Chapter-I : Semiconductors

Semiconductors are those materials whose conductivity lies between that of conductors

and insulators. Most of the electronic devices are based on semiconductors. At absolute

zero(0˚K) temperature. The semiconductors behaves as insulators. As temperature

increases,the conductivity increases.

Advantages of semiconductor devices are:-

Semiconductor devices have a small size and their cost is less.

Semiconductor devices do not require any heating filaments.

Semiconductor devices can be operated at low voltages.

Semiconductor devices consume very small amount of power.

Basic explanation of Atomic structure

An Atom consists of a nucleus surrounded by orbiting electrons. Protons and

neutrons are present in the nucleus. A proton has positive charge and a neutron is

electrically neutral. Therefore the nucleus has a positive charge and it attracts the

electrons. The number of electrons is the same as the number of protons. As electrons

has negative charge, it balances the nuclear attraction. The electrons are arranged in

the orbits or shells. An orbit can contain a maximum of 2n2 electrons, where ‘n’ is the

number of orbits.

Atomic Structure for germanium

In a germanium atom, the nucleus contains 32 protons and 41 neutrons. The

atomic number of germanium is 32. There are 2, 8, 18 and 4 electrons in the first, second,

third and fourth orbit respectively.

The electrons present in the outermost orbit are called ‘valence electrons’

Atomic structure for silicon

The atomic number of silicon is 14. There are 14 protons and 14 neutrons in

the nucleus of silicon. There are 2,8 and 4 electrons in the first,second and third orbit

respectively. The electrons present in the outermost orbit are called ‘valence electrons’

therefore in silicon there are 4-valence electrons.

Energy levels and energy bonds

Energy levels:-Electrons cannot revolve in an orbit of any radius. But only certain orbit

sizes are permitted. Each orbit represents particular energy level. It takes energy to move

an electron from a smaller to a larger orbit because work has to be done to overcome

the attraction of the nucleus.

---Electrons in the orbit close to the nucleus are tightly bounded, and those who are in

the last orbit are loosely bound to the nucleus.

---If certain amount of external energy like heat, light etc is given to an atom, the atom is

said to be in a state of excitation. Therefore electron lifts to a higher energy level or

larger orbit.

Energy bands

1. All electrons travelling in first orbits have different energy levels. Since there are

billions of first orbit electrons, the slightly different energy levels form a band

similarly second and third energy bands are formed.

2. Hence in an atom electrons present in every orbit has its own energy band.

For eg:- In silicon(si) there are 2 , 8 and 4 electrons in first , second , and third orbit

respectively. Therefore for silicon atom there will be three energy bands and the

third energy band is called valence band. These three bands are shown dark

indicating filled or saturated bands. Electrons in these bands cannot move easily

because there are no empty orbits. Beyond the valence band is the conduction

band. There is an energy gap between valence band and conduction band. An

electron can be lifted from the valence band to the conduction band by giving

some energy in form of heat, light etc. the forbidden energy gap Eg for silicon is

1.12 ev and for germanium Eg is 0.72ev.

At absolute zero temperature, the conduction band is empty. Therefore there is no

current. If valence electron is lifted in to the conduction band, it becomes free electron.

A free electron can movefrom one atom to the next and provide conduction.

Types of semiconductors:-

Semiconductors are of 2 types

1. Intrinsic type of semiconductors

2. Extrinsic type of semiconductors

Intrinsic type of semiconductors

1. Pure form of germanium or silicon is called intrinsic semiconductor.

2. An intrinsic semiconductor behaves as an insulator at absolute zero (0˚K)

temperature.

3. When germanium or silicon atoms combine to form a solid, they arrange

themselves in an orderly pattern called a crystal.

4. Ge and si are tetravalent, i.e there are four valence electrons. For stable atom

eight valence electrons are necessary. By sharing one electron each with

neighbouring four atoms the central atom gets eight electrons in its valence orbit.

Therefore it becomes stable.

If the temperature is increased beyond (0˚K), the heat energy breaks some covalent

bonds. Some valence electrons go from valence band to conduction band and they

become free electrons. When an electron goes from valence band to the conduction

band, a hole is created in the valence band. The number of holes and free electrons

always remains same in an intrinsic semiconductor. The conduction si provided by free

electrons in conduction band and by holes in valence bands.

