Transistor BC547: all you need to know

Transistor BC547

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

Later it would have a new plastic package and renamed with the code BC148. And it was evolving from BC108, BC238, to what we know today as BC548 with a cheaper TO-92, package and from here the variants like BC547 emerged. The differences between the series were basically the packages, being equal inside them. Furthermore, by its acronym BC it shows that it is a Common Base topology, achieving a high voltage gain in its output without inverting the output signal. Unlike EC (Common Transmitter), which is the only one to achieve both voltage and current gains, and CC (Common Collector) very useful to adapt stages with very different impedances respectively.

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

BC5xx family overview:

 

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

The name 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 some element with less 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 the family, the differences between all members are quite slight. 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 a low noise level for somewhat more critical or sensitive electronic noise applications. For example, high fidelity sound systems.
  • BC550: similar to the first two, that is, for high voltage (45v) but has been improved to offer low noise.

They all have three pins, as is logical with transistors. To identify them, we must look at them from the chamfered or flat face of the package, that is, leaving the rounded face for the other side. Thus, from left to right the pins are: collector – base – emitter.

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

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

Remember that the maximum current of 100mA is only for the direct current, since for the alternating current where there are punctual peaks of short duration it could be increased up to 200mA without destroying the transistor. However, some manufacturers like the mythical and historical Fairchild have come to build models of BC547 that can reach 500mA, although it is not standard. So you can find perhaps BC547 datasheets with somewhat variable voltages to what is specified here …

BC547’s particularities:

bc548 pins and symbol

After knowing some things in common with the family members, let’s focus on some magnitudes and characteristics specific to BC547.

Gain:

The current gain, when talking about the common base, is approximately the current gain from the emitter to the collector in the direct active region. In the case of 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 indicating the gain range considering the tolerance of the device. If there is no such letter, then it could be any one 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 to say, the manufacturer estimates that it will be between those ranges, but it is not known exactly what the real gain is, so we must put ourselves in the worst case when we design the circuit. That way it is guaranteed that the circuit is functional even if the gain is the minimum of the range, and it is also guaranteed that the circuit will continue to work if we replace the transistor. Imagine that you have designed the circuit to work 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 may not work… If you make it to work with 110, either one will work.

Frequency response:

The frequency response is very important for amplifiers. It depends on the frequency response of the transistor whether you can work with one frequency or the other. This will remind you something if you have studied topics like high pass and low pass frequencies, 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, the manufacturers’ datasheets give all the details of the transistor, including a graph of the frequency response. These documents can be downloaded in PDF from the official websites of the manufacturers of the devices, and there you will find the values. You will see the frequency response with the acronym fT.

These maximum frequencies will guarantee that the transistor amplifies at least 1, since the higher the frequency, the lower the amplification of the transistor due to the capacitive part of it. Above these acceptable frequencies, the transistor may have very low or almost no gain, so it does not compensate.

Equivalences and complementation:

You may be faced with the dilemma of having to use a different or complementary type of transistor to the BC547 in a circuit. That’s why we’re going to show some equivalencies or antagonists.

  • Equivalents:
    • Similar: a plate-mounted transistor with equivalent holes would be the 2N2222 or PN2222 to which we will dedicate another special article. But beware! In the case of the legendary 2N2222 the pins of the transmitter and collector are reversed. That is, it would be emitter-base-collector instead of collector-base-emitter. Therefore, you have to solder it or place it turned 180° from the way you had the BC547.
    • SMD: if you want a surface mount equivalent to the BC547 for smaller printed circuits or PCBs, then what you are looking for is a BC487 encapsulated under SOT23. That would avoid having a board 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 look at BC846, BC848, BC849 and BC850. That is, replace BC4xx with the equivalent BC8xx.
  • Complementaries: another situation that can happen is that you want the opposite, that is, a PNP instead of an NPN. In that case, the right one would be BC557. To look for complementary for the rest of the family members you can use the BC5xx like: BC556, BC558, BC559 and BC560.

 

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

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