Parts

BC547 transistor: pinout, gain groups, ratings and substitutes

By Bulan Sarkar

The BC547 is C-B-E: with the flat face toward you and the leads down, the collector is on the left and the emitter on the right. That is the reverse of the 2N3904 and the PN2222A, the two parts it most often replaces or is replaced by.

The letter after the number is the gain group. A, B and C cover gains of 110 to 220, 200 to 450 and 420 to 800 at 2 mA, and a plain BC547 can be any of them. Every figure below comes from the Diotec and MCC datasheets, which disagree in a few places; the page shows both.

Pinout by package

BC547 TO-92 · Diotec1=C 2=B 3=E

Flat face toward you, leads pointing down: C, B, E from left to right.

The reverse of the 2N3904 and PN2222A: a BC547 dropped into their footprint swaps collector and emitter.

Source: Diotec Semiconductor BC546 ... BC549 General Purpose NPN Transistors, p. 1 (TO-92 drawing with leads 1, 2, 3 and symbol: C (1), B (2), E (3))

BC547 TO-92 · MCC1=C 2=B 3=E

Flat face toward you, leads pointing down: pins 1, 2, 3 from left to right.

Source: Micro Commercial Components BC546/BC547/BC548 NPN Silicon Amplifier Transistor (TO-92), p. 1 (TO-92 outline numbered 1 2 3; 1 collector, 2 base, 3 emitter)

Which one do you have?

Read the print on the flat face. BC547 with a B or C after it tells you the gain group; with no letter, the seller has not sorted them and you should assume group A, the lowest. The BC546 and BC548 come from the same family with the same pins: the 546 is the 65 V part, the 548 the 30 V one.

The pin order is the same from both makers checked here, C-B-E. If a board was laid out for a 2N3904 (E-B-C), the BC547 goes in with its flat face turned the other way.

Absolute maximum ratings

Both makers rate the BC547 for 45 V collector to emitter and 100 mA. They disagree on two lines. Diotec rates the emitter-base junction at 5 V and MCC at 6 V, and Diotec rates dissipation at 500 mW where MCC allows 625 mW. When a part may come from either maker, design to the lower figure: 5 V reverse on the base and 500 mW.

Absolute maximum ratings at 25 °C, from each maker's datasheet
Parameter	BC547TO-92	BC547TO-92
Collector-emitter voltage, base open VCEO	45 V	45 V
Collector-emitter voltage, base shorted VCES	50 V	not given
Collector-base voltage VCBO	50 V	50 V
Emitter-base voltage VEBO	5 V	6 V
Collector current, continuous IC	100 mA (200 mA peak)	100 mA
Power dissipation at 25 °C ambient Ptot	500 mW (leads at ambient 2 mm from the body)	625 mW
Thermal resistance, junction to air RθJA	under 200 K/W	200 °C/W
Junction temperature TJ	−55 to 150 °C	−55 to 150 °C

100 mA is an absolute maximum, not a working current. Diotec allows 200 mA only as a peak. A continuous load at 100 mA leaves no margin at all.

Gain groups, saturation and noise

Limits at 25 °C (min/max unless marked typical)
Parameter	BC547TO-92	BC547TO-92
DC current gain, group A hFEIC = 2 mA, VCE = 5 V	110 to 220	110 to 220
DC current gain, group B hFEIC = 2 mA, VCE = 5 V	200 to 450	200 to 450
DC current gain, group C hFEIC = 2 mA, VCE = 5 V	420 to 800	420 to 800
DC current gain, typical hFEIC = 100 mA, VCE = 5 V	120 / 200 / 400 typ (A / B / C)	not given
Saturation voltage VCE(sat)IC = 10 mA, IB = 0.5 mA	0.25 V max	not given
Saturation voltage VCE(sat)IC = 100 mA, IB = 5 mA	0.6 V max	0.3 V max
Base-emitter voltage in saturation VBE(sat)IC = 100 mA, IB = 5 mA	0.9 V typ (no max)	1 V max
Base-emitter voltage VBE(on)IC = 2 mA, VCE = 5 V	0.58 to 0.70 V	0.55 to 0.70 V
Transition frequency fTIC = 10 mA, VCE = 5 V, f = 100 MHz	300 MHz typ	150 MHz min, 300 MHz typ
Noise figure FIC = 200 µA, VCE = 5 V, RG = 2 kΩ, 1 kHz	10 dB max (2 dB typ)	not given

Figure 1. The A, B and C groups at 2 mA. They overlap at the edges, so a high A and a low B can measure the same.

