UNIT IV
BJT
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Bipolar Junction Transistors
• The transistor is a three-layer semiconductor
device consisting of either two n- and one p-
type layers of material or two p- and one n-
type layers of material.
• The former is called an npn transistor, while
the latter is called a pnp transistor
• So, there are two types of BJT-
i) pnp transistor ii) npn transistor
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Bipolar Junction Transistors
In each transistor following points to be noted-
i) There are two junction, so transistor can be
considered as two diode connected back
to back.
ii) There are three terminals.
Ie=Ib+Ic
iii)The middle section is thin than other.
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BJT -Naming of Transistor Terminals
• Transistor has three section of doped
semiconductor.
• The section one side is called “emitter” and the
opposite side is called “collector”.
• The middle section is called “base”.
Transistor
symbol
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Naming of Transistors Terminals
1) Emitter:
The section of one side that supplies
carriers is called emitter.
Emitter is always forward biased wr to
base so it can supply carrier.
For “npn transistor” emitter supply electrons
to its junction.
For “pnp transistor” emitter supply
holes to its junction.
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Naming of Transistors Terminals
2) Collector:
The section on the other side that collects
carrier is called collector.
The collector is always reversed biased wr to
base.
For “npn transistor” collector receives
electrons to its junction.
For “pnp transistor” collector receives
holes to its junction.
27/01/2025 VIKRAM NATRAJAN AP/ ECE
Naming of Transistors Terminals
3) Base:
The middle section which forms two pn
junction between emitter and collector is
called Base.
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Features of Transistors Terminals
• The transistor has three region named emitter,
base and collector.
• The Base is much thinner than other region.
• Emitter is heavily doped so it can inject large
amount of carriers into the base.
• Base is lightly doped so it can pass most of the
carrier to the collector.
• Collector is moderately doped.
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Features of Transistors Terminals
• The junction between emitter and base is called
emitter-base junction(emitter diode) and junction
between base and collector is called collector-base
junction(collector diode).
• The emitter diode is always forward biased and
collector diode is reverse biased.
• The resistance of emitter diode is very
small(forward) and resistance of collector diode is
high(reverse).
27/01/2025 VIKRAM NATRAJAN AP/ ECE
Transistor Operation
1) Working of npn transistor:
Forward bias Is
applied to emitter-
base junction and
reverse bias is
applied to collector-
base junction.
The forward bias in the emitter-base junction
causes electrons to move toward base. This
constitute emitter current, IE
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Transistor Operation
1) Working of npn transistor:
As this electrons flow toward p-type base,
they try to recombine with holes. As base is
lightly doped only few electrons recombine
with holes within the base.
These recombined electrons constitute small
base current.
The remainder electrons crosses base and
constitute collector current.
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Transistor Operation
2) Working of pnp transistor:
Forward bias is
applied to emitter-
base junction and
reverse bias is
applied to collector-
base junction.
The forward bias in the emitter-base junction
causes holes to move toward base. This
constitute emitter current, IE
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Transistor Operation
2) Working of pnp transistor:
As this holes flow toward n-type base, they
try to recombine with electrons. As base is
lightly doped only few holes recombine with
electrons within the base.
These recombined holes constitute small base
current.
The remainder holes crosses base and
constitute collector current.
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Transistor symbols
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Transistor Operating modes
• Active Mode
Base- Emitter junction is forward and Base-
Collector junction is reverse biased.
• Saturation Mode
Base- Emitter junction is forward and Base-
Collector junction is forward biased.
• Cut-off Mode
Both junctions are reverse biased.
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Transistor Connections
• Transistor can be connected in a circuit in
following three ways-
1) Common Base (CB)
2) Common Emitter (CE)
3) Common Collector (CC)
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Common Base (CB) Connections
• The common-base terminology is derived
from the fact that the base is common to both
the input and output sides of the
configuration.
• First Figure shows common base npn configuration and second
figure shows common base pnp configuration.
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Common Base (CB) Connections
•
Current amplification factor ( ) :
The ratio of change in collector current to the
change in emitter current at constant VCB is known
IC at constant
as current amplification factor, .
