BC547 Transistor Pinout, Specifications, and Circuit Applications Explained

The BC547 is an NPN bipolar junction transistor with a maximum collector current of 100 mA, a collector-emitter voltage rating of 45 V, and a DC current gain (hFE) ranging from 110 to 800. Its three pins, from left to right when viewing the flat face, are collector, base, and emitter. This general-purpose transistor handles switching, amplification, and signal processing in low-power circuits.

BC547 Transistor Pinout: Collector, Base, and Emitter Identification

The BC547 comes in a TO-92 plastic package with three leads. When you hold the transistor with the flat side facing you and the leads pointing downward, the pins from left to right are:

• Pin 1 (left): Collector, the terminal where current flows in from the external circuit
• Pin 2 (centre): Base, the control terminal that triggers transistor operation when a small current is applied
• Pin 3 (right): Emitter, the terminal where current flows out to ground

This C-B-E pinout applies specifically to the BC547. Other transistors in the same TO-92 package, such as the 2N2222 or 2N3904, use different pin arrangements (E-B-C), so you should always verify pinout from the datasheet before wiring. Misidentifying the 547 transistor pinout is the most common cause of circuit failure in beginner projects.

To confirm pin identity with a multimeter, set it to diode test mode. Place the red probe on the suspected base pin and the black probe on each of the other two pins. Both readings should show a forward voltage drop between 0.6 V and 0.7 V. Reversing the probes (black on base) should show open-line readings on both remaining pins. This confirms the base location, and the collector and emitter can then be identified by their physical position relative to the flat face.

BC547 Specifications Table: Complete Electrical Parameters

Parameter	Symbol	Value	Unit
Collector-Emitter Voltage	VCEO	45	V
Collector-Base Voltage	VCBO	50	V
Emitter-Base Voltage	VEBO	6	V
Collector Current (continuous)	IC	100	mA
Peak Collector Current	ICM	200	mA
Power Dissipation (25°C)	PD	500	mW
DC Current Gain	hFE	110 to 800	—
Transition Frequency	fT	300	MHz
Collector-Emitter Saturation Voltage	VCE(sat)	0.25 (typical)	V
Base-Emitter Voltage	VBE	0.6 to 0.7	V
Collector-Emitter Cutoff Current	ICEO	15 (max)	nA
Operating Temperature Range	TJ	-65 to +150	°C
Package Type	—	TO-92	—

The BC547 is subdivided into three gain groups: BC547A (110 to 220 hFE), BC547B (200 to 450 hFE), and BC547C (420 to 800 hFE). When you order generic BC547 units, the gain varies widely across this full range. For circuits requiring predictable amplification, specify the suffix letter to get a tighter gain bracket. The BC547B variant is the most commonly stocked and suits the majority of switching and amplifier circuits.

How the BC547 Transistor Works: Operating Modes

The transistor bc547 operates in three distinct modes depending on the voltages applied to its terminals:

Cutoff Mode (Transistor Off)

When the base-emitter voltage is below 0.6 V, no base current flows, and the collector-emitter junction behaves as an open switch. The leakage current (I_CEO) is just 15 nA maximum, making the BC547 an effective digital switch with negligible off-state current draw. You use cutoff mode in logic circuits and sensor-triggered switching applications where the transistor must remain fully off until activated.

Active Mode (Linear Amplification)

When the base-emitter junction is forward-biased (V_BE between 0.6 V and 0.7 V) and the collector-base junction is reverse-biased, the transistor operates in its linear region. Collector current equals base current multiplied by the DC current gain: I_C = h_FE x I_B. With a gain of 300 (typical BC547B), a 10 microamp base current produces 3 mA of collector current. This proportional relationship is the basis of all transistor amplifier circuits.

Saturation Mode (Transistor Fully On)

When you drive enough base current to push the collector-emitter voltage down to its saturation value (0.25 V typical), the transistor acts as a closed switch. The rule of thumb for ensuring saturation is to supply base current equal to at least 1/10th of the desired collector current. For a 50 mA load, you need at least 5 mA of base current. In saturation, the BC547 drops only 0.25 V across collector to emitter, wasting minimal power as heat.