The gain groups are tested at 2 mA, and that is the only current where either datasheet guarantees a minimum. At 100 mA Diotec gives typical figures only (120, 200 and 400 for A, B and C), and MCC gives nothing. For a switch, ignore gain and drive the base hard, as the example below does.

The makers also differ on saturation at 100 mA with 5 mA of base current: Diotec allows up to 0.6 V across the transistor, MCC 0.3 V. Use 0.6 V for heat sums. In audio stages its noise matters more than its current: Diotec gives a noise figure of 2 dB typical and 10 dB maximum at 200 µA.

What it is good for

Audio preamplifiers and small-signal stages at a few milliamps, where its gain and noise are best. The common-emitter lesson shows the stage most circuits use.
Switching small relays, buzzers and LEDs up to about 30 to 50 mA from a microcontroller or a 555.
Logic inverters, touch switches and the input stage of hobby circuits.

It is the wrong part for anything near 100 mA. A 2N3904 (200 mA, see the 2N3904 page) or a PN2222A (1 A, see the 2N2222 page) handles more, but both have the opposite pin order.

Worked example: driving a 12 V relay

A 12 V relay whose coil measures 400 Ω draws 30 mA. The BC547 switches it from a 5 V pin: emitter to ground, the coil between +12 V and the collector, a resistor from the pin to the base.

1. Pick the base current. Both datasheets specify saturation with the base current at a twentieth of the collector current. 30 mA / 20 = 1.5 mA.

2. Take the worst-case base-emitter voltage. MCC gives V_BE(sat) as 1 V at most. R ≤ (5 − 1) V / 1.5 mA = 2.67 kΩ.

3. Round down to a standard value. The next E12 value below is 2.2 kΩ, which gives between 1.82 mA and 2.02 mA of base current (the upper figure assumes V_BE as low as 0.55 V).

4. Do not lean on gain. No datasheet promises any gain at 30 mA, so the design relies on the 1:20 drive the saturation figures were measured with. A group B part would probably switch with a tenth of the drive, but nobody guarantees it.

5. Check the heat. At most 30 mA × 0.6 V plus 2.02 mA × 1 V = 20 mW. At 200 °C/W that is about 4 °C above the air. The resistor dissipates 9 mW.

Figure 2. The relay driver from the example. The diode across the coil takes the switch-off spike; the 10 kΩ keeps the transistor off while the pin floats.

From a 3.3 V pin: R ≤ 1.53 kΩ, so 1.5 kΩ, and a base current of 1.53 to 1.83 mA. That is well within what an ESP32 or Arduino pin can supply.

Figure 3. Base resistor and base current for the 30 mA relay from a 5 V pin and a 3.3 V pin.

Fit the diode across the coil, cathode to +12 V. Without it the switch-off spike can pass the 45 V rating; see the flyback diode and relay driving.

Worked example in numbers

Base resistor: 2.2 kΩ
Base current: 1.82 to 2.02 mA
Transistor heat: 20 mW
Junction rise: 4 °C

Tips and checks

Buy BC547B rather than unsorted BC547. An unsorted part is only promised a gain of 110; the B group's minimum of 200 leaves room in a circuit drawn around "a BC547".
Matching two transistors for a differential pair or current mirror? Take them from the same gain group and the same bag, then measure hFE on a meter and pair the closest.
A relay that clicks but chatters usually means too little base drive. Halve the base resistor before you blame the relay.
Use the 5 V emitter-base limit for any circuit where the base can be pulled below the emitter, such as an AC-coupled input. A small diode with its anode on the emitter and its cathode on the base clamps it.
Diotec's 500 mW assumes the leads stay at room temperature 2 mm from the body. On a crowded board with short, warm leads, budget less.

