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    Bipolar Junction Transistor (BJT)

    A Bipolar Junction Transistor is an active semiconductor device formed by two

    P-N junctions wose function is amplification of an electric current!

    Bipolar transistors are made from " sections of semiconductor material

    (alternatin# P-type and N-type)$ wit % resultin# P-N junctions!

    &ne P-N junction is between te emitter and te base' te oter P-N junction is

    between te collector and te base!

    Bipolar transistors are classified as eiter NPN or PNP accordin# to te

    arran#ement of teir N-type and P-type materials!

    n a transistor$ flow of current is due to bot electrons oles and ence tename bipolar junction transistor!

    Teir basic construction and cemical treatment is implied by teir names! *o an

    NPN transistors is formed by introducin# a tin re#ion of P-type material between two

    re#ions of N-type material!

    NPN TransistorAn NPN transistor is formed by introducin# a tin re#ion of P-type material between

    two re#ions of N-type material!

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    PNP TransistorA PNP transistor is formed by introducin# a tin re#ion of N-type material between two

    re#ions of P-type material!

    Terminals+,e#ions of a transistor

    BJT consists of " re#ions all of wic ave a different dopin# concentration! Tey are

    .! /ollector

    %! Base

    "! 0mitter

    /ollector

    /ollector re#ion as a lar#er area tan te oter two re#ions!

    t as moderate dopin#!

    /ompared to te oter two re#ions$ te collector carries more current and

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    #enerates a lar#er amount of eat!

    n order to dissipate te eat$ collector is made of a lar#er area!

    Base

    Base as te least area amon# te " re#ions!

    t as very low dopin# levels!

    Tis terminal is also 1nown as control terminal!

    Base terminal is used to control te collector output current of te transistor!

    0mitter

    0mitter as moderate area!

    t as i# dopin# level!

    Te emitter re#ion as more number of car#e carriers! 2ence it as i# dopin#!

    3 metods for biasin# te transistor

    .! 0mitter-Base and Base-/ollector junction Bot reverse biased Te transistor

    does not conduct current i!e! transistor is in cut-off sta#e!

    %! 0mitter-Base and Base-/ollector junction Bot forward biased Te transistor

    conducts ma4imum current i!e! transistor is in saturation mode!

    "! 0mitter-Base junction forward biased and Base-/ollector junction reverse biased

    Te transistor is operatin# in active mode! Tis confi#uration is useful for te purpose

    of amplification! Te transistor action ta1es place in tis mode!

    3! 0mitter-Base junction reverse biased and Base-/ollector junction forward

    biased Tis is te inverse mode of operation of te transistor! Transistor conducts but

    te current carryin# capability of te transistor is very por! t can be used for

    attenuatin# te si#nals!

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    /onfi#urations of a transistor

    /ommon Base (/B) /onfi#uration

    n te /ommon Base or #rounded base confi#uration$ te BA*0 connection is

    common to bot te input si#nal AN5 te output si#nal wit te input si#nal bein#

    applied between te base and te emitter terminals!

    Te correspondin# output si#nal is ta1en from between te base and te collector

    terminals as sown wit te base terminal #rounded or connected to a fi4ed reference

    volta#e point!

    Te input current flowin# into te emitter is 6uite lar#e as its te sum of bot te

    base current and collector current respectively terefore$ te collector current output is

    less tan te emitter current input resultin# in a current #ain for tis type of circuit of7.8 (unity) or less$ in oter words te common base confi#uration 7attenuates8 te

    input si#nal!

    /ommon Base /aracteristicsnput /aracteristics

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    &utput /aracteristics

    /ommon 0mitter (/0) /onfi#uration

    n te /ommon 0mitter or #rounded emitter confi#uration$ te input si#nal is

    applied between te base$ wile te output is ta1en from between te collector and te

    emitter as sown!

    Tis type of confi#uration is te most commonly used circuit for transistor based

    amplifiers and wic represents te 7normal8 metod of bipolar transistor connection!

    Te common emitter amplifier confi#uration produces te i#est current and

    power #ain of all te tree bipolar transistor confi#urations!

    Tis is mainly because te input impedance is 9&: as it is connected to a

    forward biased PN-junction$ wile te output impedance is 2;2 as it is ta1en from a

    reverse biased PN-junction!

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    /ommon 0mitter /aracteristics

    nput /aracteristics

    &utput /aracteristics

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    /ommon /ollector (//) /onfi#uration

    n te /ommon /ollector or #rounded collector confi#uration$ te collector is

    now common trou# te supply!

    Te input si#nal is connected directly to te base$ wile te output is ta1en from

    te emitter load as sown!

    Tis type of confi#uration is commonly 1nown as a