Basic Electrical Engineering (IEEE102L)
BEEE – Dr. RRS
FALL Semester (2023-24)
Dr. R. Raja Singh
Sr. Associate Professor
Department of Energy and Power Electronics
School of Electrical Engineering
VIT Vellore, Tamil Nadu, India
rrajasingh@vit.ac.in | +91 98942 50650
30-10-2023
DR. R. RAJA SINGH/ SR. ASSOCIATE PROFESSOR/ VIT VELLORE
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BEEE – Dr. RRS
Syllabus
Module 6
Pre-requisite
Guest lectures from Industry, Research and Development Organizations
NIL
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MODULE 6
BEEE – Dr. RRS
Semiconductor Devices and Applications
Semiconductor Devices and Applications
Characteristics
• PN Junction diode
• Zener diode
• BJT
• MOSFET
Applications
• Rectifier
• Voltage regulator
• Operational amplifier
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Introduction to Semiconductor Devices
BEEE – Dr. RRS
A semiconductor is a material which has an electrical conductivity value falling between that of a
conductor and insulator (copper and glass).
Its resistivity falls as its temperature rises; metals behave in the opposite way. Its conducting properties
may be altered in useful ways by introducing impurities ("doping") into the crystal structure. When two
differently doped regions exist in the same crystal, a semiconductor junction is created. The behavior of
charge carriers, which include electrons, ions, and electron holes, at these junctions is the basis of
diodes, transistors, and most modern electronics.
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BEEE – Dr. RRS
Some examples of semiconductors are silicon,
germanium, gallium arsenide, and elements near the socalled "metalloid staircase" on the periodic table. After
silicon, gallium arsenide is the second-most common
semiconductor and is used in laser diodes, solar cells,
microwave-frequency integrated circuits, and others.
Silicon is a critical element for fabricating most electronic
circuits.
silicon
gallium arsenide
germanium
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Introduction to Semiconductor Devices
BEEE – Dr. RRS
Semiconductor devices can display a range of different useful properties, such as
passing current more easily in one direction than the other, showing variable
resistance, and having sensitivity to light or heat. Because the electrical properties of
a semiconductor material can be modified by doping and by the application of
electrical fields or light, devices made from semiconductors can be used for
amplification, switching, and energy conversion.
The conductivity of silicon is increased by adding a small amount (of the order of 1 in
108) of pentavalent (antimony, phosphorus, or arsenic) or trivalent (boron, gallium,
indium) atoms. This process is known as doping, and the resulting semiconductors
are known as doped or extrinsic semiconductors. Apart from doping, the
conductivity of a semiconductor can be improved by increasing its temperature. This
is contrary to the behavior of a metal, in which conductivity decreases with an
increase in temperature.
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Introduction to Semiconductor Devices
BEEE – Dr. RRS
Doping greatly increases the number of
charge carriers within the crystal. When
a doped semiconductor contains free
holes, it is called "p-type", and when it
contains free electrons, it is known as
"n-type". The semiconductor materials
used in electronic devices are doped
under precise conditions to control the
concentration and regions of p- and ntype dopants.
A single semiconductor device crystal can have many p- and n-type regions; the p–n junctions between these
regions are responsible for the useful electronic behavior. Using a hot-point probe, one can determine quickly
whether a semiconductor sample is p- or n-type.
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Introduction to Semiconductor Devices
BEEE – Dr. RRS
A few of the properties of semiconductor materials were observed throughout the mid-19th
and first decades of the 20th century. The first practical application of semiconductors in
electronics was the 1904 development of the cat's-whisker detector, a primitive
semiconductor diode used in early radio receivers. Developments in quantum physics led in
turn to the invention of the transistor in 1947 and the integrated circuit in 1958.
cat's-whisker in 1904
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Transistor in 1947
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Integrated circuit in 1958
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Transistor in 1947
BEEE – Dr. RRS
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PN Junction Diode
BEEE – Dr. RRS
A P-N junction is an interface or a boundary
between two semiconductor material types, namely
the p-type and the n-type, inside a semiconductor.
In a semiconductor, the P-N junction is created by
the method of doping. The p-side or the positive
side of the semiconductor has an excess of holes,
and the n-side or the negative side has an excess of
electrons. The process of doping is explained in
further detail in the next section.
PN Junction Power Diode
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Formation of P-N Junction
BEEE – Dr. RRS
If we add a small amount of pentavalent
impurity In a thin p-type silicon semiconductor
sheet, a part of the p-type Si will get converted
to n-type silicon. This sheet will now contain
both the p-type region and the n-type region
and a junction between these two regions. The
processes that follow after forming a P-N
junction are of two types – diffusion and drift.
