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Lecture 19 Electronics I 1

Common-Emitter Characteristics

Transistor operating in active region

Operating Point Q

Large-signal or dc β

 βdc=ICQ/IBQ

Reerred to in data s!eets as !"E

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Lecture 19 Electronics I #

Common-Emitter Characteristics

Incre$ental or ac β

%CEQ re$ains i&ed' iB is c!anged slig!tl(

)lig!t c!ange occurs in iC

 βac=∆iC/∆iB

 βac diers ro$ βdc *( 1+, - #+,

Reerred to in data s!eets as !e

 lso .non as s!ort-circuit co$$on-e$ittercurrent gain as it is $easured it! 0ero

c!ange in vCE

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Lecture 19 Electronics I

Saturation Voltage & Resistance

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Lecture 19 Electronics I 2

Saturation Voltage

In saturation

ICsat 3 β"IB

ICsat is esta*lis!ed *(

circuit designer 

)aturated transistor

is operated at

orced β βorced = ICsat/IB

!ere βorced 3 β"

4"IB

ICsat

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Lecture 19 Electronics I 5

Saturation Voltage

Overdrive actor

= β" /βorced

6reater t!e

overdrive actordeeper t!e

transistor in

saturation

Loer %CEsat

4"IB

ICsat

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Lecture 19 Electronics I 7

Saturation Voltage

In saturation

iC 8 vCE 

c!aracteristics are

steeper  Loer collector-to-

e$itter resistance

RCEsat 

RCEsat ranges ro$

e o!$s to e

tens o o!$s 

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Lecture 19 Electronics I

BJT as an Amplifier and a Switch

To $a:or areas o application

)ignal a$pliier  Operated in active region

)$all c!anges in BE voltage result in largec!anges in C current ;trans-conductancea$pliier<

%oltage a$pliication o*tained *( connecting a

resistance RC  igital circuit sitc!

Operated in saturation and cut-o region

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Lecture 19 Electronics I >

Large-Signal peration

Transfer Characteristics Co$$on-E$itter C.t? vi input voltage

Bias voltage )ignal voltage

vo output voltage Bias voltage )ignal voltage

RC

Bias voltage at collector  Converts iC into output

voltage %CC )uppl( voltage

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Lecture 19 Electronics I 9

Voltage Transfer Characteristics

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Lecture 19 Electronics I 1+

Voltage Transfer Characteristics

vo = %CC 8 RCiC In active $ode

 

  s!arp slope @A

In saturation $ode

 

RCEsatis lo ;o/presistance<

TBE V!"

SC   e#i   =

Ti V!"SCCC   e#R V"   −=

C

CEsatCC

Csat R 

VV#

  −=