Extrinsic semiconductor

1. In an intrinsic semiconductor, there aren’t enough free electrons and holes to

produce sufficient current.

2. When a crystal of intrinsic semiconductor has been doped, it is called an extrinsic

semiconductor.

3. The process of adding impurity atoms to a crystal to increase either the number of

free electrons or holes is called doping.

There are two types of extrinsic semiconductors:-

1. P-type semiconductor

2. N-type semiconductor

P- type semiconductor

---There are four valence electrons in germanium and silicon atoms, therefore they are

tetravalent atoms , when the impurity of trivalent material for e.g. Boron (B) , aluminum

(A1) , Indium (In) , Gallium (Ga) gets added to the pure semiconductor , such doped

semiconductor is called p- type semiconductor.

---In a trivalent aterial three electrons are present in the outer orbit, they form three

covalent bonds. Absence of electron in fourth bond is called hole. By controlling the

amount of impurity added, the number of holes can be controlled.

--- The conductivity of p-type semiconductors is mainly due to holes.

---In p-type semiconductor holes are majority charge carriers and free electrons are

minority charge carriers. Each hole may accept an electron during recombination.

Therefore trivalent impurity atoms are known as acceptor atoms.

N-type semiconductor

---- If impurities of pentavalent material are added to pure semiconductors it becomes

n-type semiconductor. Arsenic (As) , antimony (Sb) , phosphorus (p) are pentavalent

materials.

----In a pentavalent material, five electrons are present in the outer orbit. They form four

covalent bonds. Remaining electron goes to theconduction band and becomes free

electron. There are large number of electrons produced mostly by doping. There are

only a few holes, created by thermal energy.

---- In n-type semiconductor, free electrons are majority charge carriers and holes are

minority charge carriers.

----Pentavalent atoms are called donor atoms because they produce free electrons.

Concept of Hole and Hole Current:-

--- When outside energy is given to a valence electron, it is lifted to a higher energy level

the departing electron leaves a vacancy in the valence orbit. This vacancy is called a

hole.

Hole current:- When external energy is given an electron from the valence band moves

into the conduction. This leaves a hole in the valence band. With a slight change in

energy, the valence electron at Acan move into the hole. Therefore the original

holedisappears and a new hole appears at A. The valence electron at B can move into

the new hole with a slight change in energy. Thus due to movement of valence electron

through EDCBA path, there is movement of hole through the valence band along the

ABCDE path. Thus whole current can be obtained in the valence and free electrons

provide current in the conduction band.

+ hole

-Free electron

Positive ion

Negative ion

--- One side of a single crystal of germanium or silicon is doped with acceptor (P-type)

impurity atoms and the other side is doped with donor (n-type) impurity atoms. The

junction is formed where the p-type and n-type regions meet.

--- Free electrons on the n-side diffuse or spread in all directions. When a free electrons

diffuses across the junction, a positive ion is produced in the n-region. After entering the

p-region the free electron combines with a hole , therefore that hole disappears and the

associated atom becomes a negative ion , thus each time an electron diffuses across

the junction , a pair of ion’s is produced, the region near the junction is free from holes

and electrons. This region is called depletion layer.

--- the strength of the depletion layer goes on increasing with each electron crossing the

junction , but at a point the internal repulsion of the depletion layer stops further diffusion

of free electrons across the junction. The depletion layer acts like a barrier. The

difference of potential across the depletion layer is called barrier potential (vo). This is

also called internal potential barrier Vo=0.7v for Si diode and Vo=0.3v for Ge diodes.

Forward and Reverse biasing of diode

Forward Bias:-The positive terminal of a battery is connected to p-region and negative

terminal to the n-region. Then the junction is said to be forward biased.

The negative terminal of battery repels the free electrons in the n-region towards the

junction. These electrons cross the junction and fall into holes. This recombination occurs

near the junction and a free electron now become a valence electron. It travels

through the p-region as a valence electron. When valence electrons reach the left end

of the crystal , they leave the crystal and flow into the positive terminal of the battery.

Thus due to forward biasing the potential barrier reduces. In forward biased

condition the resistance of a diode is very low and diode acts as a closed switch.

