What Is a BJT (Bipolar Junction Transistor)?
A BJT (Bipolar Junction Transistor) is a three-terminal (base, collector, emitter) transistor that controls a larger collector current with a small base current. It's a current-controlled component.
Operating Principle
When a small current is fed into the base, a much larger current flows between the collector and emitter. When the base current is cut off, the transistor goes to cutoff. So the base, like a tap, manages the large current.
NPN and PNP
| Type | Base for conduction | Current direction |
|---|---|---|
| NPN | Positive (+) | Collector→emitter |
| PNP | Negative (−) | Emitter→collector |
Current Gain (β / hFE)
The ratio of collector current to base current is called the gain (β or hFE). For example, with β=100, a 1 mA base current gives ~100 mA of collector current.
Two Operating Modes
Amplifier: it enlarges a weak signal (analog). Switch: with enough base current it fully conducts, and with none it isolates (digital). A series resistor is needed to limit the base current.
Where Is It Used?
Audio amplifiers, signal stages, relay/LED driving and low-power switching.
Common Mistakes
- Forgetting the base resistor: if the base current flows unlimited the transistor is destroyed.
- Mixing up the terminals: the same package can have a different pinout; check the datasheet.
- Confusing saturation and cutoff: when used as a switch the transistor must be driven into full saturation.
Frequently Asked Questions
BJT or MOSFET?+
Is the β value constant?+
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