Electronics Slides

7/24/2019 Electronics Slides

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EE212-Electronics

Dr Mohsin Jamil

[email protected]


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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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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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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



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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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Power Supply design


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Power Supply Block Diagra


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Electronics


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

[email protected]

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



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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



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Binary Adder


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Electronics

Umar Ansari

[email protected]

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

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