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EE212-Electronics
Dr Mohsin Jamil
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Semiconductors are materials
whose electrical properties liebetween Conductors and
Insulators.
Ex : Silicon and Germanium
Semiconductor
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Atomic Structure
Elements are made of atoms
110 Elements; each has an atomic
structure Today, quarksand leptons, and their
antiparticles, are candidates for being thefundamentalbuilding blocks from whichall else is made!
Bohr Model Atoms have planetary structure
Atoms are made of nucleus(Protons (+)& Neutrons) and electrons(-)
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Atoms go around the nucleolus in their orbitsdiscrete distances
Each orbit has some energy level
The closer the orbit to the nucleus the less energy ithas
Group of orbits called shell
Electrons on the same shell have similar energy level
Valence shellis the outmost shell
Valence shell has valence electronsready to be
freed Number of electrons (Ne) on each shell (n)
First shell has 2 electrons
Second shell has 8 electrons (not shown here)
Ne = 2n2
Atomic Structure
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Atoms are made of valence shell
and core
Core includes nucleus and other
inner shells
For a Carbon atom the atomic
number is 6
Core charge = 6 P + 2 e = (+6)+(-2)=(+4)
Remember the first shell has 2
electrons
Valence Shell
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Basic categories Conductors
Examples: Copper, silver One valence electron , the ecan
easily be freed
Insulators Valence electrons are tightly
bounded to the atom
Semiconductors
Silicon, germanium (singleelement) Gallium arsenide, indium
phosphide (compounds) They can act as conductors or
insulators
Conduction bandis where
the electron leaves the
valence shell and becomes
free
Valence bandis where the
outmost shell is
Always free
electrons
Free electrons
Elements
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Semiconductors
Remember the further away from thenucleus the less energy is required tofree the electrons
Germanium is less stable Less energy is required to make the
electron to jump to the conduction band
When atoms combine to form a solid,they arrange themselves in asymmetrical patterns
Semiconductor atoms (silicon) formcrystals
Intrinsiccrystals have no impurities
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Conduction Electrons and Holes
Electrons exist only within prescribedenergy bands
These bands are separated by energy
gaps When an electron jumps to the
conduction band it causes a hole
When electron falls back to its initialvalence recombinationoccurs
Consequently there are two differenttypes of currents
Hole current (electrons are the
minority carriers) Electron current (holes are the
minority carriers)
Remember: We are interested in electrical current!
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Doping
By adding impurities to the intrinsicsemiconductor we can change the conductivityof the materialthis is called doping
N-type doping
P-type doping
N-type:pentavalent (atom with 5 valenceelectrons) impurity atoms are added
[Sb(Antimony) + Si]
Negative charges (electrons) are generated
N-type has lots of free electrons
P-type: trivalent (atom with 3 valence
electrons) impurity atoms are added [B(Boron) + Si]
Positive charges (holes) are generated
P-type has lots of holes
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Diodes
N region has lots of free electrons
P region has lots of holes
At equilibrium: total number positive and negative
charges is the same (@ room temp) At the pn junction the electrons and holes with different
charges form an electric field
In order to move electrons through the electric field(generate current) we need some force (voltage)
This potential difference is called barrier voltage
When enough voltage is applied such that electrons aremoved then we are biasingthe diode
Two layers of positive and negative charges for depletionregionthe region near the pn-junction is depleted ofcharge carriers)
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Biasing Types of a Diode
Forward bias
Bias voltage VBias> barrier voltage VBar
Reduction in +andions smaller
depletion region
VBar Depends on material, doping, temp,etc. (e.g., for silicon it is 0.7 V)
Reverse bias
Essentially a condition that preventselectrons to pass through the diode
Very small reverse break down current Larger depletion region is generated
Cathode
n regionAnode
p region
Connected to the
negative side of
the battery
Connected to the
positive side of
the battery
A K
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Electronics
Umar [email protected]
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I-V Characteristics of Idea Dio
If the voltage across anode and cathode is greater than zero, the reof an ideal diode is zero and current becomes infinite However if th
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Diode first approximation
This models the diode as being ideal.