There is a difference in the concentration of
holes and electrons at the two sides of a
junction. The holes from the p-side diffuse to
the n-side, and the electrons from the n-side
diffuse to the p-side. These give rise to a
diffusion current across the junction.
The pentavalent impurity atom has five valence electrons
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Unbiased P-N Junction
BEEE – Dr. RRS
when an electron diffuses from the n-side to the p-side, an ionized donor is left behind on the n-side,
which is immobile. As the process goes on, a layer of positive charge is developed on the n-side of the
junction. Similarly, when a hole goes from the p-side to the n-side, an ionized acceptor is left behind on
the p-side, resulting in the formation of a layer of negative charges in the p-side of the junction. This
region of positive charge and negative charge on either side of the junction is termed as the depletion
region. Due to this positive space charge region on either side of the junction, an electric field with the
direction from a positive charge towards the negative charge is developed. Due to this electric field, an
electron on the p-side of the junction moves to the n-side of the junction. This motion is termed the drift.
Here, we see that the direction of the drift current is opposite to that of the diffusion current.
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Biasing of P-N Junction
BEEE – Dr. RRS
Biasing Conditions for the P-N Junction Diode
There are two operating regions in the P-N junction diode:
• P-type
• N-type
There are three biasing conditions for the P-N junction diode, and this is based on the voltage
applied:
Zero bias: No external voltage is applied to the P-N junction diode. (Unbiased P-N Junction)
Forward bias: The positive terminal of the voltage potential is connected to the p-type while
the negative terminal is connected to the n-type.
Reverse bias: The negative terminal of the voltage potential is connected to the p-type and the
positive is connected to the n-type.
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Zero bias
BEEE – Dr. RRS
No external voltage is applied to the P-N junction diode.
The potential barrier that now exists discourages the
diffusion of any more majority carriers across the junction.
However, the potential barrier helps minority carriers (few
free electrons in the P-region and few holes in the Nregion) to drift across the junction.
Then an “Equilibrium” or balance will be established when
the majority carriers are equal and both moving in opposite
directions, so that the net result is zero current flowing in
the circuit. When this occurs the junction is said to be in a
state of “Dynamic Equilibrium“.
The minority carriers are constantly generated due to
thermal energy so this state of equilibrium can be broken
by raising the temperature of the PN junction causing an
increase in the generation of minority carriers, thereby
resulting in an increase in leakage current but an electric
current cannot flow since no circuit has been connected to
the PN junction.
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Forward bias
BEEE – Dr. RRS
The positive terminal of the voltage potential is connected to the p-type while
the negative terminal is connected to the n-type.
When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-type material and a
positive voltage is applied to the P-type material. If this external voltage becomes greater than the value of the
potential barrier, approx. 0.7 volts for silicon and 0.3 volts for germanium, the potential barriers opposition will be
overcome and current will start to flow.
This is because the negative voltage pushes or
repels electrons towards the junction giving them
the energy to cross over and combine with the holes
being pushed in the opposite direction towards the
junction by the positive voltage. This results in a
characteristics curve of zero current flowing up to
this voltage point, called the “knee” on the static
curves and then a high current flow through the
diode with little increase in the external voltage as
shown below.
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Reverse Bias
BEEE – Dr. RRS
The negative terminal of the voltage potential is connected to the p-type and
the positive is connected to the n-type.
When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type material and a
negative voltage is applied to the P-type material.
The positive voltage applied to the N-type
material attracts electrons towards the positive
electrode and away from the junction, while the
holes in the P-type end are also attracted away
from the junction towards the negative electrode.
The net result is that the depletion layer grows
wider due to a lack of electrons and holes and
presents a high impedance path, almost an
insulator and a high potential barrier is created
across the junction thus preventing current from
flowing through the semiconductor material.
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BEEE – Dr. RRS
PN junction as a practical device or as a
rectifying device we need to firstly bias the
junction, that is connect a voltage potential
across it. On the voltage axis above, “Reverse
Bias” refers to an external voltage potential
which increases the potential barrier. An
external voltage which decreases the
potential barrier is said to act in the “Forward
Bias” direction.
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Important characteristics of PN Junction Diode
BEEE – Dr. RRS
The PN junction region of a Junction Diode has the following important characteristics:
Semiconductors contain two types of mobile charge carriers, “Holes” and “Electrons”.
The holes are positively charged while the electrons negatively charged.
A semiconductor may be doped with donor impurities such as Antimony (N-type doping), so that it contains mobile
charges which are primarily electrons.
A semiconductor may be doped with acceptor impurities such as Boron (P-type doping), so that it contains mobile
charges which are mainly holes.
