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Can a photo transistor be considered as a photodiode?
In other words, a photo transistor can be considered as a Photodiode with current amplifier. A phototransistor converts photons to charge directly, just like a photodiode, and in addition to this, a phototransistor also provides a current gain. The symbol of phototransistor is shown below.
What happens when there is no light falling on a phototransistor?
When there is no light falling on the phototransistor, a small amount of leakage current known as Dark Current flows from collector to emitter. When there is enough light falling on the base terminal, a base current is produced, which is proportional to the intensity of the light.
How is the sensitivity of the phototransistor determined?
How linearly the output varies according to the intensity of the light is an important consideration. Sensitivity of the phototransistor is the ratio of the output signal to the input intensity of the incident light. Also, how quickly the output responds to the changes in the intensity of the light will determine the response time.
Can a base terminal be used as a phototransistor?
As mentioned earlier, phototransistors are also available as three-terminal devices. In this case, the usage of Base terminal is optional. When used, it acts as a regular BJT and when not used, it acts as a Phototransistor.
What happens when the phototransistor is turned off?
As a result, Q2 is turned ON and the Relay is energized. With slight modifications to the Light Operated Relay Circuit, you can implement a Darkness Operated Relay. When it is dark or the intensity of light is less, the phototransistor is OFF and this allows proper biasing of transistor Q2.
What are the characteristics of a phototransistor transistor?
As phototransistors are basically Bipolar NPN Transistors with large base – collector junction, the characteristics of a Phototransistor are similar to that of a simple BJT. Phototransistors are available as two-leaded or three-leaded devices.
When does reverse bias occur in a phototransistor?
When a photon of sufficient energy (hence, the dependency of the wavelength of the light) hits the photodiode, an electron is freed with an energy to pass the barrier i.e. during reverse bias, when light strikes the junction, there is an increase in the current. Keeping this mind, a Phototransistor is a Photojunction…
How does a phototransistor turn on a transistor?
There are two other things to consider which has nothing to do with the characteristics of the phototransistor i.e. its size and cost. When there is enough light falling on the phototransistor Q1, it turns ON and provides a base current to transistor Q2. As a result, Q2 is turned ON and the Relay is energized.