BC547 Switching Circuit: Driving LEDs and Relays

The most common application for the BC547 is as a digital switch controlled by a microcontroller or sensor output. Here is how you wire a basic LED switching circuit:

1. Connect the LED anode to your supply voltage (5 V or 3.3 V) through a current-limiting resistor. For a standard red LED drawing 20 mA at 5 V, use a 150 ohm resistor: (5 V – 2 V LED drop – 0.25 V V_CE(sat)) / 20 mA = 137.5 ohms, rounded up to 150 ohms.
2. Connect the LED cathode to the BC547 collector pin.
3. Connect the BC547 emitter pin to ground.
4. Connect a base resistor between your control signal and the BC547 base pin. For a 5 V control signal driving 20 mA collector current: base current required = 20 mA / 100 (using conservative gain) = 0.2 mA. Base resistor = (5 V – 0.7 V) / 0.2 mA = 21.5 k ohms. Use a 10 k ohm resistor to guarantee saturation with margin.

For relay driving, the same topology applies, but you must add a flyback diode (1N4148 or 1N4007) across the relay coil with the cathode connected to the positive supply. This diode protects the BC547 from voltage spikes generated when the relay coil de-energises. Without the flyback diode, inductive kickback can exceed 45 V and destroy the transistor. Most 5 V relays draw 70 to 90 mA, which is within the BC547’s 100 mA continuous rating but leaves little headroom. For relay coils drawing more than 80 mA, consider upgrading to a BD139 transistor rated at 1.5 A.

BC547 Amplifier Circuit: Common Emitter Configuration

The common emitter amplifier is the standard configuration for audio and signal amplification using the BC547. This circuit provides both voltage gain and current gain, with a 180-degree phase inversion between input and output.

Component Values for a Single-Stage Audio Amplifier

For a single-stage common emitter amplifier operating from a 9 V supply with a voltage gain of approximately 50:

• R_C (collector resistor): 4.7 k ohms, sets the collector voltage swing and load line
• R_E (emitter resistor): 1 k ohm, provides thermal stability and sets the DC operating point
• R1 (upper bias resistor): 56 k ohms, forms the voltage divider that sets the base bias voltage
• R2 (lower bias resistor): 10 k ohms, the second half of the bias voltage divider
• C_in (input coupling capacitor): 10 microfarads, blocks DC from the signal source
• C_out (output coupling capacitor): 10 microfarads, blocks DC from reaching the next stage or speaker
• C_E (emitter bypass capacitor): 100 microfarads, bypasses R_E at signal frequencies to maximise AC gain

The voltage divider sets the base voltage at approximately 1.36 V (9 V x 10k / (56k + 10k)), placing the emitter at about 0.66 V and establishing a collector current of 0.66 mA. This positions the quiescent collector voltage near the midpoint of the supply rail, allowing maximum symmetrical output swing before clipping. The voltage gain equals R_C / R_E at DC (4.7), but the bypass capacitor C_E increases the AC gain to approximately 50 at audio frequencies by effectively shorting R_E for AC signals.

BC547 vs BC548 vs BC549: Which Transistor to Choose

The BC547, BC548, and BC549 share the same TO-92 package, identical pinout, and similar gain ranges, but differ in voltage ratings and noise performance:

Parameter	BC547	BC548	BC549
VCEO	45 V	30 V	30 V
VCBO	50 V	30 V	30 V
IC	100 mA	100 mA	100 mA
hFE Range	110 to 800	110 to 800	110 to 800
Noise Figure	Standard	Standard	Low noise (2 dB typical at 1 kHz)
Best Application	General switching, 12 V+ circuits	5 V logic switching	Audio preamplifiers, low-noise circuits

Choose the BC547 when your circuit operates above 30 V or when you need maximum voltage headroom. The BC548 works identically in 5 V and 3.3 V systems and is often cheaper due to higher production volumes. The BC549 is specifically optimised for low-noise amplifier design with its 2 dB noise figure, making it the correct choice for microphone preamplifiers, guitar pedal input stages, and sensitive measurement circuits. For basic switching at any voltage up to 30 V, all three are interchangeable.

Interfacing BC547 with Arduino and Raspberry Pi

Both Arduino (5 V logic) and Raspberry Pi (3.3 V logic) can drive the BC547 directly through a base resistor. The calculation differs slightly for each platform:

Arduino 5 V Output to BC547

Arduino digital pins source up to 20 mA at 5 V. For switching a 60 mA load (small motor or LED strip segment), the base current required for guaranteed saturation is 60 mA / 100 = 0.6 mA. Base resistor = (5 V – 0.7 V) / 0.6 mA = 7.2 k ohms. Use a standard 4.7 k ohm resistor for reliable saturation. This draws only 0.9 mA from the Arduino pin, well within its 20 mA limit.