The first approximation ignores leakage current
potential and bulk resistance.
When an ideal diode is forward biased, the mo
closed switch.
When an ideal diode is reverse biased, the mod
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Diode first approximation
The Ideal Diode Model
Example:Assume the diode in the circuit below is ideal. Determine th
of ID if a) VA= 5 volts (forward bias) and b) VA=5 volts (reverse bias)
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Diode second approximation
This model assumes that no diode current flow
the forward bias across the diode reaches 0.7 vo
This model ignores the exact shape of the knee.
This model ignores the diodes bulk resistance.
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Diode second approximation
The Ideal Diode with Barrier Potential
Example: To be more accurate than just using the ideal diode model
the barrier potential. Assume V= 0.3 volts (typical for a germanium
Determine the value of IDif VA= 5 volts (forward bias).
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Diode third approximation
This model assumes that no diode current flow
the forward bias across the diode reaches 0.7 vo
This model ignores the exact shape of the knee.
This model does account for the diode
resistance.
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Diode third approximation
Ideal Diode with Barrier Potential and Linear Forward Resistance
Example: Assume the diode is a lowpower diode with a forward res
value of 5 ohms. The barrier potential voltage is still: V= 0.3 volts (typ
a germanium diode) Determine the value of IDif VA= 5 volts.
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Diode third approximation
Theseare
the
values
found
in
the
examples
on
previo
slides where the applied voltage was 5 volts the bar
IdealDiodeModel
IdealDiodeModel
withBarrierPotential
Voltage
IdealDiodeM
withBarrierPot
andLinearFor
Resistance
100
mA 94
mA 85.5
mA
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Diode resistance
Bulk
resistance
Forwardresistance
DCorstaticresistance
ACordynamicresistance
Average resistance
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Bulk resistance
With
forward
bias,
diode
current
increases
rapid
beyondthekneevoltage.
Small
increases
in
voltage
cause
large
increasescurrent.
Theohmicresistance ofthepandnmaterialisc
the
bulk
resistance.
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Bulk resistance
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Forward resistance
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DC resistance
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AC resistance
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Average AC resistance
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V-I Curve
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V-I Curve
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V-I Curve
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Electronics
Umar [email protected]
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Load Line
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Q point of diode
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Power Supply design
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Diode Rectifier
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Diode Rectifier
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Diode Rectifier
Halfwaverectifiersignals
Thedcvalueoftheoutputistheaverag
value.
Vdc=VP/
fout=fin
Second approximation:
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Transformer
Whentheturnsratio(N1/N2)isgreater
1,theprimaryvoltageissteppeddown.
Whentheturnsratioislessthan1,the
primaryvoltageissteppedup.
Dottedendshavethesameinstantaneo
phase.
F ll tifi i i di
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Center Tap Transformer
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Full wave Rectifier
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Diode Rectifier
Fullwaverectifier
Thedcvalueoftheoutputisthe
averagevalue.
Vdc=2VP/
fout=
2fhalfwave
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Bridge Rectifier
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Diode Rectifier
Bridgerectifier
Vdc=2VP/
fout=2fhalfwave
Secondapproximation:
VP(out)=VP(in)1.4V
Full wave Rectifier
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Filtration
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Capacitor-input Filter
Most widely used in power supplies
The peak value of the rectified signal pa
to the load resistor
With a large capacitor, ripple is small
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Capacitor-input Filter
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Power Supply design
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Capacitor-input Filter
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Capacitor-input Filter
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Capacitor-input Filter
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Power Supply Block Diagra
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Electronics
Umar [email protected]
R l t
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Regulator
The regulator is a circuit that helps maintain
a fixed or constant output voltage.
Changes in the load or the AC line voltagewill cause the output voltage to vary.
Most electronic circuits cannot withstand the
variations since they are designed to work
properly with a fixed voltage. The regulator fixes the output voltage to the
desired level then maintains that value
despite any output or input variations.
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The ripple created by the rectifier can be
unacceptable to sensitive load; therefore, a regulator is
required to obtain a very stable output.
