5. BJT

B

C

E

Transistors


•
They are unidirectional current carrying
devices like diodes with capability to control
the current flowing through them

•
Bipolar Junction Transistors (
BJT
)
control
current by current

•
Field Effect Transistors (
FET
)
control current
by voltage

•
They can be used either as
switches

or as
amplifiers



•
A transistor allows you to
control

the current, not
just block it in one direction.

•
A good analogy for a transistor is a pipe with an
adjustable gate.

•
A transistor has
three

terminals.

•
The main path for current is between the collector and emitter.

•
The base controls how much current flows, just like the gate
controls the flow of water in the pipe.

BIPOLAR JUNCTION TRANSISTOR

•
Two back to back P
-
N junctions

•
Emitter

–
Heavily doped

–
Main function is to supply majority carriers to base

•
Base

–
Lightly doped as compared to emitter

–
Thickness 10
-
6

m

•
Collector

–
Collect majority carriers from emitter through base

–
Physically larger than the emitter region


E

B

N

P

N

C

E

B

P

N

P

C

The BJT
–

Bipolar Junction Transistor

The Two Types of BJT Transistors

npn

pnp

n

p

n

E

B

C

p

n

p

E

B

C

Cross Section

Cross Section

B

C

E

Schematic
Symbol

B

C

E

Schematic
Symbol

LP4

6

NPN Bipolar Junction
Transistor

LP4

7

PNP Bipolar Junction
Transistor

•
The

collector

surrounds

the

emitter

region,

making

it

almost

impossible

for

the

electrons

injected

into

the

base

region

to

escape

being

collected,

thus

making

the

resulting

value

of

α

very

close

to

unity,

and

so,

giving

the

transistor

a

large

β

STRUCTURE

•
Energy Band diagram of an unbiased transistor

–
N
-
region moves down and P
-
region moves up due to
diffusion of majority carriers across junction.

–
The displacement of band and carrier migration
stops when Fermi levels in the three regions are
equalized

Biasing of Transistor

–
Base and emitter current when collector is open

•
EB is forward biased
-

electron diffusion from
emitter to base and hole diffusion from base to
emitter

•
Hence I
B

will be large and is equal to I
E


•
Collector is open so no current flows into collector




•
Base and Collector current when the Emitter
is open (I
CBO
)

•
CB is reverse biased
-

electron from base flow into
collector region and holes from collector flow
into base

•
This current is known as reverse saturation
current

•
The base current I
B

will be small and is equal to
I
CBO


•
Four Ways of Transistor biasing

–
Both EB and CB junctions are fwd biased
-

Huge
current flows through base. The transistor is said to
be operating in Saturation region (mode)

–
Both EB and CB junctions are reverse biased
-

The
transistor is said to be operating in cut off region
(mode)

–
EB junction is fwd biased and CB junction is reverse
biased. The collector current is controlled by emitter
current or base current
-

The transistor is said to
operate in Active region (mode)

–
EB junction in reverse biased and CB junction in fwd
biased
-

inverted region (mode)



Transistor Biasing
-
Active Region

When both Emitter and Collector are closed


•
Emitter
-
base junction is forward biased

•
Collector
-
base junction is reverse biased

•
DC emitter supply voltage (V
EE
)
-

Negative terminal of
V
EE

is connected to emitter

•
DC collector supply voltage (V
CC
)
-

Positive terminal of
V
CC

is connected to collector

•
I
B
becomes very small and I
C
will be as large as I
E


N

P

N

V
EE

V
CC

I
E

I
C

I
B

Transistor currents

•
Forward biasing from base to emitter narrows the BE
depletion region

•
Reverse biasing from base to collector widens the
depletion CB region

•
Conduction electrons diffuse into p
-
type base region

•
Base is lightly doped and also very thin
-

so very few
electron combine with available hole and flow out of the
base as valence electrons (small base electron current)






N

P

N

V
EE

V
CC

I
E

I
C

I
B

•
Sufficient holes are not avail in base
–

remote possibility
of joining of electrons with holes

•

Electron concentration is large on emitter side and nil
on collector

•
Electrons swiftly move towards collector

•
At CB junction they are acted upon by strong electric
field due to reverse bias and are swept into collector



