23CCE114 ELECTRONICS ENGINEERING

UNIT-2

Course Instructor: Sujith Kalluri

k_sujith@av.amrita.edu

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UNIT-2 Syllabus

• Bipolar Junction Transistors (BJT): Transistor construction and

working principle (qualitative)

• BJT Characteristics; Modes of operation, Input and output

characteristics of CB, CE, and CC Configurations

• BJT Biasing; Fixed bias without and with emitter resistance, collector
to base bias, voltage divider bias and emitter bias

• Transistor as an amplifier, switch

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Introduction to Bipolar Junction Transistor
• A bipolar junction transistor is a three-terminal semiconductor device that
consists of two p-n junctions which are able to amplify or magnify a signal.
• It is a current controlled device.
• The three terminals of the BJT are the base, the collector, and the emitter.
• A signal of a small amplitude applied to the base is available in the amplified
form at the collector of the transistor.
• This is the amplification provided by the BJT.
• Note that it does require an external source of DC power supply to carry out the
amplification process.

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Construction of Bipolar Junction Transistor
• BJT is a semiconductor device that is constructed with 3 doped semiconductor
Regions, i.e., Base, Collector & Emitter, separated by 2 p-n Junctions.
• Bipolar transistors are manufactured in two types, PNP and NPN, and are
available as separate components, usually in large quantities.
• The prime use or function of this type of transistor is to amplify current. This
makes them useful as switches or amplifiers.
• They have a wide application in electronic devices like mobile phones,
televisions, radio transmitters, and industrial control.
Operation of Bipolar Junction Transistor
There are three operating regions of a bipolar junction transistor:
• Active region: The region in which the transistors operate as an amplifier.
• Saturation region: The region in which the transistor is fully on and operates as a
switch such that the collector current is equal to the saturation current.
• Cut-off region: The region in which the transistor is fully off, and the collector
current is equal to zero. 4
Types of Bipolar Junction Transistor

There are two types of bipolar junction transistors:

• PNP bipolar junction transistor
• NPN bipolar junction transistor

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Working of NPN Transistor

Active mode
• Junction J1 is forward bias
• Junction J2 is reverse bias 6
Function of Bipolar Junction Transistor
• The flow of charge in a Bipolar transistor is due to the diffusion of charge
carriers between the two regions belonging to different charge
concentrations.
• Regions of BJT are known as the base, collector, and emitter.
• The emitter region is highly doped when compared to other layers. Both
collector and base layers have the same charge carrier concentrations.
• Among these junctions, the base-emitter junction is forward-biased, and the
base-collector junction is reverse-biased. Forward biased means the p-doped
region has more potential than the n-doped side.
Voltage, Charge Control and Current
• The base-emitter current is controlled by the collector-emitter current. This
conclusion is drawn by the current-voltage relation of the base-emitter junction.
• Collector current has a base region where minority carriers are concentrated.
• Transistor models such as the Glenn poon model are responsible for the
distribution of the charge which explains the behavior of a transistor. 7
Configuration of Bipolar Junction Transistor
Since a Bipolar Junction Transistor is a three-terminal device, there are three
ways to connect it within an electric circuit while one terminal is the same for both
output and input. Every method of connection responds differently to the input
signals within a circuit.

• Common Emitter Configuration – has both voltage and current gain

• Common Collector Configuration – has no voltage gain but has a current gain
• Common Base Configuration – has no current gain but has a voltage gain

Applications of BJT
We know that a bipolar junction transistor is used as a switch, as an amplifier, as a
filter, and even as an oscillator. Below is the list of other applications of bipolar
junction transistors:
• BJT is used as a detector or also known as a demodulator.
• BJT finds application in clipping circuits to shape the waves.
• Logic circuits and switching circuits use BJT. 8
Characteristics of different transistor configurations

Common Common Common

Characteristics
Base Emitter Collector
Power Gain low Very high medium
Current gain low medium high
Voltage gain High Medium low
Phase angle 0 180 0
Output
Very high high low
impedance
Input
Low medium high
Impedance

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Characteristics of Transistor

Any two-port network which is analogous to transistor configuration circuits

can be analyzed using three types of characteristic curves. They are:

• Input Characteristics: The curve describes the changes in the values of

input current with respect to the values of input voltage, keeping the output
voltage constant.
• Output Characteristics: The curve is obtained by plotting the output
current against the output voltage, keeping the input current constant.
• Current Transfer Characteristics: This characteristic curve describes the
variation of output current in accordance with the input current, keeping the
output voltage constant.

