Bipolar Junction Transistor Xx2

7/25/2019 Bipolar Junction Transistor Xx2

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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

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Bipolar Junction Transistor Xx2