A Bipolar Junction Transistor is a three-terminal semiconductor device consisting 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 BJT is a type of transistor that uses both electrons and holes as charge carriers

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BJT Properties
& Applications
by Emroz Sardar

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03
02
01
04
Operation of Bipolar Junction Transistor
Introduction
Types of Mode and Application
TABLE OF CONTENTS
Conclusion

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What is a BJT ?
A Bipolar Junction Transistor is a three-terminal
semiconductor device consisting 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 BJT is a
type of transistor that uses both electrons and holes as
charge carriers.
A signal of small amplitude if applied to the base is
available in the amplified form at the collector of the
transistor. This is the amplification provided by the BJT
provided an external source of DC power supply to carry
out the amplification process.

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TYPES OF Bipolar Junction Transistor
There are two types of bipolar junction transistors –
NPN transistors and PNP transistors. A diagram of these two types of
bipolar junction transistors is given below.

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Construction of Bipolar Junction Transistor
The BJT is constructed with three doped semiconductor regions separated by
two pn junctions, as shown in the epitaxial planar structure in Figure below The
three regions are called emitter, base, and collector. One type consists of two n
regions separated by a p region (npn), and the other type consists of two p
regions separated by an n region (pnp). The term bipolar refers to the use of
both holes and electrons as current carriers in the transistor structure.

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Transistors have three terminals namely emitter, collector and
base. We have explained the functionalities of each of these
terminals below:
● Emitter – In a transistor, emitter supplies a large section of majority
charge carriers. The emitter is always forward biased with respect to the
base so that it supplies the majority charge carrier to the base. The
emitter of a transistor is heavily doped and moderate in size.
● Collector – In a transistor, the section that collects the majority of the
charge carrier supplied by the emitter is called a collector. The collector-
base junction is always reverse biased. The collector section of the
transistor is moderately doped, but larger in size so that it can collect
most of the charge carrier supplied by the emitter.
● Base – The middle section of the transistor is known as the base. The
base forms two circuits, the input circuit with the emitter and the output
circuit with the collector. The emitter-base is forward biased and offers
low resistance to the circuit. The collector-base junction is in reverse bias
and offers higher resistance to the circuit. The base of a transistor is
lightly doped and very thin due to which it offers the majority charge
carrier to the base.

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Operation of Bipolar Junction Transistor
Cutoff Region: Base-emitter junction is reverse biased. No current flow.
Saturation Region: Base-emitter junction is forward biased and Collector-base
junction is forward biased.
Active Region: Base-emitter is junction forward biased and Collector-base junction is
reverse biased.
Breakdown Region: IC and VCE exceed specifications and can cause damage to the
transistor.

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Modes of Operation
1. Common Base (CB) mode
2. Common Emitter (CE) mode
3. Common Collector (CC) mode

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Common Base (CB) mode
The configuration in which the base of the transistor is common between
emitter and collector circuit is called a common base configuration. The
common base circuit arrangement for NPN and PNP transistor is shown in
the figure below. In common base-emitter connection, the input is
connected between emitter and base while the output is taken across
collector and base.

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Input Characteristic
The curve plotted between emitter current IE
and the emitter-base voltage VEB at constant
collector base voltage VCB is called input
characteristic curve. The input characteristic
curve is shown in the figure below.
Output Characteristic Curve
In common base configuration, the curve
plotted between the collector current and
collector base voltage VCB at constant
emitter current IE is called output
characteristic. The CB configuration of PNP
transistor is shown in the figure below. The
following points from the characteristic
curve are taken into consideration.

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Voltage gain
In the common-base circuit, we follow another basic
transistor parameter: the ratio between collector current
and emitter current, which is a fraction always less than 1.
This fractional value for any transistor is called the alpha
ratio, or α ratio
Current gain
Since the common base amplifier can not operate as a current
amplifier (Ai ≅ 1), it must therefore have the ability to operate as a
voltage amplifier. The voltage gain for the common base amplifier
is the ratio of VOUT/VIN, that is the collector voltage VC to the
emitter voltage VE.

