The BC548 is a general-purpose NPN bipolar junction transistor commonly used in European and American electronic equipment. It is notably often the first type of bipolar transistor hobbyists encounter and is often featured in designs in hobby electronics magazines where a general-purpose transistor is required. The BC548 is low in cost and widely available. History and usage The BC548 is a part of a family of NPN and PNP epitaxial silicon transistors that originated with the metal-cased BC108 family of transistors. The BC548 is the modern plastic-packaged BC108; the BC548 article at the Radiomuseum website describes the BC548 as a successor to the BC238 and differing from the BC108 in only the shape of the package. Datasheets for the BC548 give specifications that are identical to, or exceed, those of the BC108, BC148 and BC238 predecessors. Thus the BC548 (or BC546 to 550) is a valid substitute in any circuit designed for the older BC108 (or BC148), which includes many Mullard and Philips published designs. Pinout and specifications The BC548 is supplied in a standard TO-92 3-pin package. The assignment of transistor elements (b,c,e) to leads, i.e. the "pinout", uses the same convention used by some - but not all - other TO-92 devices. As viewed in the top-right image, going from left to right, the pinout is as follows: lead 1 (left in diagram) is the collector, lead 2 is the base, and lead 3 is the emitter. Sometimes the middle pin is supplied bent to form a triangle of leads (as found in TO-18 case transistors and, for example, the ZTX108-L) to match the pinout of the BC108 more exactly. The BC548 part number is assigned by Pro Electron, which allows many manufacturers to offer electrically and physically interchangeable parts under one identification. Devices registered to this Pro Electron number must have the following minimum performance characteristics: Breakdown voltage, collector-to-emitter with base open-circuit VCEO = 30 V (see below) Rated continuous collector current IC = 100 mA (Fairchild's BC548 at one time had a higher rating) Rated total power dissipation Ptotal = 500 mW (some manufacturers may specify 625 mW - see below) Transition frequency (gain-bandwidth product) ft = 150 MHz minimum (300 MHz typical) Variants The BC548 was released with, and often is found together in datasheets with, the BC547 (higher voltage) and BC549 (lower noise) devices, that correspond to the original BC107 and BC109 variants of