The junction region itself has no charge carriers and is known as the depletion region.
The junction (depletion) region has a physical thickness that varies with the applied voltage.
When a diode is Zero Biased no external energy source is applied and a natural Potential Barrier is developed across a
depletion layer which is approximately 0.5 to 0.7v for silicon diodes and approximately 0.3 of a volt for germanium
diodes.
When a junction diode is Forward Biased the thickness of the depletion region reduces and the diode acts like a short
circuit allowing full circuit current to flow.
When a junction diode is Reverse Biased the thickness of the depletion region increases and the diode acts like an
open circuit blocking any current flow, (only a very small leakage current will flow).
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BEEE – Dr. RRS
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BEEE – Dr. RRS
Types of Diode
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BJT Bipolar Junction Transistors
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Transistor Basics
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Bipolar
BEEE – Dr. RRS
Transistor
Current Controlled Device
Power Bipolar Junction Transistor (BJT)
Power BJT is used traditionally for many applications. However, IGBT (Insulated-Gate Bipolar Transistor) and MOSFET
(Metal-Oxide-Semiconductor Field-Effect Transistor) have replaced it for most of the applications but still they are used in
some areas due to its lower saturation voltage over the operating temperature range. IGBT and MOSFET have higher input
capacitance as compared to BJT. Thus, in case of IGBT and MOSFET, drive circuit must be capable to charge and discharge
the internal capacitances.
Power n-p-n transistors
are widely used in
high-voltage and highcurrent applications
NPN BJT
PNP BJT
The BJT is a three-layer and two-junction npn or pnp semiconductor device. Although BJTs have lower input capacitance as
compared to MOSFET or IGBT, BJTs are considerably slower in response due to low input impedance. BJTs use more silicon for
the same drive performance. Power BJT is different in configuration as compared to simple planar BJT. In planar BJT, collector
and emitter is on the same side of the wafer while in power BJT it is on the opposite edges as shown. This is done to increase
the power-handling capability of BJT.
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Bipolar
BEEE – Dr. RRS
Transistor
Current Controlled Device
Power Bipolar Junction Transistor (BJT)
Why BJT is called transistor?
That uses both electrons and holes as charge carriers.
The construction of the Power Transistor is different from the
signal transistor as shown in the following figure. The n- layer is
added in the power BJT which is known as drift region.
• A Power BJT has a four layer structure of alternating P and N
type doping as shown in above NPN transistor.
• It has three terminals labeled as Collector, Base, Emitter.
• In most of Power Electronic applications, the Power Transistor
works in Common Emitter configuration. ie, Base is the input
terminal, the Collector is the output terminal and the Emitter
is common between input and output.
• In power switches NPN transistors are most widely used than
PNP transistors.
• The characteristics of the device is determined by the doping
level in each of the layers and the thickness of the layers.
• The thickness of the drift region determines the breakdown
voltage of the Power transistor.
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Power BJT NPN Structure
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Bipolar
BEEE – Dr. RRS
Transistor
Current Controlled Device
Power Bipolar Junction Transistor (BJT)
Input Characteristics and Output
Characteristics for the Common-Emitter
Configuration of planar BJT respectively
VCE Configuration
• The VI characteristics of the Power BJT is different from signal level transistor.
• The major differences are Quasi saturation region & secondary breakdown region.
• The Quasi saturation region is available only in Power transistor characteristic not
in signal transistors.
• It is because of the lightly doped collector drift region present in Power BJT.
• The primary breakdown is similar to the signal transistor’s avalanche breakdown.
• Operation of device at primary and secondary breakdown regions should be
avoided as it will lead to the catastrophic failure of the device.
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The primary breakdown shown takes place
because of avalanche breakdown of
collector base junction. Large power
dissipation normally leads to primary
breakdown.
The second breakdown shown is due to
localized thermal runaway. Secondary
breakdown is a failure mode in bipolar
power transistors. In a power transistor
with a large junction area, under certain
conditions of current and voltage, the
current concentrates in a small spot of the
base-emitter junction. This causes local
heating, progressing into a short between
collector and emitter.
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Bipolar
BEEE – Dr. RRS
Transistor
Current Controlled Device
Power Bipolar Junction Transistor (BJT)
Power BJT Output Characteristics Curve
This region appears due to the insertion of lightly-doped collector drift
region where the collector base junction has a low reverse bias. The
resistivity of this drift region is dependent on the value of the base
current. In the quasi-saturation region, the value of ß decreases
significantly. This is due to the increased value of the collector current
with increased temperature. But the base current still has the control
over the collector current due to the resistance offered by the drift
region. If the transistor enters in hard saturation region, base current
has no control over the collector current due to the absence of the
drift region and mainly depends on the load and the value of VCC.
