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Electr Electroni onics cs Devic Devices es and and Circui Circuits ts Theor Theory y Experiment No.5
Experiment No. 5
Bipolar Junction Transistors I. OBJECTIVES 1. To get acquainted with the transistor-diode relationship. . !denti"y the emitter# $ase and collector terminals o" the di%erent types o" transistors. &. !denti"y whether the transistor is 'N' or an N'N. (. )easure and graph the collector characteristics curves "or a $ipolar *unction transistor transistor.. 5. +se the characteristic curves to determine the , DC o" the transistor at a given point. II. BACKGOUN IN!O"#ATION IN!O"#ATION transistor is a three-layer semiconductor device made up o" either two doped '-type and one N-type or two doped N-type and one '-type device capa$le o" ampli"ying C signal. !t has three terminals namely the emitter# $ase and collector. The emitter is highly doped while the collector is the largest in sie. The $ase is the thinnest and the least doped material. The emitter-$ase *unction ma/e ma/es s up one diode# diode# and the collector collector-- $ase *unction *unction ma/es ma/es the other. other. Either o" these *unctions may "unction as a diode# and each has its own characteristics . Di%erent doping gives the collector-$ase diode a higher $rea/down voltage than the emitter $ase diode. Two Two diodes placed placed $ac/-to-$ac/ $ac/-to-$ac/ do not not ma/e ma/e a transistor transistor. The three-layer three-layer transistor has a current that 0ows according to a certain pattern. The emitter-$ase *unction is is normally normally "orward "orward $iased# with a small $ase-curre $ase-current. nt. The collector collector -$ase *unction is is normally normally reverse-$iased reverse-$iased # with with larger current# current# owing to the current current amplication control o" the collector current $y the $ase-current. The larger the $ase-current# the larger the collector-current within certain limits. The control control o" a large large current current $y a small small current current is considered considered as curr current ent gain. 2ince there are three terminals to the transistor# any o" these may $e considered as the common terminal to $oth the input and output currents. 3or this reason# there are three ways o" measuring current gain. The most "requently "requently discussed discussed current current gain gain is that o" the the common-emitter common-emitter circuit. 2ince the gain is a "orward current gain in a common-emitter circuit# it is generally re"erred to as hy$rid parameter h "e where 4"4 stands "or "orward and 4e4 "or common emitter conguration. Capital letters would indicate it as a DC gain. The h"e is a comparison o" collector current with the $ase current controlling it.
,DC h" e !c 6 !7
Electronics Devices and Circuits Theory Experiment No.5
The value o" h"e changes with the amount o" collector current and is generally in the range o" 8 to 88. III. #ATE"IA$S 1 - 188 9 : ; 1 - && /9 : ; 1- N&<8( N'N =eneral 'urpose Transistor >or equivalent? 7read$oard @aria$le DC power supply D)) 2olid wire ;ire stripper Connecting wires Extension Cord IV. %"OCEU"ES %A"T I 1. Examine all the given transistors and write their codes on the ta$le. . Draw the $ottom view or isometric view o" each transistor on the ta$le. &. Complete the required data o" Ta$le 5.1 $y identi"ying the transistor terminals as well as the type >'N'6N'N?
Electronics Devices and Circuits Theory Experiment No.5
%A"T II. 1. )easure and record the ohmic values o" resistors listed on the ta$le $elow. $iste& Value
#easure& Value
A1
&& /9
&.5 /9
A
1889
<<.B 9
. Connect the common emitter conguration circuit shown in 3igure 5.1. 2tart with $oth power supplies set to 8 @. The purpose o" A1 is to limit $ase current and to allow the determination o" the $ase current !7. 2lowly increase @77 until @A1 is 1.5 @. This sets up a $ase current o" 58 # which can $e shown $y applying hmFs Gaw to A1.