Reverse Bias:-

---The positive terminal of a battery is connected to n-region and negative terminal to

the p-region. Then the junctions is said to be reverse biased.

---In reverse biased condition, the resistance of a diode is very high and diode acts as an

open switch.

----The free electrons in the n-region are forced away from the junction towards the

positive terminal of source (battery). Holes in the p-region move away from the junction

towards the negative terminal. Therefore the width of depletion layer increases.

---Minority carriers are produced by the thermal energy and the current caused by the

minority carriers is called the reverse saturation on current(Io).

---If we increase the reverse voltage at a particular voltage, the diode can conduct very

high reverse current is called breakdown voltage.

+

-

Forward characteristics

--- The circuit is connected as shown in fig.

--- The voltage is increased from 0v and current is recorded by the milliameter.

----When applied voltage increases small current flows as shown by the curve 0A.

Beyond Vo small increase in voltage produces a large increase in current.

---The voltage at which the current starts to increase rapidly is called the knee or offset or

cut-in voltage. This voltage is equal to the barrier potential.

---Resistance of diode is very low in the forward biased condition.

---A current limiting resistor is always used in the circuit in series with a diode. This limits the

current to less than the maximum current rating of the diode.

Reverse characteristics:-

---When the reverse voltage is increased from zero volt in suitable steps and the current is

measured by using micro-ammeter.

---As reverse voltage is increased,slowly reverse current increases called as reverse

saturation current Io. Further if the voltage is increased current increases rapidly this is

called breakdown voltage.

Application of P-N junction diode:- ( self study)

CHAPTER 2

STUDY OF TRANSISTOR

Introduction:

The transistor is a solid state device made up of silicon or germanium. There

are two main types of transistors, the bipolar junction transistor (BJT), the Field

Effect Transistor (FET). The transistor can be used as an amplifier or as an

electronic switch.

Q.) What are the types of BJT? Explain amount of doping and area of each

region in a transistor.

Ans. Types of BJT are: - (1) NPN and (2) PNP

- In bipolar junction transistor (BJT) three regions are present. These are emitter(E),

base(B) and collector(C).

- There are two junctions. One junction is between emitter and base and other

junction is between the base and collector.

- Emitter: - The function of emitter is to emit electrons in a NPN transistor and to

emit holes in a PNP transistor. As emitter is the source of charges, it is heavily

doped. The area of emitter region is smaller than that of collector.

-Base : - The base is very lightly doped. The area of base region is minimum to

prevent recombination of holes and electrons in the base.

- Collector: - The doping of the collector region is between the heavy doping of

emitter and the light doping of the base. The area of collector region is maximum

since it is required to dissipate more heat.

Q). Explain in brief about the different ways of biasing the transistor.

A. A transistor can be biased in three ways

(1) Both junctions forward biased: - In this method the emitter and collector

currents are large. The transistor is in saturation state.

(2) Both junctions reverse biased: - In this method the current is negligence. The

transistor is in ‘cut off’ state. Negligence reverse current is obtained because of

minority charge carriers.

(3) Emitter – base junction forward biased and collector – base junction reverse

biased: In this method the transistor acts as an active device because it can

amplify an input signal to produce a larger output signal. The transistor is in active

state.

Q.) Explain the operation of a NPN transistor.

- Emitter Base junction is Forward biased and the collector base junction is

reverse biased.

- Therefore, under this condition a stream of electrons leaves the negative

terminal of battery VEE and enters the emitter region.

- As electrons are majority charges here. Therefore many electrons cross the

junction J1 and enter the base. At the same time a few holes from the

base go to the emitter.

Here emitter current IE

Where InE = current due to electrons

IPE = current due to holes

As IPE Is is very small.

- When electrons from emitter diffuse through base, few electrons

recombine with holes in base, about 2% only remaining about 98%

electrons reach the depletion region.

Where

Inc: is the current due to electrons from emitter

Ico: is the reverse leakage current due to thermally generated charge carriers.

Where Inc is the current due to electrons from emitter and Ico is the reverse

leakage current due to thermally generated charge carriers. The collector

current is nearly equal to the emitter current

Q.) Define and of a Transistor and also define de and de of a transistor.

Obtain the relation between and .

A. Alpha and Beta of Transistor:

Amplification is the process of Increasing the voltage, current or power of an

input signal.

