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BC548 From Wikipedia, the free encyclopediaJump to:navigation, search BC548 transistor The BC548 is a general purpose silicon, NPN, bipolar junctiontransistor foundcommonly in European electronic equipment. It is electrically similar to the NorthAmerican2N3904and Japanese 2SC1815 but has different lead assignments.If theTO-92package is held in front of one's face with the flat side facing toward youand the leads downward, (see picture) the order of the leads, from left to right is collector, base, emitter . Note that the pin assignment of the complementary PNP device BC558 is exactly the same. Contents [hide] ã 1 Specifications ã 2 Relationships to the Family of BC Devices ã 3 Historical Roots ã 4 References [edit] Specifications 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 Telefunkenare two manufacturers of the BC548.Vcbo = 30 Volts, Ic = 100mA, Ptotal = 50 mW and ft = 300 MHz [1] [edit] Relationships to the Family of BC Devices The BC548 is a member of a larger group of similarly numbered transistors. Itscomplement is the BC558, which is similar to the North American2N3906and the Japanese 2SA1015. The BC548 is flanked by two similar transistors, the BC547 and theBC549. These are similar to the BC548 but the 547 has a greater Vcbo of 50 volts and the549 has the same Vcbo of 30 volts but a lower noise figure. The 547 and 549 havecomplementary PNP versions numbered 557 and 559. A 556 device also exists with aVcbo of 80 volts, which device finds extensive use in thecurrent mirror input stages of medium quality audio amplifiers with relatively high rail voltages. A family of older BC transistors predates the TO-92 BC54x series, the BC107, 108 and 109, (withcomplements BC177, 178 and 179). These are generally housed in the TO-18 metal package, the same as what the North American2N2222uses. These older transistors havesimilar characteristics as the TO-92 BC5xx devices and are generally interchangeable.For example, a damaged BC178 could be replaced with a BC558, taking the usual precautions to ensure that the three leads are correctly oriented.The BC337, 338 and 339 are a range of higher current, slower devices withcomplementary PNP versions BC327, 328 and 329. These are similar to the NorthAmerican 2N2222 and 2N2907 in Ic and ft values and have the same Vcbo ratings as theBC547, 548 and 549. The BC635, 637 and 639 possess an Ic value of 1A, a Vcbo of between 45 and 80 volts and an ft of 50 to 130 MHz. These devices have a different leadconfiguration, with the collector lead in the middle. The complementary PNP versions areBC636, BC638 and BC640. There are many other devices based on the BC54x family,such as the surface-mount versions of the BC547, 548 and 549, the BC847, 848 and 849. [edit] Historical Roots Before the silicon BC devices were developed, the older germaniumtechnology OC series devices were used. These generally date between about 1958-1970. The mostcommonly encountered are the glass encapsulated OC44, OC45, OC71 and OC75. These were very low power devices with a Vcbo generally in the 12-16 volt range and Ic valuesof less than 50mA. The OC44 and OC45 were the first common British/Europeanradiofrequencydevices with an ft around 1 MHz. The OC71 and OC75 were audio deviceswith an ft of about 150 kHz. A power version, the OC25, with an Ic of 3 Amperes wassometimes seen in aTO-3 package. All of these earlier germanium devices were generally PNP, although NPN versions were made. These older germanium devicescontainedindium, a metal with a very low melting point which limited the power dissipation of the devices to a very low level and rendered them unreliable in harshenvironments, such as use in aircraft where wide temperature variations are encountered.The silicon technology based BC devices appeared and superseded the older germanium based devices. The doped silicon from which the newer devices werefabricated could withstand much greater temperature variations and allowed much greater power dissipations. The main limiting factor of the newer siliconBJTswas thermal runaway,a condition where the current gain ( beta ) of a BJT increases as it gets hotter.This increases the collector current (Ic) despite the base current being constant. Anincrease in Ic makes the chip die hotter, increasing the beta and thus Ic, and so onuntil the transistor is cooled externally or it burns out. Thisthermal runaway can be overcome by using an emitter resistor in combination with a voltage divider providing the base bias current, or by using a resistor between the collector and the base (sliding bias),we call all these measures against thermal runaway Bipolar transistor biasing.
