BC547 Transistor: Pinout, Datasheet, Equivalent, SMD and A/B/C Guide

The BC547 transistor is the small NPN part I reach for first when I need to switch an LED, drive a small relay, or build a simple amplifier stage. It is cheap, available everywhere and well documented, but most pages about it give you a bare table and little else. This guide pulls everything into one place: the pinout, the datasheet numbers that matter, the A, B and C gain groups, the SMD version, and the equivalents you can safely use when you cannot find one.

Quick answer: The BC547 is an NPN transistor in a TO-92 package rated for 45 V and 100 mA, with a current gain (hFE) of 110 to 800 depending on the A, B or C version. Hold the flat face towards you with the legs down and the pins are collector, base, emitter from left to right. The SMD equivalent is the BC847. Direct swaps with the same pinout are the BC548, BC549 and BC337.

Why trust this guide: I am an electrical engineer, and I have used the BC547 Transistor in real builds: LED and relay drivers, sensor interfaces and small audio stages. The specifications below are cross-checked against the manufacturer datasheets, and the practical notes marked “From my bench” come from that hands-on work. Please verify any value against the datasheet of the exact part you buy before designing a product around it.

BC547 Transistor at a glance

1. What is the BC547 transistor?

The BC547 transistor is a low-power, general-purpose NPN bipolar junction transistor. It has three terminals: collector, base and emitter. A small current flowing into the base controls a much larger current flowing from the collector to the emitter. That single behaviour lets it work as an electronic switch or as an amplifier.

It is an NPN device, which means it conducts when the base is made about 0.6 to 0.7 V more positive than the emitter. If you need the opposite polarity, the matching PNP part is the BC557. People sometimes search “BC547 NPN or PNP” because the number does not tell you; the answer is NPN.

How the BC547 works: three operating regions

For switching, you push the transistor into saturation. For amplification, you keep it in the active region with a stable bias. Most beginner mistakes come from mixing the two: driving a switch with too little base current so it only half turns on, which makes it hot.

Typical uses

• Driving LEDs, buzzers and small relays from microcontroller pins (Arduino, ESP32, PIC)
• Signal buffering and level shifting between 3.3 V and 5 V logic
• Preamplifier and small-signal audio stages
• Oscillators, multivibrators and timer circuits
• Sensor interfaces, where its high-gain C version helps

From my bench: I treat 100 mA as a practical ceiling and rarely go past 50 mA. When a load needs more, I move to a BC337 or a MOSFET instead of stressing the BC547 Transistor .

2. BC547 pinout and pin configuration

Hold the BC547 with the flat face towards you and the legs pointing down. From left to right the pins are collector (1), base (2) and emitter (3). If you are ever unsure, the flat face with the printed part number is the front.

BC547C pinout

The BC547C pinout is identical to the BC547, BC547A and BC547B: collector, base, emitter from left to right with the flat side facing you. The suffix letter describes the gain group, not the pin order. The same C-B-E order applies to the BC548, BC549 and BC337.

Warning: The 2N3904, 2N2222 (plastic), S9014, 2SC1815 and C945 all look the same in TO-92 but use different pin orders. Dropping one into a BC547 footprint without flipping the legs is the most common way these parts get destroyed. Always check the datasheet for the exact part in your hand.

How to identify the pins without a datasheet

1. Put your multimeter in diode mode.
2. Find the pin that gives a reading of about 0.6 to 0.7 V to both other pins with the red probe on it. That is the base.
3. Once the base is found, the flat-face rule tells you which of the other two pins is collector and which is emitter. For a part with no clear markings, a transistor tester is the reliable way to confirm.

3. BC547 datasheet specifications

The BC547 datasheet (also written BC547 data sheet or Datenblatt in German) is short, but a few numbers decide whether the part works in your design. These are typical values from the Philips/Nexperia and onsemi datasheets. Add your datasheet PDF link here when you publish.

Absolute maximum ratings

Electrical characteristics

Answers to the most common datasheet questions

• BC547 current rating: 100 mA continuous, 200 mA peak.
• BC547 maximum voltage: 45 V collector-emitter, 50 V collector-base, only 6 V on the base-emitter junction in reverse.
• BC547 base current: the base needs far less than the collector. Design for 0.5 to 5 mA, then confirm the resulting collector current.
• BC547 frequency: fT is about 300 MHz, but for clean switching I keep signals in the low-MHz range.
• BC547 hFE and beta: 110 to 800 depending on group, and it falls at very low and very high currents.

From my bench: Never design around the typical hFE. I use the minimum for the group, which for the B version is 200, then add margin. That way the circuit still works with a different batch or a warm transistor.

4. The BC546, BC547, BC548 and BC549 family

The BC547 Transistor is one of four close relatives. They differ mainly in voltage rating and noise, which is why one often replaces another.

All four share the same C-B-E pinout in TO-92, so swapping within the family needs no rewiring. The PNP partners are the BC556, BC557, BC558 and BC559.

5. BC547A, BC547B and BC547C

The suffix letter on a BC547 is its gain group. The A version has the lowest hFE and the C version the highest. All three are the same silicon family with the gain sorted at the factory.

BC547B vs BC547C

For switching, either works if you drive the base hard enough. For small-signal amplification, the BC547C gives more gain but its gain varies more with temperature and current. If you are unsure, buy the BC547B. If a seller lists a plain “BC547”, expect any group; buy the lettered version when gain matters.

What do BC547BTA, BC547CBU and similar codes mean?

