What Is a Darlington Transistor?
A Darlington transistor is a transistor structure that achieves very high current gain by cascading two BJTs (one driving the other).
How Does It Work?
The output of the first transistor drives the base of the second. Since the two gains multiply, the total gain becomes very high (e.g. 100 × 100 = 10,000). So a very small base current can drive a large load.
Advantages and Disadvantages
| Advantage | Disadvantage |
|---|---|
| Very high gain | High saturation voltage (Vce) |
| Small drive current | Slower switching |
| Single package | More heat |
Where Is It Used?
Driving high-current loads such as relays, motors, solenoids or lamps from the weak output of a microcontroller. Ready-made Darlington arrays like the ULN2003 are common.
Ready-Made Darlington Arrays
Instead of wiring transistors one by one, integrated chips that pack several Darlington pairs into a single package (e.g. ULN2003) are used. These make it easy to drive relays/motors from several outputs of a microcontroller at once, and they include protection diodes inside.
Common Mistakes
- Forgetting the high Vce: a Darlington drops ~1 V+ even while conducting, which is a problem with low-voltage loads.
- Expecting fast switching: it's slow because of the two stages; not suited to high frequency.
- Ignoring heating: the high saturation voltage produces heat under load.
Frequently Asked Questions
Why does a Darlington have such high gain?+
What's the drawback?+
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