ANALOG ELECTRONIC CIRCUITS

Faculty : Harini Vaikund

Course Outline

UNIT 1:

Diode Circuits

Diode Resistance, Diode Equivalent circuits,

transition and diffusion capacitances

Clippers and Clampers, Rectifiers.

Course Outline

UNIT – 2 :

Transistor Biasing

Operating point, analysis and design of

Fixed bias circuits, Emitter stabilized

biased circuits,

Voltage divider bias and Collector voltage

feedback bias.

Transistor switching circuits. Bias

stabilization: stability factor of different biasing circuits.

Course Outline

UNIT – 3: Transistor Modelling and Frequency

Response:

Transistor as two port network, low frequency hybrid model., relation between h– parameter model of CE, CC and

CB modes.

Millers theorem and its dual. General frequency

considerations, low frequency response.

Miller effect capacitance, high frequency response.

Course Outline

UNIT – 4: Multistage, Feedback and Power

Amplifiers

a) Multistage Amplifiers: Cascade

and cascade connections,

Darlington circuits, analysis and

design.

b) Feedback Amplifiers:

Feedback concept, different type of

feedback circuits-block diagram

approach, analysis of feedback

circuits.

c) Power Amplifiers:

Amplifier types, analysis and design

Course Outline

UNIT 5: Oscillators & FET

a) Oscillators: Principle of operation, analysis of phase shift oscillator, Wien bridge

oscillator, RF and crystal oscillator. (BJT versions)

b) Field Effect Transistors: Construction, working and

characteristics of JFET and MOSFET. Biasing of JFET. Analysis and design

of JFET (only common source configuration with

fixed bias)

Text Books

❑ Robert L. Boylestad and Louis Nashelsky,

“Electronic Devices and Circuit Theory”,

11th Edition, Pearson Education, 2015.

❑ Millman and Halkias, “Electronic Devices and

Circuits”, 4th Edition, Mc Graw Hill, 2015.

❑ David A Bell, “Electronic Devices and

Circuits”, 5th Edition, Oxford University

Press, 2008.

Basic Electronics Questions

What are Circuits?

What is the difference between analog and digital circuits?

Why do we need analog and digital circuit?

What are the applications we are using these circuits?

Why do we have to study these as an electrical and electronics engineer?

Where will we be using these in future?

Basic Electronics

• Application - Home Appliance, Medical Applications, Robotics, Mobile

Communication, Computer Communication etc.

• Atom is the basic building block of all the elements.

• Atom = central nucleus of +ve charge around which –ve charged electrons

• Electrostatic force - attraction between electrons and nucleus which holds

electrons in different orbits.

• Valence electrons - Outermost orbit electrons.

• Free electrons - Valence electrons loosely attached to nucleus.

Basic Electronics• Definite energy - single isolated atom an

electron in any orbit.

• Solids are atoms are brought together, an

atom is influenced by the forces from

other atoms.

• So an electron in any orbit can have a

range of energies rather than single

energy.

• Range of energy levels are known as

Energy bands.

• Any material there are two distinct energy

bands in which electrons may exist –

Valence band and conduction band.

Basic Electronics

• Range of energies possessed by valence

electrons is called valence band.

• Range of energies possessed by free

electrons is called conduction band.

• Valence band and conduction band are

separated by energy gap in which no

electrons normally exist this gap is called

forbidden gap.

Basic Electronics

• Electrons in conduction band are either

escaped from their atoms (free electrons)

or only weakly held to the nucleus.

• So electrons in conduction band may be

easily moved around within the material

by applying relatively small amount of

energy. (either by increasing the

temperature or by focusing light on the

material etc. )

• This is the reason why the conductivity of

the material increases with increase in

temperature.

• Classification of materials based on

Energy band theory:

Based on the width of the

forbidden gap, materials are broadly

classified as

• Conductors

• Insulators

• Semiconductors.

Classification of Materials

Conductor

• Conductors are those substances, which

allow electric current to pass through

them.

• Example: Copper, Al, salt solutions, etc.

• In terms of energy bands, conductors are

those substances in which there is no

forbidden gap.

