This article describes the BC547 NPN transistor, its pinout, equivalents and operating principles, and its application in various electronic devices. It comes primarily in a TO-92 or advanced TO-226 package. The maximum output current this semiconductor device can handle is 100mA.
It also has very good gain (up to 800 hFE) and low noise (up to 10 dB), making it ideal for primary signal amplification stages. Its ability to operate in the 300 MHz band allows it to be called high-frequency. Typical saturation voltage is only 90mV, which is a definite advantage when used in circuits as a switch.
Bc547 first appeared on the market in April 1966, courtesy of Philips (Holland) and Mullard (UK). It was a joint development of the then popular bc107. It was identical in its technical specifications, but was produced unlike the metal bc107 in a plastic sealed housing TO-92. It is currently the current replacement for the older BC107 or BC147, which are included in many developments by Mullard and Philips.
The BC547 pinout will be discussed in the TO-92 (TO-226AA) package, which has three flexible leads for hole mounting. Looking at the bevel from the front, the purpose of these pins is from left to right: collector, base, emitter. The figure shows the basic appearance of the device, which will vary slightly depending on the particular brand, but the specs and pin assignments remain identical.
The datasheet for bc547 usually includes a description of similar, in terms of specifications, transistors of the series: BC546, BC548, BC549 and BC550. Similar, but not exactly. They do differ from each other. For example, bc547 differs in the threshold voltages and is in the table of maximum parameters between bc546 and bc548. Also, all types of devices are divided into groups of maximum current amplification factor hFE- from A to C. Group “A” will have the smallest gain and “C” will have the largest gain.
Bc547, bc548, bc549 are the same transistors built on the same production line. During their testing process just before release, based on VBCO and VCEO measurements and noise components, they are classified as -7, -8 or -9.
A detailed description can be found in the manufacturer’s datasheet. It usually includes a table of maximum allowable operating parameters and electrical specs, at which the device is stable.
The maximum allowable operating parameters are stated by the manufacturer at the beginning of the datasheet. They include the following parameters:
- VCEO -shows the maximum potential difference that can be applied between the collector-emitter contacts. For example, the BC547 is not capable of holding more than 45 volts, so this value is listed as the safe operating voltage to be applied to the collector load.
- IC (max) is the maximum collector current that can be applied across the collector-emitter pins. For the bc547 it should not be more than 100mA, because this value will be the breakdown limit, above which the device will surely burn out. So you can see that it starts to heat up well even before reaching this limit, already at 60 mA. Therefore it is recommended to use it at values less than half of IC (max).
- PC (max) is the maximum power of the devices or the nominal load that can be connected through its collector-emitter. This value is quite consistent with and interrelated to IC(max) and is 500 mW or half a watt for the entire group.
The addition “max”, in the designation of permissible parameters, indicates their maximum values, but sometimes it is omitted in the description. The following is a complete list of bc547 operating limits, taken from Fairchild Semiconductor’s datasheet.
Now let’s take a look at the electrical parameters of the bc547. These are listed by the device manufacturer immediately after the description of the limit values. In these spec, a separate column (test condition) indicates the values at which the device was tested by the manufacturer. Generally, testing is performed at ambient temperatures of no more than 25 degrees Celsius.
The BC547 transistor has a fairly large current Gain (hFE). Group “C”, according to the hFE U classification, starts at 420 and ends at 800. These values are very important for a bipolar and are one of the first criteria for its selection. Increasing the hFE level simply assigns more sensitivity to a particular device, which means that it is capable of starting at the lowest base currents but still switching heavier loads through its collector.
The low-noise transistor, sharpened for amplification of weak high-frequency signals, almost always has a complementary with another type of conductivity and close in magnitude to the gain of hFE. This is due to the widespread use of such devices in the primary amplification stages in the pair. The complementary pair with PNP structure for it is BC557.
A complete modern equivalent to the BC547 transistor is the bc550. Also, before you look for equivalents, it is recommended to look at the datasheet neighbors with only small differences in breakdown voltage thresholds:
Some radio amateurs use: 2n3904, 2n4401, bc337, bc639, 2N3055, 2N2369, 2SC5200 as replacements.
Another, one of the most common substitutes is the 2N2222 transistor. It has similar specifications, including pinout and housing. The only differences are higher power dissipation (up to 625 mW), collector current up to 600 mA and slightly increased input and output capacitances. The input and output capacitances can only affect the circuits during high frequency operation. Thus, if more gain is needed, the 2n2222 can be used.
The device series KT3102, from the Minsk manufacturer of electronic components “Integral”, will also be suitable for replacement. There are also Russian equivalents of transistor BC547, among which are KT3102G and KT3102E, if you choose the gain (up to 1000 hFE ), they are even better than BC547c. Below is a correspondence table for the different groups in this series.
The BC547 was developed by Philips in 1966 in Holland, so its marking corresponds to the European Pro Electron system. The first letter indicates the type of semiconductor used – “B” for silicon. The second letter indicates the frequency of operation – “C” low-power, low-frequency. In spite of the fact that it is high-frequency (up to 300 MHz), the manufacturer for some reason indicated it in the marking as low-frequency. History is silent about the reasons for this designation. Sometimes they don’t write the first letter in the designation and then it turns out: c547b, c547c, c547c, c547b.
A little about standards
Manufacturers are constantly improving the manufacturing process and can change the specified specifications, but they must not be less than the values registered for bc457 in the Pro Electron standard. For example, On Semiconductor has a maximum power (at 25°C) of 625 mW, which is currently the most common. At Philips, the hFE gain for group “B” ranges from 220 to 475. Some manufacturers have appeared to support pulsed collector current (up to 200 mA). Therefore, please refer to the datasheet before using the device in your projects.
How it works
When input voltage is applied to the terminals, some current (IB) begins to flow from the base to the emitter and controls the collector current (IC). The voltage between base and emitter (VBE) for an NPN structure must be direct. That is, a positive potential is applied to the base and a negative potential is applied to the emitter. The polarity of the voltage applied to each pin is shown in the figure below.
The input signal is amplified at the base and then transmitted to the emitter. A smaller current in the base is used to excite a larger current between the collector and the emitter (IC).
Transistors of the n-p-n structure are sometimes called semiconductor devices with reverse conductivity.
When the transistor is open, it is able to pass (IC) up to 100 mA. This stage is called the saturation region. At this stage, the permissible voltage between the collector and the emitter (VBE) can be about 200 mV, and the VBE can reach 900 mV. When the base current stops flowing, the transistor turns off completely, this stage is called the cut-off region, and the VBE will be about 650 mV.
It is widely used in control and amplification mode, in various control circuits of relay drivers, LEDs, motors, as well as in amplification circuits of low and high frequency signals. Examples of schemes and ways to create simple devices, the method of mounted mounting, can be seen in the video. It provides information about the possibilities of using the BC547 in some projects: delayed shutdown with your own hands, automatic lighting, LED strobe light, the simplest alarm system and a sound amplifier.
Companies such as NXP, Philips, Micro Electronics, Fairchild, ON Semiconductor, Vishay and many others are leaders in producing this device.
You can view and download the datasheet from each manufacturer in the next section.