BC547 transistor: everything you need to know

BC547 transistor

If you are a maker, you like DIY and electronics, surely you have ever needed to use a BC547 transistor. It is a bipolar junction transistor that was originally developed by Philips and Mullard between 1963 and 1966. Initially it was named with the BC108 nomenclature and had a TO-18 type metal encapsulation (Transistor Outline package - case style 18 ). That package was considerably more expensive than the plastic equivalent TO-92, but the heat dissipation is better in the former.

Later it would have a new plastic package and renamed with the code BC148. And it evolved from BC108, BC238, to what we now know as BC548 with an encapsulation cheaper type TO-92, and from here came variants such as the BC547. The differences between the series were basically encapsulated, being the same inside. In addition, for its acronym BC It shows that it is a silicon transistor (B), for low frequency (C).

There are also other designations such as the BF, but in this case it is used to identify the transistors used for RF (radio frequency), that is, those that achieve good gains at very high frequencies.

BC5xx family overview:

NPN diagram

The BC547 belongs to the family of transistors with similar characteristics as the BC546, BC548, BC549 and BC550. They are all of the bipolar or bipolar junction type (BJT for Bipolar Junction Transistor). That is, they are not field effect transistors like FETs, light-controlled phototransistors, etc. These types of bipolar transistors are made of materials such as germanium, silicon or gallium arsenide.

The name of bipolar comes from the fact that they form 2 PN junctions, since transistors have three semiconductor layers arranged in two possible ways: NPN and PNP. In the case of BC547 we have already said that it is an NPN. That is, a semiconductor doped with an element of the periodic table that allows it to have an excess of charge carriers (electrons) for the N parts, and a semiconductor doped with an element with fewer valence electrons giving rise to a P-type semiconductor with an excess of positive charge carriers in this case (holes).

That said, if we focus on family, the differences between all the members it is quite mild. The encapsulation of all is the same, the SOT54 or TO-92. But each one has been optimized for a specific type of task:

  • BC546: for high voltage (up to 65v).
  • BC547: also for high voltage (45v)
  • BC548: for normal voltages, up to 30v.
  • BC549: similar to BC548 but with low noise for somewhat more critical applications or sensitive to electronic noise. For example, hi-fi sound systems.
  • BC550: similar to the first two, that is, for high voltage (45v) but has been improved to offer low noise.

All of them have three pins, as is logical in transistors. To identify them, we must look at it from the chamfered or flat face of the encapsulation, that is, leaving the rounded face for the other side. Thus, from left to right the pins are: collector - base - emitter.

  • Manifold: it is a metal pin or pin in contact with an area less doped than the emitter. In this case it is an N zone.
  • Base: it is the pin or metallic contact connected to the middle zone that must be very thin. In this case it is zone P.
  • Transmitter: the contact connected to the other end (zone N in this case) and which must be a highly doped region to provide the greatest amount of carriers to the current.

Once this is known, we will better understand how transistor BC works. In the specific case of the BC5xx, output currents of up to 100 mA. That is, this would be the maximum intensity that can flow between the collector and the emitter, controlled by the base as if it were a switch. In the case of the maximum accepted stresses, this varies depending on the model as we have seen.

Remember that the maximum current intensity of 100mA is only for the DC, since for the alternating current where there are point peaks of short duration it could go up to 200 mA without destroying the transistor. However, some manufacturers such as the mythical and historical Fairchild have even built BC547 models that can reach 500mA, even if it is not standard. So you can find perhaps datasheets of the BC547 with voltages somewhat variable to what is specified here ...

Features of the BC547:

bc548 pins and symbol

After learning about some things in common with family members, let's focus on some magnitudes and specific features for the BC547.

Gain:

La current gain, when we talk about the common base, it is approximately the current gain from the emitter to the collector in the direct active region, always less than 1. In the case of the BC548, like its family brothers, they have a very good gain of between 110 and 800 hFE for direct current. This is usually specified with an extra letter at the end of the nomenclature that indicates the gain range considering the tolerance of the device. If there is no such letter then it could be any within the range I have given. For example:

  • BC547: between 110-800hFE.
  • BC547A: between 110-220hFE.
  • BC547B: between 200-450hFE.
  • BC547C: between 450-800hFE.

