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
signifiesthatthisratioisdefined
fromc⁶aluesf
I
C
and I
E
•
There is also an ac
睨ichrefersto⁴heatioof
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)
•
istheeasurefⁱualityoftransistor
-
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+
)xInputcurrent
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
)andR
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
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