5. BJT

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2 Νοε 2013 (πριν από 3 χρόνια και 10 μήνες)

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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





-





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


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



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