That is, the manufacturer estimates that it will be between those ranges, but it is not known what exactly the real gain is, therefore we must worst case when we design the circuit. In this way, it is guaranteed that the circuit is functional even if the gain is the minimum of the range, as well as guaranteeing that the circuit would continue to work if we replace said transistor. Imagine that you have designed the circuit so that it works with a minimum of 200hFE and you have a BC547B but you decide to replace it with a BC547A or BC547, it may not reach that rate and it will not work ... On the other hand, if you do it so that it works with 110, either will work for you.

Frequency response:

La frequency response it is very important for amplifiers. Whether it is possible to work with one or other frequencies will depend on the frequency response of the transistor. This will remind you of something if you've studied topics like high pass and low pass frequency filters, right? In the case of the family seen here, and therefore of the BC547, they have good frequency response and can work at frequencies between 150 and 300 Mhz.

Normally, in datasheets Full details of the transistor are given from the manufacturers, including a graph of the frequency response. These documents can be downloaded in PDF from the official websites of the device manufacturers, and there you will find the values. You will see the frequency response with the initials fT.

These maximum frequencies will guarantee that the transistor amplify at least 1, since the higher the frequency, the lower the amplification of the transistor due to the capacitive part of it. Above those acceptable frequencies, the transistor could have very little or no gain, so it does not compensate.

Equivalences and complementation:

You may find yourself in the dilemma of having to use a different type of transistor or complementary to the BC547 in a circuit. That is why we are going to show some equivalences or antagonists.
  • Equivalents:
    • Similary: an equivalent hole board mount transistor would be the 2N2222 or PN2222 to which we are going to dedicate another special article. But beware! In the case of the mythical 2N2222, the emitter and collector pins are reversed. That is, it would be emitter-base-collector instead of collector-base-emitter. Therefore, you must weld it or place it rotated 180º with respect to how you had the BC547.
    • SMDIf you want a surface mount equivalent to BC547 for smaller sized printed circuits or PCBs, then what you are looking for is a BC487 encapsulated under SOT23. That would avoid having a plate with holes for mounting and soldering. By the way, if you are looking for equivalent bipolar transistors for the other members of the family, you can check out the BC846, BC848, BC849 and BC850. That is, replace BC4xx with the equivalent BC8xx.
  • Complementary: Another situation that can occur is that you want the opposite, that is, a PNP instead of an NPN. In that case, the correct one would be the BC557. To find complementary items for the rest of the family members, you can use the BC5xx such as: BC556, BC558, BC559 and BC560.

I hope this post has helped you and the next will be PN2222.


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  1.   Rodolfo said

    Hello, good afternoon, I found this article very useful as I am repairing and replacing the transistors on an old Audinac FM900 amplifier. Thank you !!!

    1.    Isaac said

      Hello, thank you very much for commenting

  2.   rafael said

    very good, just the info I was looking for, congratulations

  3.   Manuel Aguirre said

    Well, that range of variations with respect to the BC 547 transistor is very important. I wonder if you could give me a diagram with the BC547 to make a "Pre" with the Electret. That is, make a circuit with the Electret (Microphone) and connect it to a Mono amplifier. To deliver uplifting messages in favor of those who visit Facebook or other advertising media. The information that you have given is excellent and well explained clearly. I THANK YOU for your post.
    May our heavenly Father bless you along with your loving family.
    I am from the country El Salvador CA Thank you.

  4.   ren said

    Excellent article and thank you!

  5.   Tino Fernandez. said

    This document has several errors, among them the most serious is the following:
    … In addition, by its acronym BC it shows that it is a Common Base topology….

    The acronym BC for the transistor has nothing to do with what it says, since the B indicates that it is a silicon transistor, and the C that it is a low-frequency transistor.
    You can see it on this page:
    https://areaelectronica.com/semiconductores-comunes/transistores/codigo-designacion-transistores/#:~:text=En%20la%20nomenclatura%20americana%20los,facilitado%20por%20el%20fabricante%20herunterladen.

    There are more errors in this document:
    . . . The current gain, when we speak of the common base, is approximately the current gain from the emitter to the collector in the direct active region….

    When mentioning the common base, it is understood that it is a common base assembly, in which case the current gain is always less than 1.
    When talking about the gain of transistors it is never necessary to mention the type of configuration.

    I have been an electronics teacher for more than 35 years.

    A greeting.

    1.    Isaac said

      Hello,
      Sorry for the mistakes. Thank you very much for advising.
      A greeting!