A forward-biased p-n junction has two capacitances named depletion
Characteristic curves for power BJT is just the same layer capacitance and diffused capacitance. While a reverse bias
except for the little difference in its saturation region. junction has only a depletion capacitance in action. Value of these
It has additional region of operation known as quasi- capacitances depends on the junction voltage and construction of the
saturation.
transistor. These capacitances come into role during the transient
operation i.e. switching operations. Due to these capacitances,
The beta (β) of a transistor, or transistor current gain, is the
ratio of the transistor's collector current (Ic) to its base
transistor does not turn on or turn off instantly.
current (Ib), β = Ic/Ib. The β value is fixed for a given
transistor and operating condition.
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Bipolar
Transistor
Current Controlled Device
Power Bipolar Junction Transistor (BJT)
Turn-On and Turn-Off Characteristics of BJT
Switching characteristics of power BJT is shown. As the positive base
voltage is applied, base current starts to flow but there is no collector
current for some time. This time is known as the delay time (td) required
to charge the junction capacitance of the base to emitter to 0.7 V
approx. (known as forward-bias voltage). For t > td, collector current
starts rising and VCE starts to drop with the magnitude of 9/10th of its
peak value. This time is called rise time, required to turn on the
transistor. The transistor remains on so long as the collector current is at
least of this value.
For turning off the BJT, polarity of the base voltage is reversed and thus
the base current polarity will also be changed as shown. The base
current required during the steady-state operation is more than that
required to saturate the transistor. Thus, excess minority carrier charges
are stored in the base region which needs to be removed during the
turn-off process. The time required to nullify this charge is the storage
time, ts. Collector current remains at the same value for this time. After
this, collector current starts decreasing and base-to-emitter junction
charges to the negative polarity; base current also get reduced.
BEEE – Dr. RRS
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BEEE – Dr. RRS
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Unipolar
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MOSFET
Voltage Controlled Device
Metal-Oxide Semiconductor Field-Effect Transistor
MOSFET is a voltage-controlled majority carrier (or unipolar) three-terminal device. Its symbols are
shown. As compared to the simple lateral channel MOSFET for low-power signals, power MOSFET
has different structure. It has a vertical channel structure where the source and the drain are on the
opposite side of the silicon wafer as shown. This opposite placement of the source and the drain
increases the capability of the power MOSFET to handle larger power.
N-channel enhancement type MOSFET is more common due to high mobility of electrons.
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Unipolar
BEEE – Dr. RRS
MOSFET
Voltage Controlled Device
Metal-Oxide Semiconductor Field-Effect Transistor
Drift region shown determines the voltage-blocking capability of the
MOSFET.
When VGS = 0,
⇒ VDD makes it reverse biased and no current flows from drain to source.
When VGS > 0,
⇒ Electrons form the current path as shown. Thus, current from the drain
to the source flows. Now, if we will increase the gate-to-source voltage,
drain current will also increase.
Power MOSFET Structural View with Connections
The output characteristics curves, transfer
characteristics of power MOSFET is also shown
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Unipolar
BEEE – Dr. RRS
•
•
•
•
•
•
•
MOSFET
Voltage Controlled Device
Metal-Oxide Semiconductor Field-Effect Transistor
The p-type semiconductor forms the base of the MOSFET.
From the heavily doped regions of the base, the terminals source and drain originate.
The layer of the substrate is coated with a layer of silicon dioxide for insulation.
A thin insulated metallic plate is kept on top of the silicon dioxide and it acts as a capacitor.
The gate terminal is brought out from the thin metallic plate.
A DC circuit is then formed by connecting a voltage source between these two n-type regions.
The two types of the base are highly doped with an n-type impurity which is marked as n+ in the diagram.
Working Principle of MOSFET
When voltage is applied to the gate, an electrical field is generated that changes the width of the channel region, where
the electrons flow. The wider the channel region, the better conductivity of a device will be.
The classification of MOSFET based on the construction and the material used
MOSFETs are of two classes: Enhancement mode and depletion mode.
Each class is available as n-channel or p-channel; hence overall they
tally up to four types of MOSFETs.
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Unipolar
BEEE – Dr. RRS
MOSFET
Voltage Controlled Device
Metal-Oxide Semiconductor Field-Effect Transistor
Depletion Mode
When there is no voltage across the gate terminal, the channel shows maximum conductance. When the voltage
across the gate terminal is either positive or negative, then the channel conductivity decreases.