• Valence and conduction band overlap as

shown for this reason, very large number

of electrons are available for conduction

even at extremely low temperatures.

• Conduction is possible even by a very

weak electric field.

Insulator

• Insulators are those substances, which do

not allow electric current to pass through

them.

• Example: Rubber, glass, wood etc.

• In terms of energy bands, insulators are

those substances in which the forbidden

gap is very large.

• Thus valence and conduction band are

widely separated as shown therefore

insulators do not conduct electricity even

with the application of a large electric field

or by heating or at very high temperatures.

Semiconductor

• Semiconductors are those substances whose

conductivity lies in between that of a

conductor and Insulator.

• Example: Silicon, germanium, Cealenium,

Gallium, arsenide etc.

• Energy bands, in semiconductors the

forbidden gap is narrow. Thus valence and

conduction bands are moderately separated

as shown.

• Valence band is partially filled, the

conduction band is also partially filled, and

the energy gap between conduction band

and valence band is narrow.

◦ Comparatively smaller electric field is

required to push the electrons from valence

band to conduction band . At low

temperatures the valence band is

completely filled and conduction band is

completely empty. Therefore, at very low

temperature a semi-conductor actually

behaves as an insulator.

SemiconductorElectron motion:

• Free electrons in the conduction band are moved

under the influence of the applied electric field.

• Since electrons have negative charge they are

repelled by the negative terminal of the applied

voltage and attracted towards the positive

terminal.

Hole transfer:

• Hole transfer involves the movement of holes.

• Holes may be thought of positive charged

particles and as such they move through an

electric field in a direction opposite to that of

electrons.

Semiconductor◦ In a good conductor (metal) – the current

flow is due to free electrons only.

◦ In a semiconductor as shown - the current

flow is due to both holes and electrons

moving in opposite directions.

◦ The unit of electric current is Ampere (A)

and since the flow of electric current is

constituted by the movement of electrons in

conduction band and holes in valence band,

electrons and holes are referred as charge

carriers.

Classification of semiconductors:

a) Intrinsic semiconductors.

b) Extrinsic semiconductors.

Intrinsic SemiconductorIntrinsic semiconductors:

A semiconductor in an extremely pure form is

known as Intrinsic semiconductor.

• Example: Silicon, germanium.

• Both silicon and Germanium are tetravalent

(having 4 valence electrons).

• Each atom forms a covalent bond or

electron pair bond with the electrons of

neighboring atom.

At low temperature

• At low temperature, all the valence electrons

are tightly bounded the nucleus hence no

free electrons are available for conduction.

• The semiconductor therefore behaves as an

Insulator at absolute zero temperature.

Crystalline structure of Silicon (or Germanium)

Intrinsic SemiconductorAt room temperature

◦ Some of the valence electrons gain enough

thermal energy to break up the covalent

bonds.This breaking up of covalent bonds

sets the electrons free and is available for

conduction.

◦ When an electron escapes from a covalent

bond and becomes free electrons a vacancy

is created in a covalent bond as shown.

Such a vacancy is called Hole. It carries

positive charge and moves under the

influence of an electric field in the direction

of the electric field applied.

◦ Numbers of holes are equal to the number

of electrons since; a hole is nothing but an

absence of electrons.

Crystalline structure of Silicon (or Germanium) at room

temperature

Extrinsic Semiconductor

Extrinsic Semiconductor:

• When an impurity is added to an intrinsic semiconductor its conductivity changes.

• This process of adding impurity to a semiconductor is called Doping and the impure

semiconductor is called extrinsic semiconductor.

• Depending on the type of impurity added, extrinsic semiconductors are further classified

• n-type semiconductor.

• p-type semiconductor.

N-type Semiconductor

◦ When a small current of Pentavalent impurity is

added to a pure semiconductor it is called as n-

type semiconductor.

◦ Addition of Pentavalent impurity provides a large

number of free electrons in a semiconductor

crystal.

◦ Typical example for pentavalent impurities are

Arsenic, Antimony and Phosphorus etc. Such

impurities which produce n-type

semiconductors are known as Donor impurities

because they donate or provide free electrons to

the semiconductor crystal.

N-type Semiconductor

• Due to thermal energy, still hole electron pairs

are generated but the number of free electrons

are very large in number when compared to

holes.

