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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