CB
I E
V
Practical value of is less than unity, but in the
range of 0.9 to 0.99
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Expression for Collector Current
Total emitter current does not reach the collector
terminal, because a small portion of it constitute
base current. So,
I E IC I B
Also, collector diode is reverse biased, so very
few minority carrier passes the collector-base
junction which actually constitute leakage
current,I CBO .
So, collector current constitute of portion of
emitter current I and leakage current ICBO .
IC
E
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I CB
Expression for Collector Current
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Characteristics of Common Base
• Input Characteristics:
VBE vs IE characteristics is
called input
characteristics.
IE increases rapidly with
VBE . It means input
resistance is very small.
IE almost independent
of VCB.
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Chracteristics of Common Base
Output Characteristics:
•VBc vs Ic
characteristics is called
output characteristics.
• IC varies linearly
with VBc ,only when VBc
is very small.
• As, VBc increases, IC
becomes constant.
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I/p & O/P Resistance of Common Base
• Input Resistance: The ratio of change in
emitter-base voltage to the change in emitter
current is called Input Resistance.
VBE
r
I E
• Output Resistance: The ratio of change in
collector-base voltage to the change in
collector current is called Output Resistance.
VBC
r0
I C
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Common Emitter (CE) Configuration
• The common-emitter terminology is derived from
the fact that the emitter is common to both the
input and output sides of the configuration.
• First Figure shows common emitter npn configuration and second
figure shows common emitter pnp configuration.
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Common Emitter (CE) Configuration
• Base Current amplification factor ( ) :
• In common emitter connection input current is base
current and output current is collector current.
• The ratio of change in collector current to the
change in base current is known as base current
I
IC
B
• Normally only 5% of emitter current flows to base,
so amplification factor is greater than 20. Usually
this range varies from 20 to 500.
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Relation Between β & α
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Expression for Collector Current
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Characteristics of CE
• Input Characteristics: VBE vs IB characteristics
is
called input
characteristics
.
IB increases rapidly with
VBE . It means input
resistance is very small.
IE almost independent
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of VCE.
Characteristics of CE
• Output Characteristics:
•VCE vs Ic
characteristics is called
output characteristics.
• IC varies linearly
with VCE ,only when VCE
is very small.
• As, VCE increases, IC
becomes constant.
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I/P & O/P resistance of CE
• Input Resistance: The ratio of change in
emitter-base voltage to the change in base
current is called Input Resistance.
VBE
ri
I
• Output Resistance: The ratio of change in
collector-emitter voltage to the change in
collector current is called Output Resistance.
VCE
r0
I C
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Common Collector (CC) Configuration
• The common-collector terminology is derived from
the fact that the collector is common to both the
input and output sides of the configuration.
• First Figure shows common collector npn configuration and second figure
shows common collector pnp configuration.
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Common Collector (CC) Configuration
•
Current amplification factor ( ) :
• In common emitter connection input current is
base current and output current is emitter
current.
• The ratio of change in emitter current to the
change in base current
factor is known
in common as current
collector configuration.
amplification
I B
• I E gain as CE configuration
This circuit provides same
as,
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I E
Relation between ϒ & α
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Expression for Collector Current
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Comparison of CB , CE and CC
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Transistor as an Amplifier of CE config
• Figure shows CE amplifier for npn transistor.
• Battery VBB is connected with base in-order to
make base forward biased, regardless of input ac
polarity.
• Output is taken across Load R
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Transistor as an Amplifier of CE conf.
• During positive half cycle input ac will keep the emitter-
base junction more forward biased. So, more carrier
will be emitted by emitter, this huge current will flow
through load and we will find output amplified signal.
• During negative half cycle input ac will keep the
emitter-base junction less forward biased. So, less
carrier will be emitted by emitter. Hence collector
current decreases.
• This results in decreased output voltage (In opposite
direction).
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Transistor Load Line Analysis
• In transistor circuit analysis it is necessary to
determine collector current for various VCE
voltage.
• One method is we can determine the collector
current at any desired VCE voltage, from the
output characteristics.
• More conveniently we can use load line
analysis to determine operating point.
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Transistor Load Line Analysis
Consider common emitter
npn transistor ckt shown in
figure.
There is no input signal.
Apply KVL in the output
ckt-
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Transistor Load Line Analysis
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Operating Point
It is called operating point because variation of IC
takes place about this point.
It is also called quiescent point or Q-point.
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Thank you !!
27/01/2025
