How does a photodarlington transistor provide increased sensitivity to light signals compared to a regular phototransistor?
1 Answer
A photodarlington transistor, also known as a photodarlington or a darlington phototransistor, provides increased sensitivity to light signals compared to a regular phototransistor due to its unique configuration. It is a specialized type of phototransistor designed to offer higher gain and sensitivity in response to light.
To understand how it works, let's first review the basic principles of a regular phototransistor:
Regular Phototransistor:
A phototransistor is a semiconductor device that is sensitive to light. It consists of a base-collector junction (similar to a regular bipolar transistor) and an additional transparent casing on top of the base region, allowing light to reach the base-collector junction. When light strikes the base-collector region, it generates electron-hole pairs, creating a photocurrent that flows between the collector and the emitter terminals. This photocurrent is proportional to the intensity of the incident light.
The sensitivity of a regular phototransistor is limited by the transistor's inherent current gain, also known as the current transfer ratio (CTR). CTR is the ratio of the output photocurrent to the input base current. Regular phototransistors typically have moderate CTR values, which means they might not provide sufficient sensitivity for some applications.
Photodarlington Transistor:
A photodarlington transistor is essentially a combination of two transistors in a Darlington pair configuration. It consists of two bipolar transistors connected together in a way that amplifies the current gain significantly. One of the transistors acts as the input phototransistor (primary transistor), and the other one acts as the output transistor (secondary transistor).
Here's how it works:
Incident light falls on the base-collector region of the primary phototransistor, generating a photocurrent as in a regular phototransistor.
This photocurrent flows into the base of the secondary transistor, which acts as a load for the primary phototransistor.
The secondary transistor amplifies the current received from the primary phototransistor. Since it has its own current gain, it multiplies the photocurrent, resulting in a much larger output current compared to a regular phototransistor.
In summary, the photodarlington transistor increases sensitivity to light signals because it combines the current gain of two transistors, offering a higher level of amplification for the photocurrent. This increased sensitivity makes photodarlington transistors suitable for applications where very low light levels need to be detected or in situations where the output of a regular phototransistor is not sufficient. However, it's essential to note that photodarlington transistors may have slower response times due to the higher gain configuration.
To understand how it works, let's first review the basic principles of a regular phototransistor:
Regular Phototransistor:
A phototransistor is a semiconductor device that is sensitive to light. It consists of a base-collector junction (similar to a regular bipolar transistor) and an additional transparent casing on top of the base region, allowing light to reach the base-collector junction. When light strikes the base-collector region, it generates electron-hole pairs, creating a photocurrent that flows between the collector and the emitter terminals. This photocurrent is proportional to the intensity of the incident light.
The sensitivity of a regular phototransistor is limited by the transistor's inherent current gain, also known as the current transfer ratio (CTR). CTR is the ratio of the output photocurrent to the input base current. Regular phototransistors typically have moderate CTR values, which means they might not provide sufficient sensitivity for some applications.
Photodarlington Transistor:
A photodarlington transistor is essentially a combination of two transistors in a Darlington pair configuration. It consists of two bipolar transistors connected together in a way that amplifies the current gain significantly. One of the transistors acts as the input phototransistor (primary transistor), and the other one acts as the output transistor (secondary transistor).
Here's how it works:
Incident light falls on the base-collector region of the primary phototransistor, generating a photocurrent as in a regular phototransistor.
This photocurrent flows into the base of the secondary transistor, which acts as a load for the primary phototransistor.
The secondary transistor amplifies the current received from the primary phototransistor. Since it has its own current gain, it multiplies the photocurrent, resulting in a much larger output current compared to a regular phototransistor.
In summary, the photodarlington transistor increases sensitivity to light signals because it combines the current gain of two transistors, offering a higher level of amplification for the photocurrent. This increased sensitivity makes photodarlington transistors suitable for applications where very low light levels need to be detected or in situations where the output of a regular phototransistor is not sufficient. However, it's essential to note that photodarlington transistors may have slower response times due to the higher gain configuration.