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The applications of the common base amplifier circuit-
It is used in moving coil microphones Preamplifiers.
It is used in UHF and VHF RF amplifiers.
It is mainly used at high frequencies where low source resistance
is common.
It is used for impedance matching in circuits with very low output
resistances to those with a high input resistance.
Application Of Common Base

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Common Emitter (CE) mode
The configuration in which the emitter is connected between the collector
and base is known as a common emitter configuration. The input circuit is
connected between emitter and base, and the output circuit is taken from
the collector and emitter. Thus, the emitter is common to both the input
and the output circuit, and hence the name is the common emitter
configuration. The common emitter arrangement for NPN and PNP
transistor is shown in the figure below.

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Input characteristics
The input characteristics describe
the relationship between input
current or base current (IB) and
input voltage or base-emitter
voltage (VBE).
Output characteristics
The output characteristics
describe the relationship between
output current (IC) and output
voltage (VCE).

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The current gain of the common emitter amplifier is defined as the
ratio of change in collector current to the change in base current. The
voltage gain is defined as the product of the current gain and the ratio
of the output resistance of the collector to the input resistance of the
base circuits. The following equations show the mathematical
expression of the voltage gain and the current gain.
β = ΔIc/ ΔIb
Av = β Rc/Rb
Common Emitter Voltage Gain

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Application of Common Emitter
Low-frequency voltage amplifier
The input capacitor C removes any constant component of the input, and the
resistors R1 and R2 bias the transistor so that it will remain in active mode for the entire
range of the input. The output is an inverted copy of the AC component of the input
that has been amplified by the ratio RC/RE and shifted by an amount determined by all
four resistors. Because RC is often large, the output impedance of this circuit can be
prohibitively high. To alleviate this problem, RC is kept as low as possible and the
amplifier is followed by a voltage buffer like an emitter follower.
Radio
Common-emitter amplifiers are also used in radio frequency circuits, for example
to amplify faint signals received by an antenna.In this case it is common to
replace the load resistor with a tuned circuit. This may be done to limit the
bandwidth to a narrow band centered around the intended operating frequency.
More importantly it also allows the circuit to operate at higher frequencies as the
tuned circuit can be used to resonate any inter-electrode and stray
capacitances, which normally limit the frequency response. Common emitters are
also commonly used as low-noise amplifiers.

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Common Collector (CC) mode
In the Common Collector or grounded collector configuration, the collector is
connected to ground through the supply, thus the collector terminal is common to
both the input and the output. The input signal is connected directly to the base
terminal, while the output signal is taken from across the emitter load resistor as
shown. This type of configuration is commonly known as a Voltage Follower or
Emitter Follower circuit.
Parameter Characteristic
s
Voltage gain Zero
Current gain High
Power gain Medium
Input or output phase
relationship
Zero degree
Input resistance High
Output resistance Low

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Input characteristics
Input characteristics are the relationship
between the input current IB and input voltage
VCB keeping output voltage constant.
Output characteristics are the
relationship between the output current
and output voltage keeping input current
constant
Output characteristics

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Current Gain
The current gain is defined as the ratio of the load current to the input current.
Ai= il/ib= -ie/ib
From the h-parameter circuit, it can be determined that the emitter and base currents are
related through the dependent current source by the constant hfe+1. The current gain is
dependent only on the BJT characteristics and independent of any other circuit element
values. Its value is given by Ai= hfe+1
Voltage Gain
The voltage gain is the ratio of output voltage to input voltage. If the input voltage is again
taken to be the voltage at the input to the transistor, Vb.
Av= Vo/Vb
Av= (vo/il)(il/ib)(ib/vb)
Replacing each term with its equivalent expression
Av= (Re)(Ai)(1/Ri)

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Applications of Common Collector
· This amplifier is used as an impedance matching circuit.
· It is used as a switching circuit.
· The high current gain combined with near-unity voltage gain makes this
circuit a great voltage buffer
· It is also used for circuit isolation.

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Applications of BJT
The applications of the bipolar junction transistor are as
follows:
1. These are the transistors that are preferred in the logic
circuits.
2. It is used in the circuits of amplification.
3. These are preferred in the oscillation circuits.
4. These are preferred in the multi-vibrator circuits.
5. In the clipping circuits, these are preferred for wave
shaping circuits.
6. It used in the circuits of the timer and the circuits of the
time delay.
7. These are used in the circuits of switching.
8. Used in the circuits of detector or as demodulation.

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Conclusion
To conclude, we have discussed about BJT, its types,
operations and properties, the different modes
including CB, CC and, CE modes, and their
applications. We have also seen where and how BJT
is applied in our daily lives.

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