E

c

deI

I , at constant VCB

Definition: -

The ratio of static (dc) collector current to the static current at a constant

collector voltage emitter is called ‘current amplification factor’.

B

c

deI

I , at constant VCE

Definition: -

The ratio static (dc) collector current to the static base current at a constant

collector voltage is called ‘current amplification factor’.

and are called as a small signal current amplification factor.

E

C

I

I

at constant VCB

is always larger than 1.

Definition:

The ratio of a small change in collector current to the corresponding small

change in emitter current at a constant VCB is called ‘small signal current

amplification factor’ ( )

B

C

I

I

at constant VCE

Definition:

The ratio of a small change in collector current to the corresponding small

change in base current at a constant VCE is called ‘small signal current

amplification factor’ ( )

Relation between and

Configurations of a Transistor: -

There are three configurations of a Transistor. Configuration is the method of

connecting any one terminal of transistor common to both input and output

circuits.

1. Common base configuration

2. Common emitter configuration

3. Common collector configuration (Emitter follower)

Q). Draw circuit diagram of common base configuration and explain its working.

A.

- In this configuration base is common to both the input and output. The

emitter – base junction is Forward biased and the collector base junction is

Reverse biased.

- The collector to base voltage VCB is the output voltage and it is given by

VOUT = VCB = VCC - ICRL ……….. 1

- During a positive half cycle of the input signal Vi, the forward bias voltage

VBE decreases.

- Therefore there is a decrease in IE AND IC.

- Substituting small value of IC in equation 1. Leads to increase in VOUT

voltage.

- Therefore VCB increases.

- During a negative half cycle of input signal Vi, the forward bias voltage VBE

increases

- Therefore there is increase in IE and IC.

- Substituting large value of IC in equation 1 leads to decreases in VOUT

voltage.

- Therefore VCB decreases, thus input and output voltages are in phase.

- Input resistance is lowest and output resistance is highest.

- In CB amplifier, the voltage gain is high, but current gain ( ) is less than

one.

Waveforms diagram:-

Q. Draw circuit diagram of common emitter configuration and explain its

working.

A. circuit diagram

.In the common emitter configuration input signal is applied between base and

emitter and output is obtained between the collector and emitter. the emitter-

base junction is forward biased and collector base junction is reverse biased.

.The collector to emitter voltage vce is given by

Vce=vout=Vcc-Ic.Rl---------1

. During a positive half cycle of input signal vi the forward bias voltage vBE

increase.

. Therefore there is increase in Ie and Ic.

.substituting large value of Ic in equation 1

.Vout i.e. Vce voltage decreases.

.during a negative half cycle of input signal Vi,Vbe decreases.

.there is decreases in Ie and Ic

.Vout voltage increases.

.therefore there is a phase-shift of 1800 between the input and output signals.

.

Q. Draw circuit diagram of common collector configuration and explain its

working.

A. circuit diagram

In this configuration, collector is common to both input and output circuits. But

the load (RL) is connected to emitter. The output signal obtained in the emitter

circuit follows the input signal as voltage gain is nearly one. Therefore, common

collector configuration is also known as ‘emitter follower’.

During the positive half-cycle of input signal, Vi, the forward bias voltage

VBE increases. Therefore, IE increases. This causes an increase in output voltage as

V0=IE.RL. Similarly, during the negative half cycle of input signal, the output signal

also decreases. Thus, the input and output signals are in phase.

Q. Static characteristics of A transistor in common emitter configuration.

In the common emitter configuration input signal is applied between base and

emitter and output is obtained between the collector and emitter. the emitter

base junction is forward biased and collector base junction is reverse biased.

INPUT CE characteristics-

This is plotted for VBE versus IB keeping VCE constant. when VBE=0 ,IB keeping

VCE constant. when VBE=0,IB is also zero. when VCE=0 and VBE is slowly increased at

cut-in voltage(0.2to0.3 for Ge and 0.6to0.7 for Si),the base current starts to

increase. The emitter junction behaves as a forward-biased diode. Then the

characteristics are plotted for IB versus VBE for various values of Vce eg.2v,4v etc.

The dynamic input resistance of the transistors is the reciprocal of the slope

of the input characteristics.

