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ComponentsActive / Semiconductor

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

NPN (e.g. BC547, 2N2222, 2N3904)PNP (e.g. BC557, 2N3906)Darlington (huge gain)Power BJT (TIP/TO-220)

Key specs

Current gain (hFE/β)

-

Collector current ÷ base current; often 100-300.

Collector current (Ic max)

A

Max continuous current through the device.

VCEO

V

Max collector-emitter voltage before breakdown.

Power dissipation

W

Max heat; needs a heatsink for power types.

Transition frequency (fT)

MHz

Upper frequency limit for useful gain.

Laws that govern it

  • Kirchhoff's current law (KCL)ΣI(in) = ΣI(out)

    Ie = Ib + Ic is Kirchhoff's current law with a name — the base and collector currents literally meet inside the device and leave by the emitter.

  • Shockley diode equation

    The base-emitter junction is a diode: Ic rises exponentially with Vbe, which is why you drive a BJT with current, never with a voltage.

  • Ohm's lawV = I × R

    The base resistor sets Ib = (Vin − 0.7) / Rb, and choosing Rb so Ib > Ic/β is the whole art of saturating a switch.

  • Watt's lawP = V × I = I²R = V²/R

    P = Vce × Ic is what cooks the package — worst in the linear region, minimal when fully on or fully off.

Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.

How it compares

  • MOSFET vs BJT

    Side by side with MOSFET — specs, variants and which one the job calls for.

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

flat faceCBE123

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.

Where to buy

DigiKeyMouserLCSCRobuAmazon