The BC548 is a general purpose silicon NPN BJT transistor found commonly in European electronic equipment; the part number is assigned by Pro Electron, which allows many manufacturers to offer electrically and physically interchangeable parts under one identification. The BC548 is commonly available in European Union and Commonwealth Countries and is often the first type of bipolar transistor young hobbyists encounter. The BC548 is often featured in circuit diagrams and designs published in Electronics Magazines such as "Silicon Chip" and "Elektor".
As a representative of the large family of bipolar transistors the BC548 provides a "stepping off point" to the use of more esoteric, higher voltage, current or frequency devices for beginners.
If the TO-92 package is held in front of one's face with the flat side facing toward you and the leads downward, (see picture) the order of the leads, from left to right is collector, base, emitter.
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The exact specs of a given device depend on the manufacturer. It is important to check the datasheet for the exact device and brand you are dealing with. Philips and Telefunken are two manufacturers of the BC548.
The JEDEC numbered NPN 2N3904 and PNP 2N3906 possess very similar electrical characteristics to the BC548 and BC558 respectively but the pin outs are reversed. If the TO-92 package for these devices is held in front of one's face with the flat side facing toward you and the leads downward, (see picture) the order of the leads, from left to right is emitter, base, collector.
The Electronic Industries Association of Japan (EIAJ) assigns the 2SC prefix to NPN transistors for high frequency use.[2] The NPN 2SC1815 and PNP 2SA1015 are usually the most the common general purpose devices which are encountered and also possess characteristics which are similar to the BC548 and BC558. If the TO-92 package for these devices is held in front of one's face with the flat side facing toward you and the leads downward, (see picture) the order of the leads, from left to right is emitter, collector, base.
Generally all of the above devices can be used interchangeably in low powered hobby electronic circuits operating at frequencies op to 200 MHz or so. "Elektor" magazine has coined the term "DUS TUP TUN", meaning "Diode Universal, Silicon", "Transistor universal P.N.P." and "Transistor Universal N.P.N.", where the "DUS" means something like a 1N4148 or 1N914, two very commonly encountered small signal silicon diodes, the "TUP" referring to the BC558, 2N3906 or 2SA1015 and the "TUN" referring to the BC548, 2N3904 or 2SC1815. This allowed their magazine to be sold in different geographical realms and circuit constructors need only be aware of the pin-out configurations of the devices available in their particular realm. In more demanding applications, such as power conversion or high frequency equipment, devices must be more carefully chosen depending upon the task and position in the circuit.
The BC140 and BC160 are complimentary devices which are similar to the BC548 and BC558 but possess a collector currents of 1A and a Vcebo of 80v. These are similar to the North American 2N2219 and 2N2906.
The BC639 and BC640 are similar to the above two devices but possess a greater Vcebo of 100v. The pin outs of these devices differ from most of the other "BC" family with the collector lead in the middle for heat dissipation. These devices are similar to the North American 2N2222 ans 2N2907.
There are many other devices based on the BC54x family, such as the surface-mount versions of the BC547, 548 and 549; the BC847, BC848 and BC849 which are almost electrically identical but housed in tiny surface mount TO-236 packages. These tiny packages are marked with a two or three symbol code which must be translated back into the actual number using a conversion table because the actual number is too large to fit on the tiny package top.