The TO-18 packaged BC107 (NPN) / BC177 (PNP) and the TO-92 packaged BC547 (NPN) / BC557 (PNP) pairs are well-known and loved general-purpose transistors. We call them jellybean component, as they are as common as jellybeans in a jar. A common example is the Diotec Semiconductor BC546B as shown in Figure 1.
It may seem strange to use the word “loved,” but there are three generations (nearly 60 years) of enthusiastic educators and engineers who would disagree – they just happen to live on the other side of the pond. The closest American equivalents are the 2N2222 and 2N3904 families. We say close, as the BC107 family has a few surprises not typically included in their American counterparts requiring close attention to the part numbers.
Recently I posted an ode to the 2N3904 family. There was a surprising amount of backlash from Australia and Germany, with one user commenting, “Crikey my “go to” is a BC547.” With comments like that, we have no choice but to take a deep dive to discover the growing legacy of this simple transistor.
You little beauty!
DigiKey has millions of these devices in stock. This includes the original TO-18 BC107, the TO-92 BC547, and the SOT-23 BC847 packaging, plus variants including dual packaged devices.
The BC107 family is an unquestionable member of the jellybean component club. It would be very difficult to declare a winner between the 2N3904 and BC107. We may need to wait another 60 years to see who wins this competition.
Figure 1: First section of the Diotec Semiconductor BC546 datasheet.
Tech Tip: Which side of the road do you drive on? It’s a vital question directly related to the BC547 and the 2N3904. We could argue that the Motorola 2N3904 family has the correct E-B-C pinout. By contrast, the Philips BC547 family members drive on the left-hand side with a C-B-E pinout. Depending who you talk to, one of these devices is backwards. BTW, this side-of-the-road problem is very dangerous for tourists e.g., Americans always look to the right before stepping out into the street. They also tend to smoke BC547 transistors.
Understanding the BC107, BC547, and BC847 family part numbers
The general transistor part numbers are described in the October 1970 Philips Semiconductors and Integrated Circuit Data Handbook:
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First letter: The letter “B” indicates the semiconductor material with a band gap between 1.0 to 1.3 eV (silicon).
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Second letter: The letter “C” identifies the transistor designed primarily for audio frequency applications.
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Series Number: Three figures are used to identify the individual semiconductor.
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Gain suffix: Close inspection of the BC107 datasheet reveals that a gain designator is appended to the series number. The BCXXXA has a typical DC current gain 90, the BCXXXB 150, and the BCXXXC is 270.
Tech Tip: The BC108 is a family member with a lower V_{CE} rating. The BC109 is a low voltage BC108 with superior noise characteristics. Please leave a comment if you would like to explore these components. They have been omitted as this article is already very long.
Meet the BC107’s extended family
The BC107 family can be a handful when it comes to selecting the perfect transistor for your needs. The most important piece is to recognize that the TO-18 packaged BC107 and the TO-92 packaged BC547 share the same internal silicon die. For example, a BC107B and a BC547B should have similar specifications. The same statement applies to all other family members.
The complete family includes the following complementary pairs:
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TO-18 BC107 (NPN) / BC177 (PNP) with TO-92 counterpart as BC547 (NPN) / BC557 (PNP).
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TO-18 BC108 (NPN) / BC178 (PNP) with TO-92 counterpart as BC548 (NPN) / BC558 (PNP).
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TO-18 BC109 (NPN) / BC179 (PNP) with TO-92 counterpart as BC549 (NPN) / BC559 (PNP).
As stated in the previous section, each family member is available in one of three gain bins, including the A, B, and the high gain C designation. Therefore, there are 36 variations within the family.
But wait, jellybean components often have variants and derivatives. The true nature of the BC107 family is revealed when we include the BC847 components:
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TO-92 BC547 (NPN) / BC557 (PNP) with SOT-23 counterpart BC847 (NPN) / BC857 (PNP).
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TO-92: BC548 (NPN) / BC558 (PNP) with SOT-23 counterpart as BC848 (NPN) / BC858 (PNP).
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TO-92: BC549 (NPN) / BC559 (PNP) with SOT-23 counterpart as BC849 (NPN) / BC859 (PNP).
As before, there are gain bins (A, B, and high gain C) for each family member.
How do we locate the family members in DigiKey?
The DigiKey search engine is designed to locate the needle in the haystack. For example, you may search for a “BC547”. The results are not useful, as we have not specified the gain. A better search is “BC547B” to find the mid-gain devices. The best search occurs when we hack the search engine using keywords “BC547 and BJT”. This returns a list showing all DigiKey BC547X transistors. We see that the B and C gain version tend to be the most common.
Similar searches can be done for the original TO-18 devices using keywords “BC107 BJT” and the SOT-23 devices using keywords “BC847 BJT”. Be sure to adjust your keywords to search for the PNP complementary devices e.g., “BC557 BJT”.
Identification of variants
The variants typically show up when searching “BC847 BJT.” An example is the Nexperia BC847BPN,135. This is a fun device that includes both an NPN and a PNP transistor in the same package, as shown in Figure 2.
You can determine if this Nexperia part falls under the BC107 jellybean umbrella. However, it would be very difficult to argue against legacy.
Figure 2: Image and pinout for the Nexperia BC847BPN,135.
Potential problems for the BC107, BC547, and BC847
It’s a fact that most transistors are used as switches. Therefore, it’s important to place the transistor deep into saturation using a condition known as forced beta. Problems arise if we neglect the gain specification (A, B, and high gain C).
As an example, select the BC547C to drive a 40 mA relay. Also, assume a 3.3 VDC logic is used to drive the base via a 2.2 kΩ series resistor. Suppose the board house selects a lower gain BC547B or even worse, a BC547A. There is a real chance that the base current is insufficient to fully turn on the load. This could be particularly troublesome, as the load may operate intermittently, compounded with a temperature-dependent operation.
Other precautions include providing protection against inductive flyback voltage and overheating. This is especially important for surface mount devices which are sensitive to PCB construction for proper cooling. Be sure to follow the manufacturer’s specifications to properly derate the transistor.
Parting thoughts
Like the 2N3904 family, the BC107 family has roots extending back nearly 60 years. Both jellybean families are immensely popular and will likely remain so for decades to come.
If you’re an educator, I recommend Colin Michell’s free ebook 200 Transistor Circuits. The .au bias is strong in this book with a BC547 on nearly every page. For a real challenge, have your students select a circuit, and then produce a PCB featuring SMD equivalent components. By standing the project on its head, students are forced to explore the original through-hole components as well as the SMD equivalents. It’s a challenge, but one with clear guardrails ensuring student success.
What is your favorite European jellybean?
Best wishes,
APDahlen
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About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.