What Is a Phototransistor?
A phototransistor is a transistor that senses the light falling on it and at the same time amplifies it. It can be thought of as a combination of a photodiode and a transistor.
How Does It Work?
Light creates a base current in the transistor's base region; the transistor multiplies this current by its gain to produce a much larger collector current. So it both senses and amplifies.
Difference From a Photodiode
A photodiode responds to light fast but with a small current; a phototransistor is far more sensitive (high output) but slower. If sensitivity matters, a phototransistor; if speed matters, a photodiode is preferred.
| Property | Photodiode | Phototransistor |
|---|---|---|
| Sensitivity | Low | High |
| Speed | Very fast | Slower |
| Output | Small current | Large current |
Where Is It Used?
The receiver side of optocouplers, object detection, optical encoders, line-following robots and proximity sensors paired with an IR LED.
Dark Current
A small leakage current (dark current) flows even with no light. In precise sensing it's important that this value be low and the thresholds set accordingly.
Common Mistakes
- Load resistor choice: a wrong value gives either a too-slow response or a low output.
- Ambient light: if not filtered, stray light causes false triggering.
- Speed expectation: for high-speed signals a photodiode is more suitable.
Frequently Asked Questions
Does a phototransistor have a base terminal?+
Does it work with visible light?+
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