• So in an n-type semiconductor electrons are

majority charge carriers and holes are minority

charge carriers .

• Since the current conduction is pre-dominantly

by free electrons( -vely charges) it is called as n-

type semiconductor( n- means –ve).

Energy band diagram for n-type semiconductor

P-type Semiconductor

◦ When a small amount of trivalent impurity

is added to a pure semiconductor it is called

p-type semiconductor.

◦ The addition of trivalent impurity provides

large number of holes in the semiconductor

crystals.

◦ Example: Gallium, Indium or Boron etc.

Such impurities which produce p-type

semiconductors are known as acceptor

impurities because the holes created can

accept the electrons in the semi conductor

crystal.

P-type Semiconductor

◦ Due to thermal energy, still hole-electron

pairs are generated but the number of holes

is very large compared to the number of

electrons.

◦ Therefore, in a p-type semiconductor holes

are majority carriers and electrons are

minority carriers.

◦ Since the current conduction is

predominantly by hole( + charges) it is

called as p-type semiconductor (p means

+ve)

Energy band diagram for p-type semiconductor

PN Junction Diode

PN Junction Diode:

• When a p-type semiconductor material is suitably joined to n-type semiconductor the contact

surface is called a p-n junction.

• The p-n junction is also called as semiconductor diode.

Applications of diode:

• Used as rectifier diodes in DC power suppliers

• Used as clippers and clampers

• Used as switch in logic circuit in computers

• Used as voltage multipliers.

Construction and working of a PN Junction diode Open Circuited PN Junction:

• p-type semiconductor having –ve acceptor

ions and +vely charged holes.

• n-type semiconductor having +ve donor

ions and free electrons.

• Two pieces are suitably treated to form pn

junction, then there is a tendency for the

free electrons from n-type to diffuse over to

the p-side and holes from p-type to the n-

side . This process is called diffusion.

• As the free electrons move across the junction from n-type to p-type, +ve donor ions are uncovered.

• Hence a +ve charge is built on the n-side of the junction. At the same time, the free electrons

cross the junction and uncover the –ve acceptor ions by filling in the holes.

• Therefore a net –ve charge is established on p-side of the junction.

Construction and working of a PN Junction diode

• When a sufficient number of donor and

acceptor ions is uncovered further diffusion

is prevented.

• Thus a barrier is set up against further

movement of charge carriers. This is called

potential barrier or junction barrier.

• The potential barrier is of the order of 0.1

to 0.3V.

• Outside this barrier on each side of the

junction, the material is still neutral.

• Only inside the barrier, there is a +ve

charge on n-side and –ve charge on p-side.

This region is called depletion layer.

Biasing of a PN junction diode:

Connecting a p-n junction to an external DC voltage

source is called biasing.

1. Forward biasing

2. Reverse biasing

PN Junction – No Bias (V = 0)

• Absence of external voltage across the p-n

junction is called the unbiased diode.

• Because of the density gradient electrons and

holes diffuse and they combine leaving the ions

unneutralised and are called uncovered charges.

• The uncovered charges generate an electric field

directed from n-side to p-side called as barrier

field which opposes the diffusion process

further.

• Since the vicinity of the junction is depleted of

mobile charges. Hence called a as depletion

region.

PN Junction – Reverse Bias (VD<0V)• Positive polarity of the external bias VD is

connected to n-type and negative terminal is

connected to p-type.

• The number of uncovered positive and negative

ions will increase in the depletion region causing

widening the depletion region which creates a

great barrier for the majority carrier to overcome,

effectively reducing the majority carrier flow to

zero and hence the current due to majority

carrier Imajority=0

• The minority carriers which travels down the

potential barrier remain unaffected and give a

small current called the reverse saturation

current denoted as Is.

PN Junction – Forward Bias (VD >0V)• Positive polarity of the external bias VD is connected

to p-type and negative terminal is connected to n-

type.

• External bias VD exerts a force on the mobile carriers

to move them towards the junction. At the boundary

they recombine with the ions and reduce the width of

the depletion region.

