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:
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:
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