Transistor currents

•
Most of the electrons diffuse into CB depletion region

•
These electrons are pulled across the reverse biased CB
junction by the attraction of the collector supply voltage
and form the
collector electron current.
Therefore

I
E
= I
C
+ I
B

•
1
-
2% of emitter

current goes to supply base current and
98
-
99% goes to supply collector current

•
Moreover,
I
E

flows into the transistor and I
B

& I
C

flow out
of transistor


•
Current flowing in is taken as positive and currents
flowing out are taken as negative

•
The ratio of the number of electrons arriving at collector
to the number of electrons emitted by the emitter is called
base transportation factor








Important Biasing Rule

•
Both collector and base are positive with respect to
emitter

•
But collector is more positive than base

•
Different potentials have been designated by double
subscripts as shown in the figure

•
V
CB

(Collector is more positive than base) and V
BE
(base
is more positive than emitter)





++ C

-

E

+

B

V
CB

V
BE

E

C

+

B

-

++

V
BE

V
CB

Transistor circuit configuration

•
There are of three types

–
Common base (CB) OR grounded base

–
Common emitter (CE) OR grounded emitter

–
Common collector (CC) OR grounded collector

•
Common is the term used to denote the electrode that is
common to the input and output circuits and it is
generally grounded

•
Common
-
Base Biasing (CB) :


input

= V
BE

& I
E

•






output = V
CB

& I
C


•
Common
-
Emitter Biasing (CE):


input

= V
BE

& I
B

•






output

= V
CE

& I
C


•
Common
-
Collector Biasing (CC):

input

= V
BC

& I
B

•






output = V
EC

& I
E




Common

Base

Configuration

•
Input

signal

applied

between

emitter

&

base

•
Output

is

taken

from

collector

&

base

•
Ratio

of

collector

current

to

emitter

current



is

called

dc

alpha

(

dc
)

of

a

transistor


E

C

+

B

-

++

V
BE

V
CB

•
The subscript
dc
on


signifiesthatthisratioisdefined
from⁤c⁶aluesf
I
C
and I
E


•
There is also an ac


睨ichrefersto⁴he⁲atioof
change
in collector current to the
change
in emitter current

•
For all practical purposes

dc
=

ac
=



•

I
E

is taken as positive (flowing into transistor) and I
C

is
taken as negative (flowing out of transistor)

•



istheeasurefⁱualityof⁡transistor
-

higher⁩ts
values, better is the transistor

•
Value ranges from 0.95 to 0.999


Common Emitter Configuration

•
The input signal is applied between the base and
emitter and the output signal is taken out from the
collector and the emitter

•

Ratio of collector current to base current is called dc
beta (

dc
) of a transistor

C

E

+

B

-

Relation between


and


and

using

then

becomes

or

or

or

Common

Collector

Configuration

•
The

input

signal

is

applied

between

the

base

and

collector

and

the

output

signal

is

taken

out

from

the

emitter
-
collector

circuit

•

Ratio

of

emitter

current

to

base

current

is


From the figure

C

E

+

B

-

Output current=(1+

)xInputcurrent

Relation between transistor currents



We know

and

and

because

We get

•
This

shows

that

emitter

current

initiated

by

the

forward

biased

emitter

base

junction

is

split

into

two

parts

•
(
1
-

⥉
E

which

becomes

base

current

in

the

external

circuit

•

I
E

which

becomes

collector

current

in

the

external

circuit

Therefore

Static Characteristics

•
Common Base Static characteristics

–
Input characteristics
.
I
E

varies with V
BE

when voltage V
CB

is held constant

•
V
CB
is adjusted with the help of R
1

•
V
BE
is increased and corresponding values of I
E
are
noted

•
The plot gives input characteristics

•
Similar to the forward characteristics of P
-
N diode

•
This characteristics is used to find the input
resistance of the transistor. Its value is given by the
reciprocal of its slope




