BJT (NPN / PNP)
Transistor
A current-controlled switch and amplifier with three pins. A small base current controls a much larger collector current. The classic transistor.
Schematic symbol
Bipolar transistor (NPN)
How bjt (npn / pnp) appears in a circuit diagram.
Types & variants
Key specs
Current gain (hFE/β)
-Collector current ÷ base current; often 100-300.
Collector current (Ic max)
AMax continuous current through the device.
VCEO
VMax collector-emitter voltage before breakdown.
Power dissipation
WMax heat; needs a heatsink for power types.
Transition frequency (fT)
MHzUpper frequency limit for useful gain.
Markings
Part number on the flat/body. Pinout VARIES by part - always check the datasheet (e.g. BC547 = CBE, 2N2222 = EBC).
Standard values
Jellybean: BC547/BC557, 2N3904/2N3906, 2N2222/2N2907; power: TIP120/TIP31/BD139.
How to choose
Switching: pick Ic > load, VCEO > supply, and drive the base hard (base resistor for ~Ic/10). Amplifying: choose for gain and fT. Add a heatsink for power use.
Pinout & package
TO-92 — flat face toward you, legs down
Check first: Shown for the BC547 family. Pin order is NOT universal — 2N2222 (metal can) and many others differ. Always check the part's datasheet.
3 pins: Base, Collector, Emitter. TO-92 (small signal), TO-220/TO-126 (power), SOT-23 (SMD). Pin order differs per part.
Example circuits
- Low-side switch driving a relay/LED from a logic pin
- Common-emitter amplifier
- Darlington pair for high current
- H-bridge / current source
Common failures
Shorts C-E from overcurrent/overvoltage, or opens from overheating. Often fails shorted.
How to test
Diode mode: B-E and B-C each read like a diode (~0.7V) in one direction. NPN: base is the common anode; PNP: common cathode. C-E should not conduct either way.
Substitutes
Same polarity (NPN/PNP) with equal/greater Ic, VCEO and power, similar hFE. 2N2222≈BC547 for most low-power uses.