Raspberry Pi 3.3 V GPIO to BC547

Raspberry Pi GPIO pins deliver a maximum of 16 mA at 3.3 V. For the same 60 mA load: base resistor = (3.3 V – 0.7 V) / 0.6 mA = 4.3 k ohms. A 3.3 k ohm resistor provides approximately 0.79 mA base current, drawing safely within the GPIO limit. Always add a 10 k ohm pull-down resistor between base and emitter to ensure the transistor stays off when the GPIO is in a high-impedance state during boot-up. Without this pull-down, floating GPIO pins can cause unpredictable transistor switching during Raspberry Pi GPIO setup.

Common BC547 Circuit Problems and Troubleshooting

When your BC547 circuit does not behave as expected, check these failure points systematically:

• Transistor stays off: Measure V_BE. If it is below 0.6 V, the base drive is insufficient. Reduce the base resistor value or check that your control signal is actually reaching the base pin. A disconnected or cold solder joint on the base is the most frequent cause.
• Transistor overheating: Calculate the power dissipation: P = V_CE x I_C. If the transistor is not fully saturated (V_CE above 1 V) while passing high current, it dissipates excessive heat. Reduce the base resistor to drive harder into saturation, or confirm your load current does not exceed 100 mA.
• Oscillation or noise on output: Long base wiring acts as an antenna. Keep base connections short and add a 100 ohm resistor in series with the base, close to the transistor, to dampen parasitic oscillation. A 100 nF ceramic capacitor across the power supply rails also suppresses high-frequency noise.
• Reverse-connected transistor: The BC547 will partially function with collector and emitter swapped, but with drastically reduced gain (typically below 10) and higher saturation voltage. If your circuit works poorly but not completely dead, verify the pinout orientation against the datasheet.

Where to Buy BC547 Transistors and Equivalent Replacements

The BC547 is available from all major electronics distributors: Mouser, Digi-Key, Farnell, and RS Components stock the ON Semiconductor and Nexperia versions. Pricing ranges from $0.05 to $0.15 per unit in quantities of 10 or more. For prototyping, packs of 100 BC547B transistors cost approximately $3 to $5 on Amazon and AliExpress.

If the BC547 is unavailable, these transistors serve as direct replacements with identical or superior specifications: 2N3904 (40 V, 200 mA, TO-92, E-B-C pinout, requires rewiring), S8050 (25 V, 500 mA, higher current capacity), and KSP2222A (40 V, 600 mA, TO-92 package). The 2N3904 is the closest equivalent but uses a different pinout (E-B-C versus the BC547’s C-B-E), so you must swap the collector and emitter connections when substituting. The 2N3904 vs BC547 comparison covers all the differences in detail.

Frequently Asked Questions About the BC547 Transistor

What is the maximum current a BC547 can handle?

The BC547 supports a continuous collector current of 100 mA and a peak current of 200 mA. For loads exceeding 80 mA continuous, you should consider upgrading to a higher-rated transistor like the BD139 (1.5 A) or TIP31 (3 A) to maintain thermal safety margins and long-term reliability.

Can you use a BC547 to drive a 12 V relay?

Yes, the BC547’s 45 V collector-emitter rating comfortably handles 12 V relay circuits. Ensure the relay coil current stays below 100 mA, add a 1N4148 flyback diode across the coil, and calculate the base resistor to supply at least 1/10th of the coil current. Most standard 12 V relays draw 30 to 80 mA, fitting within the BC547’s limits.

What is the difference between BC547 and BC557?

The BC547 is an NPN transistor where current flows from collector to emitter, controlled by a positive base current. The BC557 is its PNP complement where current flows from emitter to collector, controlled by a negative base current (current flows out of the base). They share similar voltage and current ratings but require opposite biasing. PNP transistors like the BC557 are used in high-side switching and complementary push-pull output stages.

Is the BC547 suitable for PWM switching from a microcontroller?

The BC547’s 300 MHz transition frequency makes it suitable for PWM frequencies up to approximately 50 kHz without significant switching losses. Arduino’s default PWM frequency of 490 Hz or 980 Hz is well within this range. For PWM frequencies above 100 kHz, consider a MOSFET like the 2N7000, which switches faster and does not require continuous base current drive.

How do you test a BC547 transistor with a multimeter?

Set your multimeter to diode test mode. Place the red probe on the base (centre pin) and the black probe on the collector (left pin): you should read 0.6 V to 0.7 V. Move the black probe to the emitter (right pin): you should again read 0.6 V to 0.7 V. Reversing the probes on both tests should show “OL” (open line). If any reading deviates, the transistor is damaged and should be replaced.