the Photodarlington to generate significantly higher output currents, and so it has a greater sensitivity to illumination levels. Due to this arrangement, the switching time of the Photodarlington transistor is considerably longer compared to a standard phototransistor.

Figure 3: NPN and PNP Photodarlington Pair

The overall current amplification in the photodarlington transistor is the product of the individual amplifications of the transistors in series. 

Let’s consider CTR as the current transfer ratio of the first phototransistor (β1), collector current (Ic1) and the incident light intensity (IL1) of the first phototransistor.

The current gain of the second BJT (output transistor) be β2. It represents the ratio of the collector current (Ic2) to the base current (Ib2) of the second transistor.

The total current gain (βtotal) of the photodarlington transistor is the product of the individual gains of the two transistors since they are connected in series:

The Darlington configuration has a higher voltage from the base of the input phototransistor to the emitter of the output second BJT.

Total voltage is given by:

VBE1 is the voltage from the input transistor and VBE2 voltage from the second transistor.

Applications of Photodarlington transistors

Photodarlington transistors find applications in various fields where high sensitivity and low-light-level amplification are required. Some common applications include:

  • Used in optical sensors for detecting and measuring light levels. They can be employed in ambient light sensors, optical encoders, and position sensors.
  • Photodarlington transistors are used in fiber optic communication systems as receivers for converting optical signals into electrical signals. They can be found in applications such as optical receivers, fiber optic data transmission, and telecommunication systems.
  • They are utilized in medical devices for imaging, such as computed tomography (CT) scanners and X-ray machines. They can also be found in diagnostic instruments and medical monitoring devices.
  • These transistors are used in industrial automation systems for light detection and control. They can be employed in light barriers, safety sensors, and object detection in manufacturing processes.
  • They are integrated into security systems for intrusion detection, motion sensors, and surveillance cameras. They are capable of detecting low light levels and capturing images in dimly lit environments.
  • The transistor can be found in automotive applications, including ambient light sensors for automatic headlights, rain sensors for windshield wipers, and optical sensors for anti-lock braking systems (ABS).