&. ;ithout distur$ing the setting o" @77# slowly increase @ CC until .8 @ is measured $etween the transistorFs collector and emitter >@ CE?. Then measure
Electronics Devices and Circuits Theory Experiment No.5
and record @ A "or the setting. Aecord @ A in Ta$le 5.& under columns la$eled 7ase Current 58 . T7GE 5.H 7ase Current 58
7ase Current 188
7ase Current 158
@CE >measure d? '.(
@A >measure d? 8.<5 @
!c >computed ? <.5 m
@A >measure d? @
!C >compute d? 8 m
@A >measure d? .II @
!C >computed ? I.I m
).(
1@
18 m
.8I @
8.I m
&.8 @
&8. m
*.(
1.85 @
18.5 m
.15 @
1.5 m
&.&1 @
&&.1 m
+.(
1.8< @
18.< m
.I @
.I m
&.51 @
&5.1 m
,(.(
1.1& @
11.& m
.(8 @
( m
&.I8 @
&I m
(. Compute the collector current # ! C# $y applying hmFs Gaw to A . +sed the measured voltage# @A# and measured resistance# A# to determine the current. Note that the current A is in the same as !C "or the transistor. Enter the computed collector current in Ta$le 5.& under the columns la$eled 7ase Current 58 .
5. ;ithout distur$ing the setting o" @77# slowly increase @CC until (.8 @ is measured $etween the transistorFs collector and emitter >@ CE?. Then measure and record @ A "or the setting. Compute the collector current # !C# $y applying hmFs Gaw in step (. Continue in this manner "or each o" the values o" @CE listed in Ta$le 5.&. . Aeset @CC "or 8 @ and ad*ust @77 until @A is &.& @. The $ase current is now 188 . I. ;ithout distur$ing the setting o" @ 77# slowly increase @CC until @CE is (.8 @. Then measure and record @A "or the setting in Ta$le 5.& under the columns la$eled 7ase Current 188 . . Compute the collector current # !C# $y applying hmFs Gaw to
A. Enter the computed collector current in Ta$le
5.&.
B. !ncrease @CC until @CE is equal to (.8 @. )easure and record @ A "or this setting. Continue in this manner "or each o" the values o" @ CE listed in Ta$le 5.&. <. Aeset @CC "or 8 @ and ad*ust @77 until @A is (.<5 @. The $ase current is now 158 . 18. Complete the ta$le 5.& $y repeating steps I and B "or 158 o" $ase current. 11. 'lot three collector characteristic curves using the data ta$ulated in Ta$le 5.& . The collector characteristicFs curve is a graph o" @CE versus !C "or a constant $ase current.
Electronics Devices and Circuits Theory Experiment No.5
Choose a scale "or ! C that allows the largest current o$served to t on the graph. Ga$el each curve with the $ase current it represents. =raph the data using the plot $elow.
1. +se the characteristic curve you plotted to determine the , DC "or the transistor at a @CE o" &.8 @ and a $ase current o" 58# 188# 158 . Then repeat the procedure "or a , DC at a @CE o" 5.8 @. Aecord your results in Ta$le 5.(
Current Gain- C VCE
IB / 5( 0A
IB / ,(( 0A
IB / ,5( 0A
&.8
18
8(
1
5.8
1(
15
15.&&
Electronics Devices and Circuits Theory Experiment No.5
Electronics Devices and Circuits Theory Experiment No.5
E%E"I#ENTA$ ISCUSSION 7ipolar transistors are constructed o" a three-layer semiconductor either 'N' or N'N. s such# transistors register as two diodes connected $ac/-to-$ac/ when tested with a multimeterKs LresistanceM or Ldiode chec/M "unction as illustrated in 3igure $elow. Gow resistance readings on the $ase with the $lac/ negative >-? leads correspond to an N-type material in the $ase o" a 'N' transistor. n the sym$ol# the N-type material is LpointedM to $y the arrow o" the $ase-emitter *unction# which is the $ase "or this example. The '-type emitter corresponds to the other end o" the arrow o" the $aseemitter *unction# the emitter. The collector is very similar to the emitter# and is also a '-type material o" the 'N *unction.
