Phototransistor equivalent circuit
A phototransistor is essentially the same thing as a common transistor, which can work in two different ways:
- As a common transistor. Using the base current (IB)
- As a phototransistor. Using the light that illuminates this element and it acts as the base current. (IP).
You can use both simultaneously, although it is mainly used with the base terminal that is not connected (IB = 0). The total base current is equal to the base current (as a common transistor) + base current (using the light as a base current): IBT = IB + IP.
If you want to raise the sensitivity of the phototransistor, due to low lighting that brights the phototransistor, we can increase the base current (IB), using an external bias.
Phototransistor = photodiode + Transistor
A common transistor with a photodiode connected to its base and the collector terminals. The cathode of the photodiode must be connected to the collector and the anode to the base of the transistor. It is widely used with light detection applications.
The phototransistor as the photodiode, has a short time response but it can handle much higher current. The picture show the equivalent circuit. You can see that the current runs from the photodiode to the base of the transistor. The current that it can deliever is ß times the photodiode current.
Note: ß is the current gain.