R
in
=


V
BE
/


I
E







BJT Input Characteristics

V
BE

I
E

2
mA

4
mA

6
mA

8
mA

0.7 V

V
CC

E

C

+

B

V
BE

V
CB

I
C

I
E

V
EE

R
1

R
2

Static Characteristics

•
Common Base Static characteristics

–
Output characteristics
.
I
C

varies with V
CB

when I
E

is held
constant

•
V
BE
is adjusted with the help of R
2

and I
E

is held constant

•
V
CB
is increased and corresponding values of I
C
are noted

•
The plot gives output characteristics

•
Then I
E

is increased to a value little higher and whole process is
repeated

•
The output resistance of the transistor is given by




R
out
=


V
CB
/


I
C







V
CC

E

C

+

B

V
BE

V
CB

I
C

I
E

V
EE

R
1

R
2



I
C

flows

even

when

V
CB
=
0

for

different

values

of

I
E
(due

to

internal

junction

voltage

at

CB

junction)



I
C

flows

even

when

I
E
=
0

(Collector

leakage

current

or

reverse

saturation

current

I
CBO
)


The

output

resistance

is

very

high

(
500
k

)

Saturation Region

I
E

I
C

V
CB

Active
Region

Cutoff

I
E

= 0

BJT Output
Characteristics

Static Characteristics

–
It can be seen that I
C

flows even when
V
CB

is zero

–
It is due to the fact that electrons are being injected
into base due to forward biased E
-
B junction and are
collected by collector due to action of internal junction
voltage at C
-
B junction

–
Another important feature is that a small amount of
collector current flows even when the emitter current I
E
is zero called collector leakage current (
I
CBO
)

–
When
V
CB
is permitted to increase beyond a certain
value,
I
C
increases rapidly due to avalanche breakdown

•
This characteristics may be used to find

ac









ac
=

I
C
/

I
E


LP4

30

Common Emitter(CE) Connection

Common Emitter Configuration

•
Transistor

is

biased

in

active

region

•
Called

CE

because

emitter

is

common

to

both

VBB

and

VCC

•
VBB

forward

biases

the

EB

junction

and

VCC

reverse

biases

the

CB





V
CC

B

C

B

E

V
BE

V
CE

I
C

I
B

V
BB

R
1

R
2

Static Characteristics

•
Common Emitter Static
characteristics

–
Input characteristics
.
I
B

varies with V
BE

when
voltage V
CE

is held constant

•
V
CE
is adjusted with the help of R
1

•
V
BE
is increased and corresponding values of I
B
are noted

•
The plot gives input characteristics

•
Procedure is repeated for different (constant) values of V
CE

•
This characteristics is used to find the input resistance of the
transistor. Its value is given by the reciprocal of its slope




R
in
=


V
BE
/


I
B











V
BE

I
B

2 mA

4 mA

6 mA

8 mA

0.7 V

Static Characteristics

•
Common Emitter Static characteristics

–
Output characteristics
.
I
C

varies with V
CE

when I
B

is held constant

•

I
B

is held constant

•
V
CE
is increased and corresponding values of I
C
are
noted

•
The plot gives output characteristics

•
Then I
B

is increased to a value little higher and
whole process is repeated

•
The output resistance in this case is very less as
compared to CB circuit and is given by




R
out
=


V
CE
/


I
C









As

V
CE

increases

from

zero,

I
C

rapidly

increases

to

saturation

level

for

a

fixed

value

of

I
B



I
C

flows

even

when

I
B
=
0

(Collector

leakage

current

or

reverse

saturation

current

I
CEO
),

the

transistor

is

said

to

be

cutoff


When

V
CB

is

permitted

to

increase

beyond

a

certain

value,

I
C

increases

rapidly

due

to

avalanche

breakdown



This

characteristics

may

be

used

to

find


ac


ac

=

I
C
/


I
B

V
CE

I
C

Active
Region

I
B

Saturation Region

Cutoff Region

I
B

= 0

Region of
Operation

Description

Active

Small base current
controls a large
collector current

Saturation

V
CE(sat)

~ 0.2V, V
CE

increases with I
C

Cutoff

Achieved by reducing
I
B

to 0, Ideally, I
C

will
also equal 0.

where

Therefore,

in

general

Common Base

Common Emitter

(Reverse saturation current)

where

(Reverse saturation current)