• The depletion region will continue to decrease in

width as the voltage is increased further and a heavy

flood of electrons will move from n-side to p-side

giving the Imajority an exponential rise from p-side to

n-side.

• The minority carrier flow will not be affected by this

because the conduction level is determined by the

limited number of impurities in the material and the

current is denoted by Is.

◦ The total current is given by

ID=IForward+IReverse =Imajority - Iminority

(Direction opposite)

◦ In terms of reverse saturation current, ID

can be written as

ID=Is (exp(𝑽𝑫

ƞ𝑲𝑻-1)) is called the

Shockley’s equation.

Where e- Charge of an electron

K-Blotzman’s Conatant

T-Temperature in Kelvin

η- Quality factor depends upon the

diode material (η=2 for Si and 1 for Ge)

𝑽𝑫- Supplied voltage across the

junction

Breakdown ConditionZener Breakdown

• Too much of reverse bias across a p-n junction exert a

strong force on a bound electron to tear it out from the

covalent bond.

• Thus a large number of electron and hole pair will be

generated through a direct rupture of the covalent bonds

and they increase the reverse current and gives sharp

increase in the characteristics. It is called zener

breakdown.

• Diode employing the unique portion of the characteristics

of a p-n junction is called zener diode.

• Maximum reverse voltage potential that can be applied

before entering the zener region is called the peak inverse

voltage (PIV) or peak reverse voltage (PRV).

Avalanche Breakdown:• With increasing reverse bias voltage,

the electric field across the junction

of a diode increases.

• At a certain value of the reverse

voltage, the electric field imparts a

sufficiently high energy to a thermally

generated carrier.

• The carrier on colliding with an ion

on its way disrupts a covalent bond

and gives a new hole electron pair.

• This process is cumulative and gives

an avalanche of carriers in a very

short time. It is called avalanche

multiplication.

V-I Characteristics

Diode Resistance DC or Static Resistance

• The application of a dc voltage to a circuit containing a semiconductor diode will result in an operating

point on the characteristic curve that will not change with time.

• The resistance of the diode at the operating point can be found simply by finding the Corresponding

levels of VDand ID

• The dc resistance levels at the knee and below will be greater than

the resistance levels obtained for the vertical rise section of the

characteristics. The resistance levels in the reverse-bias region

will naturally be quite high. Since ohmmeters typically employ

a relatively constant-current source, the resistance determined

will be at a preset current level (typically, a few mill amperes).

•DC or Static Resistance

•AC or Dynamic Resistance

•Average Resistance

Determining the dc resistance of a diode at

a particular operating point

AC or Dynamic Resistance

• Static resistance that the dc resistance of a diode is independent of

the shape of the characteristic in the region surrounding the point

of interest.

• If a sinusoidal rather than dc input is applied, the situation will

change completely.

• The varying input will move the instantaneous operating point up

and down a region of the characteristics and thus defines a specific

change in current and voltage.

• With no applied varying signal, the point of operation would be the

Q-point appearing on figure determined by the applied dc levels.

• The designation Q-point is derived from the word quiescent, which

means ―still or unvarying.

• A straight line drawn tangent to the curve through the Q-point will

define a particular change in voltage and current that can be used to

determine the ac or dynamic resistance for this region of the diode

characteristics.

Defining the ac resistance of a diode

Average Resistance

• If the input signal is sufficient enough to produce a large swing,

then the resistance related to the diode for this region is called as AC

average resistance.

• It is determined by the straight line that is drawn linking the

intersection of the minimum and maximum values of external input

voltage.

Average AC Resistance

Diode Equivalent Circuits

• Equivalent circuit is a combination of element properly chosen to best represent the actual

terminal characteristics of a device or system in a particular operating point.

• Diode replaced by equivalent circuit in many practical electronic circuits for analysis purpose.

• Such equivalent circuit is called circuit model.

• Methods of replacing diode by circuit model

• Piecewise-Linear Equivalent Circuit : One technique for obtaining an equivalent circuit for a diode is

to approximate the characteristics of the device by straight-line segments. The resulting equivalent

circuit is naturally called the piecewise-linear equivalent circuit.

• Simplified Equivalent Circuits:

• Ideal Equivalent Circuits

THANK YOU!