The BC548 is a general purpose NPN bipolar junction transistor found commonly in European electronic equipment and present-day designs in Australian and British electronics magazines where a commonly-available low-cost NPN transistor is required. It is a part of a family of NPN and PNP epitaxial silicon transistors that include higher-quality variants, originating in 1966 when Philips introduced the metal-cased BC108 family of transistors which became the most used transistors in Australia and taken up by many European manufacturers. The BC548 is the modern plastic packaged BC108, and can be used in any circuit designed for the BC108 or BC148, which includes many Mullard and Philips published designs.
The BC546 and BC547 are essentially the same as the BC548 but selected with higher breakdown voltages while the BC549 is low noise version, and the BC550 both high-voltage and low-noise. The BC556 to BC560 are the PNP counterparts of the BC546 to BC550, respectively.
The BC548 is low cost and is available in most European Union and many other countries. It is 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 part number is assigned by Pro Electron, which allows many manufacturers to offer electrically and physically interchangeable parts under one identification. As viewed in the image to the right, and going from left to right, lead 1 (left in diagram) is the collector, lead 2 is the base, and lead 3 is the emitter. Note that not all transistors with TO-92 cases follow this arrangement.
The type number of any of the devices in this "family" may be followed by a letter to indicate a narrow range of gain (hFE) spread (although it is not so common for a BCxx7 or BCxx8 part to be available with a "C" gain grouping).
- "A" indicates low gain (110 to 220 at 2 mA),
- "B" indicates medium gain (200 to 450)
- "C" indicates high gain (420 to 800)
So a BC548 might have a current gain anywhere between 110 and 800, but the gain of a BC548A would be within the range of 110 to 220.
Devices registered to this Pro Electron number must have 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)
In the summer of 2013, the manufacturer's budgeted cost for the part is less than US $0.03 in lots of 1000.
The rated power dissipation for transistors is the total power developed across both junctions of the transistor that will raise the internal temperature to the maximum permitted (i.e. not something that should be maintained in normal use), and will be specified for a given ambient temperature for low-power transistors such as these, in this case 25 degrees Celsius. In practice factors such as the proximity of the transistor to the printed circuit board will influence how well heat can be removed from the transistor and proximity to other heat-generating components will increase the ambient temperature - and probably reduce the permissible dissipation below the 500-625 mW ideal-conditions specification.
The BC548 and BC549, and their PNP counterparts (BC558 and BC559) can be used in circuits where voltages reach no more than 30 Volts, limited mainly by their VCEO rating. The VCBO rating refers to the maximum voltage between collector and base with the emitter open-circuit (not typical operation), and their predecessors, the BC108 and BC109, while having VCBO or VCES ratings of 30 V have only a 20 VCEO) rating, meaning a BC548 (or BC549) can replace a BC108 but a BC108 might not be a safe replacement for a BC148.
The BC546 and BC547 have higher voltage ratings; the BC549 has lower noise, and the BC550 has both higher voltage and lower noise, and the last digit of the type number follows a pattern common to several other transistors tabulated for the BC108 family of transistors.
Some manufacturers specify their parts with higher ratings, for example the Fairchild 1997 datasheet (547ABC, Rev B) for the BC547, sourced from Process 10 gave 500mA as the maximum collector current, while their datasheets dated 2002 have dropped the current rating to the standard 100mA.
PNP Versions of BCxxx
The PNP counterparts of the BC546 to BC550 are the BC556 to BC560 respectively, i.e. the type numbers are higher by ten.
The surface-mount package versions are the BC846 to BC850 (and PNP versions: BC856 to BC860).
The noise figure of the BC548 is less than 10 dB and typically 2 dB at a collector current of 0.2 mA; the low-noise counterparts: BC549 and BC550 are specified to have a noise figure of less than 4 dB and typically 1.4 dB under the same conditions, while the low-noise PNP complements - BC559 and BC560 - have a slightly lower typical noise figure of 1.2 dB.
- ""MiniWatt:" DIGEST" (PDF). Vol 7 Number 2. Philips Australia. February 1968. Retrieved 17 March 2014.
- "BC546 through BC550".
- Motorola Small Signal Transistor Data Book (1984 ed.). pp. 2–97,8–3.
- World's Transistor Comparison Tables, Tech/ECA, 1993, ISBN 981-214-444-7
- "BC548 − NPN Epitaxial Silicon Transistor". Retrieved 28 June 2013.
- BC546/7/8/9 data sheet
- BC556/7/8/9 data sheet