Enhancement Mode
When there is no voltage across the gate terminal, then the device does not conduct. When there is the maximum
voltage across the gate terminal, then the device shows enhanced conductivity.
Operating Regions of MOSFET
Cut-Off Region
The cut-off region is a region in which there will be no conduction and as a result, the MOSFET will be OFF. In this
condition, MOSFET behaves like an open switch.
Ohmic Region
The ohmic region is a region where the current (IDS)increases with an increase in the value of VDS. When MOSFETs are
made to operate in this region, they are used as amplifiers.
Saturation Region
In the saturation region, the MOSFETs have their IDS constant in spite of an increase in VDS and occurs
once VDS exceeds the value of pinch-off voltage VP. Under this condition, the device will act like a closed switch
through which a saturated value of IDS flows. As a result, this operating region is chosen whenever MOSFETs are
required to perform switching operations.
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BEEE – Dr. RRS
Drain Current (ID)
Drain-to-Source Voltage (VDS)
Gate-to-Source Voltage (VGS)
Threshold Voltage (VTH)
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Drain Current (ID) vs Drain-to-Source Voltage (VDS)
Characteristics Curves
For lower value of VDS, MOSFET works in a linear region where it has
a constant resistance equal to VDS / ID. For a fixed value of VGS and
greater than threshold voltage VTH, MOSFET enters a saturation
region where the value of the drain current has a fixed value.
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Unipolar
BEEE – Dr. RRS
MOSFET
Voltage Controlled Device
Metal-Oxide Semiconductor Field-Effect Transistor
Output Characteristics with Load Line
Drain Current (ID) vs Drain-to-Source Voltage (VDS)
Characteristics Curves
For lower value of VDS, MOSFET works in a linear region where it has
a constant resistance equal to VDS / ID. For a fixed value of VGS and
greater than threshold voltage VTH, MOSFET enters a saturation
region where the value of the drain current has a fixed value.
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If XY represents the load line, then the X-point
represents the turn-off point and Y-point is the turn-on
point where VDS = 0 (as voltage across the closed switch
is zero). The direction of turning on and turning off
process is also shown.
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Unipolar
BEEE – Dr. RRS
MOSFET
Voltage Controlled Device
Metal-Oxide Semiconductor Field-Effect Transistor
MOSFET applications
Radiofrequency applications use MOSFET amplifiers extensively.
MOSFET behaves as a passive circuit element.
Power MOSFETs can be used to regulate DC motors.
MOSFETs are used in the design of the chopper circuit.
Advantages of MOSFET
MOSFETs operate at greater efficiency at lower voltages.
Absence of gate current results in high input impedance producing high switching speed.
Disadvantages of MOSFET
MOSFETs are vulnerable to damage by electrostatic charges due to the thin oxide layer.
Overload voltages make MOSFETs unstable.
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Comparison
BJT – MOSFET
BEEE – Dr. RRS
Unipolar Transistor: They use a single charge
carrier i.e. either electrons or holes for the
operation. These do not have any junction as they
are made up of either N-Type or P-type.
Bipolar Transistor: As the name suggests two
poles i.e. both the electrons and holes contribute
in the operation. Made up of N-type combined
with P-type forming a junction between these
two. Common configuration is N-P-N or P-N-P.
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BEEE – Dr. RRS
Operational Amplifier
An operational amplifier (op-amp) is an integrated circuit (IC) that amplifies the difference in voltage between two inputs.
It is so named because it was developed for perform arithmetic
operations. Amplifiers, buffers, comparators, filters, etc. can be
implemented with simple external circuits.
An op-amp has five terminals: positive power supply, negative power
supply (GND), noninverting input, inverting input, and output.
Generally, these terminals are named as shown below. (Positive and
GND terminals may be omitted from the symbol of single-supply opamps)
An op-amp amplifies the difference in voltage between the
noninverting (IN(+)) and inverting (IN(-)) inputs. Its output voltage is
given by Equation 1, which indicates that the output is in the same
phase as VIN(+) and in opposite phase to VIN(-).
VOUT = A * ( VIN(+) – VIN(-) )
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(1)
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Operational Amplifier
BEEE – Dr. RRS
In the basic form of usage, an op-amp acts as a voltage amplifier or a comparator. It can also be configured as a filter, phase
shifter, buffer (voltage follower), etc. Nowadays, op-amps are commonly used to amplify weak analog signals from sensors in
a wide range of IoT devices and home appliances.
Op-amps are generally used with negative feedback to reduce product variations in gain and expand the bandwidth. Typical
applications of op-amps include noninverting amplifiers, inverting amplifiers, and voltage followers, which are configured as
shown below:
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