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Common Emitter (CE) Configuration of Transistor
The configuration in which the emitter is connected between the collector and
base is known as a common emitter configuration.

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Input Characteristics

The variation of emitter current(IB) with Base-Emitter voltage (VBE), keeping

Collector Emitter voltage(VCE) constant.

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Output Characteristics
The variation of collector current(IC) with Collector-Emitter
voltage(VCE), keeping the base current(IB) constant.

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Current Transfer Characteristics

The variation of collector current(IC) with the base current(IB), keeping

Collector-Emitter voltage(VCE) constant.
The resulting current gain has a value greater than 1.

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Common Base (CB) Configuration of Transistor

In CB Configuration, the base terminal of the transistor will be commonly

connected between the output and the input terminals.

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Input Characteristics
The variation of emitter current(IE) with Base-Emitter voltage(VBE), keeping
Collector Base voltage(VCB) constant.

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Output Characteristics
The variation of collector current(IC) with Collector-Base voltage(VCB),
keeping the emitter current(IE) constant.

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Current Transfer Characteristics

The variation of collector current(IC) with the emitter current(IE), keeping

Collector Base voltage(VCB) constant.
The resulting current gain has a value of less than 1.

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Common Collector (CC) Configuration of Transistor
In CC Configuration, the Collector terminal of the transistor will be commonly
connected between the output and the input terminals.

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Input Characteristics

The variation of emitter current(IB) with Collector-Base voltage(VCB), keeping

Collector Emitter voltage(VCE) constant.

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Output Characteristics

The variation of emitter current(IE) with Collector-Emitter voltage(VCE), keeping

the base current(IB) constant.

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Current Transfer Characteristics

The variation of Emitter current(IE) with the base current(IB), keeping Collector-
Emitter voltage(VCE) constant.

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The relationship between α, β and γ in BJT

α = Common base amplification factor = IC/IE

β = common emitter forward current amplification factor = IC/IB

γ = current gain of the common collector = IE/IB

The current relationship is, γ = 1 + β = 1/(1-α)

The ascending order of α, β and γ is α < β < γ

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BJT Regions of Operation

• When the BJT is operating in the

cutoff region, then it could be used like
an open switch.

• If it is operating in the saturation

region, the BJT could be used as a
closed switch.

• The BJT can also be used as an

amplifier if you properly bias it to
operate in the active or linear region.

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Problems

1. A common base transistor amplifier has an input resistance of 20 Ω and an

output resistance of 100 kΩ. The collector load is 1 kΩ. If a signal of 500 mV is
applied between the emitter and base, find the voltage amplification.
Assume αac to be nearly one.

Solution

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2. In a common base connection, IE = 1mA, IC = 0.95mA. Calculate the value of IB .

Solution

3. In a common base connection, current amplification factor is 0.9. If the emitter

current is 1mA, determine the value of base current.
Solution

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4. In a common base connection, the emitter current is 1mA. If the emitter
circuit is open, the collector current is 50 μA. Find the total collector current.
Given that α = 0.92.
Solution

5. In a common base connection, α = 0.95. The voltage drop across 2 kΩ

resistance which is connected in the collector is 2V. Find the base current.

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Solution
The voltage drop across RC (= 2 kΩ) is 2V.

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6. For the common base circuit shown in Figure, determine IC and VCB .
Assume the transistor to be of silicon.
Solution
Since the transistor is silicon, VBE = 0.7V.
Applying Kirchhoff’s voltage law to the
emitter-side loop, we get,

Applying Kirchhoff’s voltage law to the

collector-side loop, we have,

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7. For a transistor, β = 45 and voltage drop across 1kΩ which is connected in
the collector circuit is 1 volt. Find the base current for common emitter
connection.
Solution
The voltage drop across RC (= 1 kΩ) is 1
volt.