Electronics Devices and Circuits Theory Experiment No.5
'N' transistor meter chec/ >a? "orward 7-E# 7-C# resistance is lowO >$? reverse 7-E# 7-C# resistance is P. Qere weKre assuming the use o" a multimeter with only a single continuity range >resistance? "unction to chec/ the 'N *unctions. 2ome multimeters are equipped with two separate continuity chec/ "unctions resistance and Ldiode chec/#M each with its own purpose. !" your meter has a designated Ldiode chec/M "unction# use that rather than the LresistanceM range# and the meter will display the actual "orward voltage o" the 'N *unction and not *ust whether or not it conducts current. )eter readings will $e exactly opposite# o" course# "or an N'N transistor# with $oth 'N *unctions "acing the other way. Gow resistance readings with the red >R? lead on the $ase is the LoppositeM condition "or the N'N transistor. !" a multimeter with a Ldiode chec/M "unction is used in this test# it will $e "ound that the emitter-$ase *unction possesses a slightly greater "orward voltage drop than the collector-$ase *unction. This "orward voltage di%erence is due to the disparity in doping concentration $etween the emitter and collector regions o" the transistor the emitter is a much more heavily doped piece o" semiconductor material than the collector# causing its *unction with the $ase to produce a higher "orward voltage drop. Snowing this# it $ecomes possi$le to determine which wire is which on an unmar/ed transistor. This is important $ecause transistor pac/aging# un"ortunately# is not standardied. ll $ipolar transistors have three wires# o" course# $ut the positions o" the three wires on the actual physical pac/age are not arranged in any universal# standardied order.
Electronics Devices and Circuits Theory Experiment No.5
Tested with a multimeter in the LresistanceM or Ldiode chec/M modes# a transistor $ehaves li/e two $ac/-to-$ac/ 'N >diode? *unctions. The emitter-$ase 'N *unction has a slightly greater "orward voltage drop than the collector-$ase 'N *unction# $ecause o" heavier doping o" the emitter semiconductor layer. The reverse-$iased $ase-collector *unction normally $loc/s any current "rom going through the transistor $etween emitter and collector. Qowever# that *unction $egins to conduct i" current is drawn through the $ase wire. 7ase current may $e thought o" as Lopening a gateM "or a certain# limited amount o" current through the collector.
ANS6E"S TO 7UESTIONS AN SO$UTIONS TO %"OB$E#S 1. Qow are you going to determine the transistor terminals when the transistor is de"ective
Electronics Devices and Circuits Theory Experiment No.5
- !t is impossi$le to determine the transistor terminal when the transistor is de"ective $ecause there will $e no resistance $etween any o" the pairs during test "or all the steps. The transistor is shorted. . ;hat happens to the value o" ! c as @CE increases - s the @CE increases the ! c decreases. &. ;hat can you say o" the relationship $etween ! c and !7 - ;hen !$ is increasing also ! c increases. (. Do the experimental data indicate that , DC is constant at all points - No $ecause ,DC is direct proportional to !c and inversely proportional to ! $ 5. ;hat e""ect would a higher , DC have on the characteristic curves you measured - !t will have a high gain and the $ipolar *unction transistor can wor/ very well. . ;hat value o" @CE would you expect i" the $ase terminal o" a transistor were open Explain your answer.
- ;henever we o$serve the terminals o" a 7UT and see that the emitter$ase *unction is not at least 8.-8.I volts# the transistor is in the cuto% region. !n cuto%# the transistor appears as an open circuit $etween the collector and emitter terminals. @ CEV 8.@. This is /nown as the saturation voltage# or @ CEsat .
CONC$USION !n this experiment we are as/ed to identi"y the $ase# the collector and the emitter o" the di%erent types o" transistors. +sing the digital meter# we test each o" the three pins. 'utting the positive pro$e on the assumed $ase and measure the other two pins o" the transistor using the negative pro$e. !" we get low resistance values "rom the other two pins# then our assumed $ase is correct. ;e also "ound out that i" one low resistance values is higher than the other pin# it is the emitter. therwise# it is the collector. ;e can also conclude that in identi"ying the transistor i" it is 'N' or N'N# i" the Emitter to 7ase is greater than the 7ase to Collector# it is 'N'. therwise it is an N'N. 7ase on the graph weKve plotted# it shows the relationship o" @ ce and !c with constant !$. as we increase @ce# !calso increases. There"ore# they are directly proportionl when ! $is constant. The "ormula o" $eta is , !c6!$ $ut "irst# we must "ind !c. ;e pro*ected the curve downward# with a given @ce# and pro*ected it to the le"t so we can get !c.