Relationship between

dc

and

dc

and

Common Base Formulas

V
CC

E

C

B

V
BE

V
CB

I
C

I
E

I
B

V
EE

R
L

R
E

Where

V
BE
=
0
.
3

V

for

Ge

and

0
.
7

V

for

Si

Generally

V
EE
>>V
BE

so

I
E
=V
EE
/R
E

and

Common Emitter Formulas

and

V
CC

E

C

B

V
BE

V
CE

I
C

I
B

I
E

V
BB

R
L

R
B

DC


and DC





†
=Co浭mn
-
e浩瑴敲捵rr敮琠g慩n




†
=Com浯n
-
b慳攠捵rr敮琠g慩n






=I
C





=I
C




I
B



I
E



The relationships between the two parameters are:





=







=






+1




ㄠ
-





No瑥t


慮d


慲攠獯me瑩浥猠r敦敲r敤瑯慳a

dc

and


dc


because the relationships being dealt with in the BJT

are DC.

BJT Example

Using Common
-
Base NPN Circuit Configuration

+

_

+

_

Given: I
B

= 50


A,I
C

= 1 mA

Find: I
E

,


,慮d




卯lu瑩on

I
E

= I
B

+ I
C

= 0.05 mA + 1 mA = 1.05 mA



= I
C

/ I
B

= 1 mA / 0.05 mA = 20



=I
C

/ I
E

= 1 mA / 1.05 mA = 0.95238



捯uld慬獯b攠捡c捵l慴敤u獩ng瑨攠癡vu攠o映


睩瑨瑨攠景rmula晲om瑨攠pr敶eou猠獬id攮






=


=㈰2=〮㤵9㌸





+ㄠ㈱

I
C

I
E

I
B

V
CB

V
BE

E

C

B

Transistor as an amplifier

Transistor as an amplifier

•
An electronic circuit that causes an increase in the
voltage or power level of a signal

•
It is defined as the ratio of the output signal voltage to
the input signal voltage

V
EE

V
CC

I
E

I
C

I
B

R
L


In the figure we see that an output voltage is developed
across R
L



The dc voltage V
EE

is a fixed voltage and causes a dc
current I
E
to flow through EB junction


When the ac voltage V
i

is super
-
imposed on V
EE
, the
emitter base voltage varies with time


Say if V
EE
=10V and the peak voltage of V
i

is
is

1V, the
EB voltage swings from 9V to 11V


The causes corresponding variations in I
E

and I
C
which
gives V
o


The emitter variation due to EB voltage variation can be
expressed as







The collector current I
C

changes by


This current

I
C
flows through R
L

causing a voltage
drop


Hence

as


Where r
i

is very small (100

)andR
L
is of the order of
kilo
-
ohms. It means
V
o
is larger than
V
i
indicating that
the transistor has amplified small V
i

to a larger V
o

Problems

•
In the CE Transistor circuit V
BB
= 5V, R
BB
=
107.5 k

Ⱐ
R
CC
= 1 k

,V
CC
= 10V. Find
I
B
, I
C
,
V
CE
,


and the transistor power dissipation


In

the

CE

Transistor

circuit

shown

earlier

V
BB
=

5
V,

R
BB
=

107
.
5

k

,

R
CC

=

1

k

,

V
CC

=

10
V
.

Find

I
B
,

I
C
,

V
CE
,



and

the

transistor

power

dissipation

using

the

characteristics

as

shown

below

By Applying KVL to the base emitter circuit

By

using

this

equation

along

with

the

i
B

/

v
BE

characteristics

of

the

base

emitter

junction
,

I
B

=

40


A

By

Applying

KVL

to

the

collector

emitter

circuit

By

using

this

equation

along

with

the

i
C

/

v
CE

characteristics

of

the

base

collector

junction
,

i
C

=

4

mA,

V
CE

=

6
V

Transistor

power

dissipation

=

V
CE
I
C

=

24

mW

We

can

also

solve

the

problem

without

using

the

characteristics

if



慮d

V
BE

values

are

known

i
B

100

A

0

㕖

v
BE

Input Characteristics

Output Characteristics

i
C

10
m
A

0

v
CE

100

A

㠰8

A

㘰6

A

㐰4

A

㈰2

A