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8. A transistor is connected in a common emitter (CE) configuration where the
collector supply is 8 V and the voltage drop across resistance RC connected in
the collector circuit is 0.5 V. The value of RC = 800 Ω. If α = 0.96, determine (i)
collector-emitter voltage (ii) base current.
Solution

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9. Using diagrams, explain the correctness of the relation
ICEO = (β + 1)ICBO.

Solution

The leakage current ICBO is the current that flows through the base-collector
junction when the emitter is open, as shown is Figure below.

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When the transistor is in CE arrangement, the base current (i.e. ICBO) is
multiplied by β in the collector as shown in Figure below.

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10. Determine VCB in the transistor circuit shown in Figure.
The transistor is silicon and has β = 150.
Solution

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BJT Biasing

• Electronic circuits with amplification capabilities can perform more efficiently if

the BJT undergoes biasing.
• Generally, this process involves applying an external voltage to its terminals
that switch the device to the desired state.
• Many circuit designs commonly feature resistors to distribute correct input
current and voltage levels.
• Varying BJT biasing techniques provide specific characteristics, while others
prevent thermal runaway.
• In effect, this makes them very useful for amplification applications.
• Transistor biasing involves applying a specific amount of voltage to a BJT’s
base and emitter terminals, improving its efficiency and performance. In this
case, the process enables a transistor to amplify an AC input signal in a
transistor circuit. So, biasing the BJT will set the emitter-base junction in a
forward-biased state. Meanwhile, the base-collector intersection will configure
to a reverse-biased state. Thus, it will operate in the active region. 38
Beta BJT

• Beta (β) refers to the device’s overall

sensitivity between the base current and
its collector amplification level.
• It can also identify the device’s gain.
• For example, a transistor’s base current
will amplify by 100 if the β value matches
that value.
• Of course, this factor is generated while
the bipolar junction transistor operates in
the forward-active state.

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BJT Biasing Circuits
Fixed Bias
As you can see in the circuit diagram, a base
resistor (RB) connects to the VCC and base
terminal. In this case, a voltage drop across
RB causes the base-emitter junction to set to a
forward-biased state.

The fixed base bias circuit relies on minimal

components with a simplistic design. By adjusting
the RB value in the course, users can change the
active region’s operating point. In addition, the
source does not have a load since the base-emitter
junction features no resistors. As a result, this circuit
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has switching applications.
Fixed bias with emitter resistor

Here, if IC rises due to an increase in

temperature, the IE also increases,
increasing the voltage drop across RE.
This results in the reduction of VC, causing
a decrease in IB, bringing IC back to its
normal value. Thus, this kind of biasing
network offers better stability compared to a
fixed base bias network.

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Collector-to-base bias
The base resistor RB is connected across the collector and
the base terminals of the transistor.
This means that the base voltage, VB, and the collector
voltage, VC are inter-dependent because

• From these equations, it is seen that an increase in IC

decreases VC, which results in a reduced IB.
• This indicates that, for this type of biasing network, the Q-
point (operating point) remains fixed irrespective of the
variations in the load current, causing the transistor to
always be in its active region regardless of β value.
• Furthermore, this circuit is also referred to as a self-biasing
negative feedback circuit, as the feedback is from output
to input via RB. 42
Voltage divider bias or potential divider

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• Here, the common emitter transistor configuration is
biased using a voltage divider network to increase
stability.
• The name of this biasing configuration comes from the
fact that the two resistors RB1 and RB2 form a voltage
or potential divider network across the supply with
their center point junction connected to the transistors
base terminal, as shown.
• This voltage divider biasing configuration is the most
widely used transistor biasing method.
• The emitter diode of the transistor is forward biased
by the voltage value developed across resistor RB2.
• Also, voltage divider network biasing makes the
transistor circuit independent of changes in beta as
the biasing voltages set at the transistor base, emitter,
and collector terminals are not dependent on external
circuit values. 44
Emitter bias
• The circuit, as shown above, relies on two power
supply sources known as VCC and VEE to operate.
These feature equal but opposite polarities.
• VEE sets the base-emitter junction to a forward-
biased state. Meanwhile, VCC forms the collector-
base intersection to a reverse bias state.
• In this kind of biasing, IC can be independent of both
β and VBE by choosing RE >> RB/β and VEE >> VBE,
respectively, which results in a stable operating
point.