Extra letters after the group, such as BU or TA, describe the packaging (bulk or tape and ammo), not electrical differences. The electrical part is the same as the plain BC547B or BC547C.

6. BC547 SMD (SOT-23) and the BC847

There is no official BC547 SMD part number. When people search for BC547 SMD, they usually want the surface-mount version of the same transistor, and that is the BC847 in a SOT-23 package. It keeps the same NPN type, 45 V rating, 100 mA current and A, B, C gain groups, but it handles less power because of the tiny package.

BC547 vs BC847: what changes

Note the SOT-23 pin order is different from TO-92. If you are converting a design, redraw the footprint rather than assuming the pins line up. Marking codes on SMD parts can be ambiguous across manufacturers, so confirm against the datasheet before trusting a code. Search terms like “1AM”, “J3Y” or “1am smd transistor equivalent” often point to different parts than the BC847.

7. BC547 equivalent and substitute transistors

A BC547 Transistor equivalent is any NPN transistor that meets or exceeds its voltage, current and gain requirements in your specific circuit, with the pins connected correctly. Here are the substitutes I reach for, grouped by whether they are drop-in.

How to choose a replacement in five checks

1. Make sure the polarity is NPN.
2. Check VCEO is at least equal to the BC547 value for your supply, with margin for spikes.
3. Check collector current rating covers your load.
4. Compare hFE at your operating current, not just the headline range.
5. Verify the pinout on the datasheet and flip the legs if needed.

BC547 vs 2N2222

The 2N2222 is the stronger transistor. It switches several hundred milliamps, which a BC547 cannot. For loads under 100 mA both behave similarly, and the real trap is the pin order. Searches such as “BC547 vs 2N2222” or “difference between BC547 and 2N2222” mostly come down to current capability and pinout.

BC547 vs 2N3904

The 2N3904 sits between them at 200 mA and 40 V. It is a good alternative in most small-signal designs, but the E-B-C pinout means you must flip the legs compared with the BC547.

BC547 vs C945, S9014 and 2N5551

The C945 and 2SC1815 are 50 V, 150 mA parts with E-C-B pins. The S9014 is the closest ratings match at 45 V and 100 mA, with E-B-C pins. The 2N5551 is a 160 V part for high-voltage stages; it is not a general replacement, but it is the right answer when the BC547 voltage limit is the issue.

BC547 PNP equivalent

For a PNP complement use the BC557 (45 V, 100 mA). For higher current use the BC327, the PNP partner of the BC337. The BC558 and BC559 pair with the BC548 and BC549.

Not equivalents: BC140 is a medium-power TO-39 part, and the BC212 and BC307 are PNP types. Mistyped searches such as BC457 or BC5457 also appear; they are not BC547 variants.

If you search in another language, the same ideas apply: BC547 equivalente (Spanish, Italian), BC547 zamiennik (Polish), BC547 muadili (Turkish) and BC547 Datenblatt (German) all lead to the same substitutes listed above.

8. How to use the BC547 transistor in a circuit

Example: switching an LED from a 3.3 V pin

Take a 5 V supply, an LED with a 2 V forward drop and a 10 mA target current. The collector resistor is (5 V minus 2 V minus 0.1 V) divided by 10 mA, which is 290 ohm, so use a standard 270 ohm. For the base, drive about one tenth of the collector current, so 1 mA. From a 3.3 V pin the base resistor is (3.3 V minus 0.7 V) divided by 1 mA, which is 2.6 kohm, so use 2.2 kohm.

Driving a small relay

A 5 V relay coil often draws 70 to 90 mA, which is right at the edge of a BC547. I normally use a BC337 for relays, or keep the BC547 Transistor only for coils under about 50 mA. Always place a 1N4148 or 1N4007 diode across the coil, cathode to the supply side.

Simple amplifier note

In a common-emitter stage, bias the collector at about half the supply and add an emitter resistor for stability. With the emitter resistor un-bypassed, the voltage gain is roughly the collector resistor divided by the emitter resistor, which makes the stage far less sensitive to the wide hFE spread between BC547 parts. For quiet audio input stages, a BC549 is the better pick.

Common mistakes

• Wrong pin order when substituting another transistor
• No base resistor, or a base resistor that is too large
• Switching an inductive load without a flyback diode
• Running above 100 mA for long periods
• Forgetting a common ground between the microcontroller and the transistor circuit

9. How to test a BC547 with a multimeter

Set the meter to diode mode. For an NPN device like the BC547 Transistor , put the red probe on the base and the black probe on the emitter; you should read about 0.6 to 0.7 V. Repeat with the black probe on the collector and you should see a similar reading. Reverse the probes and both should read open (OL). Anything else points to a damaged part. This test checks the junctions only; it does not measure gain, so use a transistor tester if you need hFE.

10. Troubleshooting and buying tips

When you buy, choose a lettered version (BC547B or BC547C) from a reputable seller, since a plain “BC547” can be any gain group. Check the markings are crisp and consistent with the brand. A quick diode-mode check on a few parts from each batch catches many dead or mislabeled units.

11. BC547 transistor FAQ

Conclusion

The BC547 transistor remains the most practical small-signal NPN for hobby and light professional work: 45 V, 100 mA, three gain groups and a pinout that is easy to remember. Know the C-B-E order, pick the B or C version to suit your gain needs, treat the BC847 as its SMD twin, and use the BC548, BC549 or BC337 as direct swaps. For anything else, check the pin order before you power up.

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