Three diodes operate as a primitive regulator
Z A R l t
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Zener As Regulator
Z I V Ch t i ti
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Zener I-V Characteristic
V lt R l t
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Voltage Regulator
Zener diode is a voltage regulator device
because it is able to fix the output voltage at a
constant value (DC voltage).
R1 is to limit the zener current, IZso that it is
less than the maximum current, IZM(to avoidthe zener diode from broken).
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7
Other Applications of diodes
Clipper circuits
Clamper circuits
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Clipper Circuit
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10
Clipper Circuit
Clipper circuits have the ability to clip
off a portion of the input signal withoutdistorting the remaining part of thealternating waveform.
Cli Ci it
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Clipper Circuit
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12
Clamper Circuit
The clamping network clamp a signal to different dclevel without altering the wave-shape.
The network will have a capacitor, a diode and a resistiveelement.
The magnitude ofR
andC
must be chosen such that thetime constant t = RC is large enough to ensure that thevoltage across the capacitor does not dischargesignificantly during the interval the diode is non-conducting
Diode :- Clamper
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Diode : ClamperPositive Clamper
The circuit for a positive
clamper is shown in thefigure. During the negativehalf cycle of the input signal,the diode conducts and actslike a short circuit. The output
voltage Vo 0volts . Thecapacitor is charged to thepeak value of input voltageVm
. and it behaves like abattery. During the positive
half of the input signal, thediode does not conduct andacts as an open circuit. Hencethe output voltage Vo Vm+ VmThis gives a positivelyclamped voltage.
Vo Vm+ Vm= 2 Vm
Diode :- Clamper
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pPositive Clamper
Diode :- Clamper
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Diode : Clamper
Negative Clamper
During the positive halfcycle the diode conductsand acts like a short circuit.The capacitor charges topeak value of input voltageVm. During this interval the
output Vowhich is takenacross the short circuit willbe zero During the negativehalf cycle, the diode is open.
Vo -Vm- Vm= -2 Vm
Diode :- Clamper
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Diode :- Clamper
Negative Clamper
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Electronics
Umar Ansari
INTRODUCTION - BJT
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INTRODUCTION BJT Three terminal device
Basic Principle
Voltage between two terminals controls current flowing in the third
terminal.
Device is used in discrete and integrated circuits and can act as : Amplifier
Logic Gates
Memory Circuits
Switches
Invented in 1948 at Bell Telephone Industries
INTRODUCTION
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MOSFET has taken over BJT since 1970s for designing of
integrated circuits but still BJT performance under sever
environment is much better than MOSFET e.g. Automotive
Electronics
BJT is used in Very high frequency applications (Wireless Comm)
Very high speed digital logics circuit (Emitter Coupled Logic)
Innovative circuit combine MOSFET being high-inputresistance and low power operating devices with BJT merits
of being high current handling capacity and very high
frequency operationknown as BiMOS or BiCMOS
INTRODUCTION
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Study would include
Physical operation of BJT
Terminal Characteristics
Circuit Models
Analysis and design of transistor circuits
INTRODUCTION
A l f d f h
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A simplified structure of the npn transistor.
Device Structure & Physical Operation
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Device Structure & Physical Operation
npn & pnp Transistor
Three terminal ---- Emitter, Base, Collector
Consists of two pn junctions
np-pn -------- npn pn-np -------- pnp
Modes
Cutoff, Active, Saturation, Reverse Active
Junctions
Emitter Base Junction (EBJ)
Collector-Base Junction (CBJ)
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TWO EXAMPLES OF DIFFERENT SHAPES OF TRANSISTOR
A i lifi d t t f th t i t
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A simplified structure of the pnp transistor.
Current flow in an npn transistor biased to operate in the active mode.
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Common parameters
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Common parameters
npn & pnp BJT
EEB iii
1
1
BBE iii 1
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Electronics
Umar [email protected]
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Electronics
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l l
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Real world (lab) is analog
V
t
Computer (binary) is digital
Analog vs Digital System
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Electronics
Umar [email protected]
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Binary Adder
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Electronics
Umar Ansari
What are Karnaugh maps?