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• BJT biasing ensures that the transistor will operate correctly in a circuit,
providing AC signal amplification.

• It achieves this by selecting resistors that affect the transistor’s operating point
or Q-point.

• In bipolar transistor circuits, the Q-point is represented by ( VCE, IC ) for the NPN
transistors or ( VEC, IC ) for PNP transistors. The stability of the base bias
network and, therefore, the Q-point is generally assessed by considering the
collector current as a function of both Beta (β) and temperature.

• Additionally, the collector junction sets to a reverse bias state while the emitter-
base sets to a forward-biased state.

• Of course, the circuit design will depend entirely on the intended application and
what you want to achieve. 46
Transistor as an Amplifier

The electronic circuit that performs amplification is known as an Amplifier. The

transistor is the main component in Amplifiers. Bipolar Junction Transistor (BJT)
is the basic transistor among all transistors. So, if we use BJT in the amplifier
circuits, those are known as BJT amplifiers.

BJT Amplifier Circuit

The Amplifiers are classified into three

types according to the quantity amplified at
the output.

1. Voltage amplifiers
2. Current amplifiers
3. Power amplifiers
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Types of BJT Amplifiers

Common Base Amplifier

In this BJT Amplifier, the AC voltage waveform,

which is applied at the emitter terminal, will be
amplified and produced at the collector terminal.
This circuit has no phase shift between the input
and output waveforms. The characteristics of a CB
amplifier are mentioned below.

• Low input resistance

• High output resistance
• High voltage gain
• The current gain is approximately equal to one

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Common Emitter Amplifier

In this BJT Amplifier, the AC voltage

waveform applied at the base terminal will
be amplified and produced at the collector
terminal. However, there is a 180˚ phase
difference between the input and output
waveforms. The characteristics of the CE
amplifier are mentioned below.

• Medium input resistance

• Medium output resistance
• Medium voltage gain.
• Medium current gain.

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Common Collector Amplifier

The BJT amplifier configuration with the lowest

output resistance is the Common Collector
configuration. In this BJT Amplifier, the AC
voltage waveform, which is applied at the base
terminal, will be produced at the emitter terminal
with unity voltage gain. This circuit has no phase
shift between the input and output waveforms.
The characteristics of the CC amplifier are
mentioned below.

• High input resistance

• Low output resistance
• Voltage gain is approximately equal to one.
• High current gain.
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Comparison Between Types of BJT Amplifiers

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Transistor as a Switch

When a transistor is used as a switch, there are two operating regions, and
they are saturation region where the transistor is fully ON and the cut-off region
where the transistor is fully OFF.
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Cut-off Region

When the transistor is in the cut-off region, this is because there is no flow of
current through the transistor. This also means that the base current IB is
zero, and the collector current IC is also zero. When these currents are zero,
the collector voltage VCE would be at its maximum, which results in a large
depletion layer.

Saturation Region

In this region, the maximum base current, IB, is applied to the transistor, and
the maximum current is obtained at the collector, IC. Since the current is
maximum, the voltage at the collector would be minimum. Therefore, in this
condition, the transistor is said to be full ON.
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Reference Textbooks
T1: A. P. Malvino, “Electronic Principles”, 7th Edition, Tata McGraw Hill, 2007.
T2: D.P. Kothari, I. J. Nagrath, “Basic Electronics”, McGraw Hill Education
(India) Private Limited, 2014.
T3: David A. Bell, “Electronic Devices and Circuits”, 5th Edition, Oxford
University Press, 2008.
T4: Michael Tooley B. A., “Electronic circuits: Fundamentals and
Applications”, 3rd Edition, Elsevier Limited, 2006.
T5: R. Boylestad and L. Nashelsky, “Electronic Devices and Circuit Theory”,
Prentice Hall, Seventh Edition.

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