DYANAMIC INPUT RESISTANCE=ri=

At constant Vce as shown in fig. ri can be calculated for VCE=2V.

for common emitter configuration ri is typically 1kΩ.

Output CE characteristics –

These are plotted for collector current versus Vce for various values of input

current Ib. The output characteristics can be divided into three regions

1.saturation region

2.active region

3.cutoff region

1.saturation region- This region lies to the left of the saturation line OA. in this

region both the junction are forward biased. in this region collector current is

independent of the base current.

2. Active region- This region lies above the characteristics for Ib=0 and to the

right side of the saturation line 0A.in this region the E-B junction is forward biased

and the

C-B junction is reverse biased. This is the central region where the curves are

uniform in the spacing and slope.

In the active region Ic and times greater than Ib .therefore small input

current ,Ib produces a large output current Ic. for use as an amplifier the

transistor must be operated in active region.

3.Cut-off region- This region lies below the curve for Ib=0 region both the junction

are reverse biased. when Ib =0,Ic is not zero .It has value given by ICEO .this is

called the reverse leakage current.

for output characteristics ,dynamic output resistance can be calculated at

constant ib.

BASIC CE AMPLIFIER

basic circuit of CE amplifier using NPN transistor is shown. the emitter base

junction is forward biased by using battery Vbb of 1.5v.the collector base

junction is reverse biased by using battery Vcc of 6 or9 v. in practice instead of

using two such batteries only one battery Vcc is used and biasing network is used

to provide proper biasing to the base.

vi is the input is signal which is to be amplified .Rb is base resistance and rc is the

load resistance. the capacitors Cc1 and Cc2 allow the ac to pass though them,

but they block dc. therefore only ac voltage is obtained at the output.

DC LOAD LINE- The condition of having no input signal is called quiescent

condition. the battery Vcc produces a voltage drop across Rc.

This equation is of the form y= mx+ c which is the equation of a straight line.

Slope=m=

This straight line represented by the above equation is called the DC load line. dc

load line can be plotted on collector (output) characteristics. on the vce axis at

Vce=Vcc, Ic=0(point A) and on the Ic axis, at can be plotted .slope of the load

line is .

the operating condition of the transistor, are describe by the value of Vce and Ic.

these value fix up the operating (quiescent)point (Q)of the transistor. this point is

generally selected at the center of the load line.

TRANSISTOR AS SWITCH-

A transistor can be used as switch if we operate it at either saturation or cut off,

but nowhere else along the load line .when a transistor is cut off, it is like an open

switch. the base current is zero and transistor operates at lower end of load line.

when a transistor is saturated ,it is like a closed (on) switch from the collector to

emitter .

Transistor switching circuit is shown for designing transistor switching circuit, the

base current must be approximately one-tenth of the saturated value of the

collector current. This guarantees saturation under all operating condition.

Chapter 3

Study of Semiconductor Components

The important Characteristics of semiconductor diodes are:

Diodes shows forward and reverse resistances

Diodes conducts good amount of forward current

Behaviour in breakdown region

Junction Capacitance

PIV rating(Peak inverse voltage rating)- This is maximum reverse voltage

the diode must withstand. To avoid breakdown, the PIV rating of the diode

must be more than the peak inverse voltage.

Types of Diodes

Q) States the different types of diodes and explain in brief.

The important types of diodes are:

Signal diode

Power diode

Zener diode

Varactor diode

Schottky diode

Tunnel diode

Photo diode

Light emitting diode(LED)

Q) Write a note on Signal diode

These are germanium or silicon diodes.

These are general purpose diodes.

These can be used as radio waves detector

These can be used as an electronic switch.

These diodes are used to handle small currents and voltages

Maximum current range is up to 250 ma

PIV rating is up to 150 v

Very small junction capacitance

Q. Write a note on power diodes.

1. These are mostly silicon diodes

2. These can handle large power

3. Power diode are mostly used in rectifiers

4. Maximum forward current 30 A

5. PIV rating upto 1000 v

6. Very small forward resistance

7. Reverse resistance is very high

Q. Write a note on zener diodes.

1. This is PN junction diode which is heavily doped.

2. The depletion layer is very narrow.

3. Zener diodes can operate in any of three regions

a. Forward

b. Leakage

c. Breakdown

4. In the forward region – the zener diode acts like an ordinary PN

junction diode.