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g p
Karnaughmaps provide an alternative way of simplifyinglogic circuits.
Instead of using Boolean algebra simplificationtechniques, you can transfer logic values from a Boolean
statement or a truth table into a Karnaugh map.
The arrangement of 0's and 1's within the map helpsyou to visualise the logic relationships between thevariables and leads directly to a simplified Boolean
statement.
1Named for the American electrical engineer Maurice Karnaugh.
Karnaugh maps
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Karnaugh maps
Karnaugh maps, or K-
maps, are often used to simplify logic problems with 2, 3
or 4 variables.
BA
For the case of 2 variables, we form a map consisting of 22=4 cellsas shown in Figure
A
B0 1
0
1
Cell = 2n ,where n is a number of variables
00
10
01
11
A
B0 1
0
1
A
B0 1
0
1
BA
BA AB
BA BA
BA BA
Maxterm Minterm
0
2
1 3
Karnaugh maps
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Karnaugh maps
3 variables Karnaugh map
ABC 00 01 11 10
0
1
CBA CBA CAB CBA
CBA BCA ABC CBA
0 2 6 4
5
3
1
7
Cell = 23=8
Karnaugh maps
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Karnaugh maps
4 variables Karnaugh map
AB
CD 00 01 11 10
00
01
11
10
5
3
1
7
62
0 4
9
15
13
11
1014
12 8
Karnaugh maps
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The Karnaugh map is completed by entering a '1(or
0) in each of the appropriate cells.
Within the map, adjacent cells containing 1's (or 0s)
are grouped together in twos, fours, or eights.
g p
Example
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p
A
BY
2-variable Karnaugh maps are trivial but can be used to introducethe methods you need to learn. The map for a 2-input OR gate
looks like this:
A B Y
0 0 0
0
1
1
1 0 1
1 1 1
A
B0 1
0
1
1
1
1
B
A
A+B
Example
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p
A B C Y
0 0 0 1
0 0 1 1
0 1 0 0
0 1 1 0
1 0 0 1
1 0 1 1
1 1 0 1
1 1 1 0
CA
CAB
AB
C 00 01 11 10
0
1
1
11
1
1
B
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Design of a 2/1 Mux
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g /
2/1 mux Block Diagram Truth Table
D1
D0
S
O
S D1 D0 O
0 0 0 0
0 0 1 1
0 1 0 0
0 1 1 1
1 0 0 0
1 0 1 0
1 1 0 1
1 1 1 1
Design of 2/1 Mux Continued
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es g o / u Co t ued
K-MapSA
B00 1101 10
0
1
1
1 1 1
O = SA + SB
4/1 Mux
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/
Circuit
Uses of Multiplexers
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p
Used in data communications for several
computers to communicate over 1 line
Used in radio to select one channel from many Used to route data within a computer
Used for function generation
What is Demultiplexer (DEMUX)
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p
Typical Applications of a DEMUX
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1 to 2 Demultiplexer
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Truth Table Circuit
S D 01 O0
0 0 0 0
0 1 0 1
1 0 0 0
1 1 1 0
Encoder and Decoder
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Binary decoders Converts an n-bit code to a single active output
Can be developed using AND/OR gates
Can be used to implement logic circuits.
Binary encoders
Converts one of 2ninputs to an n-bit output Useful for compressing data
Can be developed using AND/OR gates
Both encoders and decoders are extensively used in digitalsystems
Binary Decoder
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Black box with n input lines and 2noutput
lines
Only one output is a 1 for any given input
Binary
Decoder
n
inputs2noutputs
Encoders
If h d d ' d h f bi
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If the a decoder's output code has fewer bits
than the input code, the device is usuallycalled an encoder.
e.g. 2n-to-n
The simplest encoder is a 2n-to-n binaryencoder
One of 2ninputs = 1
Output is an n-bit binary number.
.
.
.
.
.
2n
inputs
n
outputs
Binary
encoder