03.BJT-Basics.pdf

8/9/2019 03.BJT-Basics.pdf

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Bipolar Junction Transistors - I

(BJT-I)

BASICS


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A bipolar junction transistor (BJT) is a three terminal

device constructed of doped semiconductor material

BJT is used in amplifying or switching applications, in

discrete circuits and in IC design, both in analog anddigital domain

Bipolar Junction Transistors are so named because

their operation involves both electrons and holeswhich are negative and positive charge carriers


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Emitter

region

Base

region

Collector

region

P N P

Emitter

(E)

Base

(B)

Collector

(C)

J1 J2

Emitter-Base

Junction

Collector-Base

Junction

Physical Structure of PNP Transistor

Symbol

B

E

CArrow indicates the direction

in which the current flows in

the emitter


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Emitter

region

Base

region

Collector

region

N P N

Emitter

(E)

Base

(B)

Collector

(C)

J1 J2

Emitter-Base

Junction

Collector-Base

Junction

Physical Structure of NPN Transistor

Symbol

B

E

CArrow indicates the direction

in which the current flows in

the emitter


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P N P

E

B

C

Operation of a PNP Transistor in the Active mode

VEBVCB

+ - + -

IE

IB

IC

ICO

Forward Biased Reverse Biased

is the fraction of the total emitter

current which goes from the emitter,

across the base, to the collector

IE= IB+ IC

IC = IE+ ICO

IE

B

E

C


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N P N

E

B

C

Operation of an NPN Transistor in the Active mode

VEB VCB

+- +-

--

++

IE

Injected

electrons

Injected holes

Diffusing

electrons

Recombinedelectrons

Collected

electrons

IB

IC{IEIC0

Forward Biased Reverse Biased

++

+Drift Current

IB

IE

IC = IE+ ICO

IE= IB+ IC

BE

C


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C E CO E B C

C B C CO

C B CO

C B CO

I =I +I I =I +I

I =(I +I )+I

(1-)I =I +I

1I = I + I

1- 1-

( ) COBC

I1II ++=

Define =

1-


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Different Transistor configurations

A Transistor may be operated in three different

configurations :

Common Base (CB)

Common Emitter (CE)

Common Collector (CC)


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Common Base (CB) Transistor Configuration

P N PE

B

C

VEE VCC

+ - + -

IE

IB

IC

IEE C

B

IC

IB

PNP

N P NE

B

C

VEE VCC

+- +-

IE

IB

IC

IEE C

B

IC

IB

NPN


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Common Collector (CC) Transistor Configuration

PNP

E

B

C

IB

IE

IC

P

N

P

B

C

E

+

- -

+

VBB

VEE

IB

IE

IC

N

P

N

B

C

E

+

-

-

+VBB

VEE

IB

IE

IC

E

B

C

IB

IE

IC

NPN


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TheCommon Emitter (CE)configuration

is the most common configuration for

using a transistor.

We will discuss this in more detail in our

subsequent slides concentrating mainly

on the way in which this can be used asan amplifier.

To keep things simple, we will mainly

look at circuits using an NPN transistor

but the usage of a PNP transistor will bevery similar (just use voltage supplies of

opposite polarity!).

E

B

CIB

IE

IC

E

B

CIB

IE

IC

NPN

PNP

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