Substitutes, and the pin-order trap

The BC546 and BC548 drop straight in. The 2N3904 and PN2222A handle more current but use E-B-C, so they go in the other way round.

Pin order read with the flat face toward you, leads down, left to right
Part	Pins	Verdict
BC548, BC546	C-B-E	Same die family, same pins, same 100 mA. The BC548 is rated only 30 V; the BC546 takes 65 V. Either works for a BC547 in a 5 V or 12 V circuit.Diotec Semiconductor BC546 ... BC549 General Purpose NPN Transistors, p. 1
2N3904	E-B-C	Twice the current rating (200 mA) and a guaranteed gain at 50 mA, but the outer pins are swapped. Turn it round before you solder it into a BC547 footprint.Diotec Semiconductor 2N3904 General Purpose NPN Transistors, p. 1; Diotec Semiconductor 2N3904 General Purpose NPN Transistors, p. 2
PN2222A	E-B-C	The step up for loads above about 80 mA: rated 1 A. Pins reversed against the BC547, as with the 2N3904.onsemi (Fairchild) PN2222A / MMBT2222A / PZT2222A, p. 1

Figure 4. The BC547 is specified right up to its 100 mA rating, but that rating is the ceiling. The 2N3904 is rated higher and specified lower.

Finding the pins with a multimeter

Set the meter to diode test. Put the red probe on each lead in turn until you find the one that conducts to both others, about 0.6 to 0.7 V each. That is the base.
Reverse the probes: both junctions should read open, and the two outer leads should read open both ways.
With an hFE socket, try both orientations. The right one reads in the hundreds for a B or C part; the wrong one reads far lower.

The transistor testing lesson goes through it step by step, and continuity and diode test explains what the meter is doing.

What we'd do

Stock BC547B for audio stages, sensors and anything that switches 30 mA or less. Size every switch with the 1:20 base drive, not with the gain group. For 30 to 50 mA use a 2N3904, and above that a PN2222A, turning either round in a BC547 footprint and checking it with a meter before power goes on.

Questions people ask

What is the BC547 pinout?

Collector, base, emitter from left to right, with the flat face toward you and the leads pointing down. Pin 1 is the collector.

What do A, B and C mean on a BC547?

The gain group at 2 mA: A is 110 to 220, B is 200 to 450 and C is 420 to 800. The voltage and current ratings are the same for all three.

How much current can a BC547 handle?

100 mA absolute maximum, with 200 mA allowed only as a peak. Keep a switch at about half that, and size it for 0.6 V of saturation voltage.

BC547 or BC548: what is the difference?

Voltage. The BC548 is rated 30 V collector to emitter, the BC547 45 V and the BC546 65 V. Pins, current and gain groups are the same.

What is the PNP version of the BC547?

The BC557. Diotec lists the BC556 to BC559 as the complementary PNP parts for the BC546 to BC549.

Buying BC547 transistors in India

The BC547 is one of the cheapest parts in any Indian market, usually sold loose by the hundred. Ask for the gain group by name (BC547B) and for the maker; an unmarked bag can hold mixed groups. Meter a few from every bag before they go into a board. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.

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Datasheets this page was checked against

Diotec Semiconductor, BC546 ... BC549 General Purpose NPN Transistors (Version 2026-03-30)
Micro Commercial Components, BC546/BC547/BC548 NPN Silicon Amplifier Transistor (TO-92) (Rev.3-2-12012020)
Diotec Semiconductor, 2N3904 General Purpose NPN Transistors (Version 2026-03-10)
onsemi (Fairchild), PN2222A / MMBT2222A / PZT2222A (Rev. A3, August 2010)

Makers revise datasheets. If the copy you download has a newer revision and a number here disagrees with it, the datasheet wins, and we would like to hear about it.