9.3.4. Learning Outcome No. 3.
Demonstrate understanding of transistors
9.3.4.1. Learning Activities
Learning Outcome No. 3. Demonstrate understanding of transistors
Learning Activities Special Instructions
Transistors are identified as per their Follow the links below for
characteristics experiments on BJT
NPN and PNP are determined as per their characteristics
operation vlabs.iitb.ac.in/vlab/electri
P and N channels are identified as per their cal/exp4/Theory.pdf
operation
Biasing and determination of gain of http://staff.iium.edu.my/ad
transistors is performed as per their standard ah510/ManualECE1201_S
operating procedure EM%20I%202015_2016.d
Transistor configuration is performed as per their oc
application
9.3.4.2.Information Sheet No. 9/ LO3
Bipolar Junction Transistor (Bjt)
The BJT is a three terminal semiconductor device whose output current, voltage and
power is controlled by its input current. This device is applied in electronic circuits
either as
1. A linear Amplifier to boost an electric signal or
2. As electronic switch.
The word transistor was derived from two words combination, transfer resistance. It
is a device that transfers a low resistance into a circuit having a high resistance. This
semiconductor device was invented by a team of three scientists at Bell Laboratories,
USA in 1947.
Basically, the bipolar junction transistor consists of two back-to-back P-N junctions
manufactured in a single piece of a semiconductor crystal. These two junctions are
formed by either sand witching P-Type or N-Type semiconductor layer between a
layer of opposite types thus forming either NPN or PNP transistors.
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A bipolar junction transistor has three regions called
Emitter region
Base region and
Collector region.
Each of the three regions has a terminal emanating from it and named after the
respective region. The circuit symbols for PNP and NPN transistors are also shown in
Figure above where the arrow head is always at the emitter and in each case its
direction indicates the conventional direction of current flow. For a PNP transistor,
arrowhead points from emitter to base meaning that emitter is positive with respect to
base. For NPN transistor, the arrow points from base to emitter meaning that base is
positive with respect to the emitter.
Characteristics of each region
1. Emitter
majority charge carries (either electrons or holes) to the base region. It second biggest
in size.
2. Base
It forms the middle section of the transistor. It is very thin (10 6 m) as compared to
-doped.
3. Collector
Collector region is made physically larger than the emitter region because it has to
dissipate much greater power. Its main function is to collect majority charge carriers
coming from the emitter and passing through the base region.
Thus,
Due to the difference in construction of collector and Emitter regions, biasing
polarities cannot be interchanged and have the transistor function the same way.
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Transistor Biasing
For a transistor to operate, working of a transistor, voltages of correct polarity must be
applied across its two junctions. A transistor has base-emitter P.N junction and the
base collector P.N junction
For normal operation;
1. emitter-base junction is always forward-biased while
2. collector-base junction is always reverse-biased.
Transistor Action/ operation
Working of NPN transistor.
When the NPN transistor is biased properly. I.e. applying forward bias to the emitter-
base junction and reverse bias to the collector-base junction.
The forward bias cause the electrons in the n-type emitter to flow towards the base.
This constitutes the emitter current IE. As these electrons flow through the p-type
base, they tend to combine with holes. As the base is lightly doped and very thin, only
a few electrons i.e. less than 5% combine with holes to constitute base current IB. The
remainder i.e. more than 95% cross over into the collector region to constitute
collector current IC. In this way almost entire emitter current flows in the collector
circuit.
The emitter current is thus the sum of collector and base current.
IE = IB + IC
Working of PNP transistor:
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The forward bias across the base-emitter PN junction causes the holes in the p-type
emitter to flow towards the base. This constitute the emitter current (IE). As these
holes cross into n-type base, some tend to combine with the electrons. As the base is
lightly doped and very thin, only a few holes i.e. less than 5% combine with the
electrons. This constitute the base current IB. The remainder holes i.e. more than 95%
cross into the collector region to constitute collector current IC.
Transistor circuit as an Amplifier
As an amplifier, the transistor raises the strength of a weak signal.
The weak signal is applied between the emitter and base while the output is taken
across the load RC, connected between the collector and base.
To achieve faithful amplification, the input circuit should always be forward biased
regardless of the polarity of the input signal.
As the input circuit is forward biased, it has low resistance and therefore, a small
change in signal voltage causes an appreciable change in emitter current. This causes
almost the same change in collector current due to transistor action.
If the collector current is made to flow through a high resistance (RC), its produces a
large voltage across it which is an amplified version of a weak signal at the input.
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Thus, a weak signal applied in the input circuit appears in the amplified form in the
collector circuit. It is in this way that a transistor acts as amplifier.
Transistor Circuit Configurations/ Transistor Modes Of Connection
There are three types of circuit connections (called configurations) for operating a
transistor.
1. common-base (CB),
2. common-emitter (CE),
3. common-collector (CC).
output
Circuits. Because the common electrode is generally grounded, these modes of
operation are
Frequently referred to as grounded-base, grounded-emitter and grounded-collector
configurations
Since a transistor is a 3-terminal (and not a 4-terminal) device, when used as an
amplifier, one of its terminals has to be common to the input and output circuits.
CB Configuration.
In this configuration, emitter current IE is the input current and collector current IC
is the output current. The input signal is applied between the emitter and base
whereas output is taken out between the base and collector terminal
Common base characteristics.
1. Has low input impedance.
2. High output impedance.
3. The voltage is less than unity, current gain is less than one and the overall
power gain is also when compared to the other transistor configurations.
4. The input and output signals are in phase.
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Common collector configuration:
This transistor configuration is also known as the emitter follower because the emitter
voltage follows that of the base. In this transistor configuration, the collector electrode
is common to both input and output circuits.
