BC547 — General-Purpose NPN Bipolar Junction Transistor

Overview

The BC547 (and its gain sub-variants BC547A, BC547B, and BC547C) is the standard European general-purpose NPN Bipolar Junction Transistor (BJT) manufactured by STMicroelectronics, ON Semiconductor, and Fairchild. It is widely used in audio preamplifiers, sensor signal conditioning, low-current LED driving, and digital logic switching.

Housed in a 3-pin TO-92 plastic package, the BC547 is rated for a collector-emitter breakdown voltage ( $V_{CEO}$ ) of $45\text{ Volts}$ and continuous collector current ( $I_C$ ) of $100\text{ mA}$ . Its complementary PNP counterpart is the BC557.

Quick reference


Transistor Type	General-Purpose NPN Bipolar Junction Transistor (BJT)
Package	TO-92 (Flat face front, leads pointing down) / SOT-23 (BC847)
Pinout (TO-92)	Pin 1: Collector (`C`), Pin 2: Base (`B`), Pin 3: Emitter (`E`) — C-B-E
Collector-Emitter Voltage ( $V_{CEO}$ )	$45\text{ V}$ max
Collector-Base Voltage ( $V_{CBO}$ )	$50\text{ V}$ max
Continuous Collector Current ( $I_C$ )	$100\text{ mA}$ continuous
DC Current Gain ( $h_{FE}$ )	$110\dots 220$ (A) / $200\dots 450$ (B) / $420\dots 800$ (C)
Transition Frequency ( $f_T$ )	$300\text{ MHz}$ typ
Power Dissipation ( $P_D$ )	$500\text{ mW}$ at $T_A = 25^\circ\text{C}$

Pinout (TO-92 Package)

Looking at the flat face of the TO-92 package with leads pointing down:

text

        ┌─────────────┐
        │   BC547B    │  (Flat Package Face)
        └─┬───┬───┬───┘
          1   2   3
          C   B   E

Pin	Name	Description
1	`COLLECTOR` (`C`)	Collector terminal (Low-side load connection)
2	`BASE` (`B`)	Base input control terminal (Connect via resistor)
3	`EMITTER` (`E`)	Emitter terminal (Ground reference 0 V)

Important

Pinout Differences (BC547 vs 2N3904):

BC547 (European standard): Pin 1 = Collector, Pin 2 = Base, Pin 3 = Emitter (C-B-E).
2N3904 (US standard): Pin 1 = Emitter, Pin 2 = Base, Pin 3 = Collector (E-B-C).

Gain Classification Suffixes

BC547A: Low DC Current Gain ( $h_{FE} = 110 \dots 220$ ).
BC547B: Medium DC Current Gain ( $h_{FE} = 200 \dots 450$ ) — Most common kit variant.
BC547C: High DC Current Gain ( $h_{FE} = 420 \dots 800$ ) — Ideal for weak sensor amplifiers.

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Collector Breakdown Voltage	$V_{(BR)CEO}$	45	—	—	V	$I_C = 10\text{mA}, I_B = 0$
Emitter Breakdown Voltage	$V_{(BR)EBO}$	6.0	—	—	V	$I_E = 10\ \mu\text{A}, I_C = 0$
Collector Saturation Volts	$V_{CE(sat)}$	—	90	250	mV	$I_C = 10\text{mA}, I_B = 0.5\text{mA}$
Base Saturation Voltage	$V_{BE(sat)}$	—	700	900	mV	$I_C = 10\text{mA}, I_B = 0.5\text{mA}$

Common mistakes

Exceeding 100mA continuous load current: The BC547 is a small-signal transistor limited to $100\text{ mA}$ . Driving motors or relays requiring $>100\text{ mA}$ causes thermal breakdown. Use a 2N2222 ( $800\text{mA}$ ) or IRLZ44N MOSFET instead.
Direct MCU pin connection without base resistor: Always use a series base resistor ( $1\ \text{k}\Omega \dots 10\ \text{k}\Omega$ ) to limit base current.

Notes

BC547 vs BC557: BC547 is NPN; BC557 is its complementary PNP transistor.

Interface	Current-Driven Base Junction
Package	TO-92 Plastic / SOT-23 (BC847)