Common collector characteristics.
1. Has high input impedance due to the reverse biased base collector PN
junction
2. High output impedance
3. The voltage gain is less than one, although current gain is high.
4. The input and output signals are in phase.
In view of these characteristics, the emitter follower configuration is used as a buffer
circuit for impedance matching between two stages of common emitter amplifiers.
Common emitter mode of connection:
As can be seen from the diagram, the emitter electrode is common to both input and
output circuits. In common emitter mode of connection, the input current is base
current (Ib) while the output current is the collector current (Ic).
Figure 127: emitter mode
Common emitter characteristics.
1. Has low input impedance
2. Has high output impedance
3. Current gain is greater than one.
4. Voltage gain is greater than one
5. Power gain is greater than one
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6. The output is the inverse of the input, i.e. 180° phase change.
This transistor configuration is the most widely used.
Field-Effect Transistor (Fet)
The field-effect transistor (FET) is an electronic device which uses an electric field to
control the flow of current through it. FETs have 3-terminals.
Source,
gate, and
drain terminal.
FETs control the flow of current by the application of a voltage to the gate terminal,
which in turn alters the conductivity between the drain and source terminals.
FETs are also known as unipolar transistors since they involve single-carrier-type
operation. That is, FETs use electrons or holes as charge carriers in their operation,
but not both as is the case with the BJT.
Types of Field Effect Transistors (FETs)
FETS are six different types which are broadly classified in two main groups
1. Junction FET.
-N channel JFET.
-P channel JFET.
2. MOSFET or IGFET transistor
a. Enhancement MOSFET Transistor.
-N channel.
-P channel.
b. Depletion mode MOSFET transistor.
-N channel.
-P channel
N-Channel Depletion Mode MOSFET
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If the gate is connected to a positive voltage with respect to the source, electrons will
be attracted from the highly doped N-regions and move into the slightly doped N-
region. This reduces the size of the depletion layer since electrons are caused to move
into the region of the channel beneath the gate-insulator (the slightly doped N-region).
The drain current is increased since the channel resistance has been reduced. The
more positive the gate-source voltage, the smaller is the depletion layer and the more
the drain current that flows.
A negative voltage at the gate with respect to the source causes the widening of the
depletion layer by forcing electrons to move from the slightly doped N-region of the
channel beneath the gate-insulator. This results in increase of the channel resistance
and hence reduction of drain current. The more negative is the gate-source voltage,
the wider is the depletion layer and the less the drain current that flows.
P-Channel Depletion Mode MOSFET
A positive voltage at the gate with respect to the source causes the widening of the
depletion layer by forcing electrons to move into the region of the channel beneath the
gate-insulator (the slightly doped P-region). This results in reduction of drain current.
The more positive is the gate-source voltage, the wider is the depletion layer and the
less the drain current that flows.
If the gate is now connected to a negative voltage with respect to the source, HOLES
will be attracted from the highly doped P-regions and move into the slightly doped P-
region. This reduces the size of the depletion layer since HOLES are caused to move
into the region of the channel beneath the gate-insulator (the slightly doped P-region).
The drain current is increased since the channel resistance has been reduced. The
more negative the gate-source voltage, the smaller is the depletion layer and the more
the drain current that flows.
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Enhancement Mode Only MOSFET (E-Mosfet)
These types of MOSFETS are off at zero gate source voltage. The N-channel
MOSFET can be turned on by pulling the gate voltage higher than the source voltage
while the P channel MOSFET can be turned on by pulling the gate voltage lower than
the source voltage. Thus, pulling an enhancement-mode MOSFET's gate voltage
towards its drain voltage turns it ON.
Gain of transistor amplifier
When a transistor is to be connected as an amplifier, a signal is connected across the
input terminals and then obtained across the output terminals of the amplifier. The ratio
of the magnitude of the output signal to that of the input signal defines the gain of an
amplifier
DC load line.
A dc load line is a straight line that passes through two points on the output
the Dc load line where the transistor operating point is established.
AC load line.
The AC load line is a straight line with a slope equal to the AC impedance the
transistor amplifier is experiencing. The ratio of AC voltage to current in the device is
defined by this line
9.3.4.3.Self-Assessment
1. Describe a Bipolar Junction Transistor (BJT)
2. Explain regions of Bipolar Junction Transistor (BJT)
3. Define transistor biasing
4. Describe transistor circuit configurations/ transistor modes of connection
5. Describe the operation of field-effect transistor (FET
6.
output characteristics of a transistor. The two points are cutoff and saturation
point.
a) Dc load line.
b) Ac load line.
7. s a straight line with a slope equal to the AC impedance the
transistor amplifier is experiencing.
a) Dc load line.
b) Ac load line.
8. The BJT is a three terminal semiconductor device whose output current,
voltage and power is controlled by its input current
a) True
b) False
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9. the bipolar junction transistor consists of two back-to-back P-N junctions
manufactured in a single piece of a semiconductor crystal
a) True
b) False
Practical exercises
a) Identify Transistors as per their characteristics
b) Determine the operation pf P and N channels
c) Biasing and determination of gain of transistors is performed as per their standard
operating procedure
9.3.4.4. Tools, Equipment, Supplies
Assorted BJT transistors. NPN/PNP types
Assorted FETS.
Multipurpose meters
cables
dc power supplies
biasing resistors
bread boards
cupper strip boards
standard electronic tool kit
9.3.4.5. References
1. vlabs.iitb.ac.in/vlab/electrical/exp4/Theory.pdf
2. http://staff.iium.edu.my/adah510/ManualECE1201_SEM%20I%202015_2016.doc
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