5. In the break down region – the current value increases to optimum

and voltage is almost constant approximately equal to Vz. Zener

diodes are usually operated in breakdown region.

6. In the Leakage region- there is only a small leakage or reverse

current.

Q. Give ratings of zener diodes.

1. Power dissipation :

this is the product of voltage and current.

Pz = Vz * Iz. Power ratings may be from 1/4 W to more than 50 W.

2. Maximum current rating:

This is the maximum current a zener diode can handle without

exceeding its power rating.

3. Zener resistance :

Zener resistance is specified at the same test current Izt used to

measured Vz.

Q. Explain the action of zener diode as a voltage regulator.

The zener diode is used to regulate the voltage across a load. When there are

variations in the supply voltage. The zener regulator is connected between the

output of the filter and load.

The zener regulator consists of current limiting resistor Rs connected in series with

the input voltage Vs and the zener is connected in parallel with the load. The

zener diode is reversed biased.

Where Is is the current from unregulated power supply.

Effects of supply voltage variations

If there is increase in input voltage Vs, the current through zener diode increases

and the load current remains constant. The extra voltage is dropped across Rs

therefore output voltage across the load remains constant. If there is decrease in

input voltage, current Iz decreases and the voltage drop across Rs is reduced. As

load current remains constant, output voltage remains constant.

Effect of load current variation

When RL increases, IL decreases. IZ increases such that IS is kept constant

therefore output voltage remains constant.

Thus the Zener diode can be used as a voltage regulator.

Q. Write a note on varactor diodes. State its uses.

In a reverse biased silicon diode there is depletion region between P and N

region.

The P and n regions are like the plates of a capacitor and the depletion

layer is like the dielectric.

When this diode is reverse biased, valence electrons from P side get

removed and free electrons are added to the n- side.

The depletion layer capacitance is called transition capacitance CT.

Uses :

When a varactor diode is connected in parallel with an inductor, a resonant

circuit can be formed whose tuned frequency is

Such circuit can be used for tuning in TV receivers, FM radio receivers and other

communication equipments.

Q. Write a note on Schottky diode . State its uses.

This diode uses a metal like gold, silver or platinum on one side of the

junction and n- type silicon on the other side.

When a schottky diode is not biased free electrons on the n-side are in

smaller orbits then the free electrons on the metal side.

The difference in orbit side is called the schottky barrier.

When the diode is forward biased free electrons on the n-side get enough

energy to travel in larger orbits.

Electrons cross the junction and entered the metal producing a large

forward current.

The metal has no holes therefore there is no charge storage and no

reverse recovery time

The time it takes to turn off a forward biased diode is called the reverse

recovery time.

Uses:

Schottky diode can switch off faster than an ordinary diode.

It can easily rectify frequencies above 300MHz

It has very small forward voltage drop therefore it can be used in

low voltage power supplies.

These are used in digital computers and other digital devices.

Q. Write a note on Tunnel diode. State its uses.

This is type of back diode.

Doping level is high.

The forward bias produces immediate conduction.

Maximum current Ip can be obtained at peak voltage Vp . then current

decreases to minimum value Iv(valley current) at voltage Va.

The region between Peak and valley points is called a negative resistance

region, in this region an increased in voltage produces decreased in

current.

Uses:

Negative resistance property of tunnel diode is useful in high

frequency oscillator.

Q. Write a note on photo diode. State its uses.

Photo diode are called optoelctronic devices.

Germanium, silicon or cadmium sulphide are used in a photo diode

This diode is operated in reversed bias condition.

When light falls on the diode pairs of electrons and holes produced.current

obtain due to minority charge carrier. Stronger the light, the greater the

minority carriers and therefore the reverse current increases.

When light is not incident is called Dark current.

Uses:

Photo diodes are used for reading punched cards and

tapes.

A photo diode can be used as a photodetector to convert

incoming light into a electrical quantity.

Q. Write a note on light emitting diode. State its advantages.

When the diode is forward bias, free electrons cross the junction and fall

into holes.

When electrons falls from higher energy level to lower energy level, they

radiates energy in the form of heat. But in LED, energy is radiated in the

form of light by using compounds of gallium, arsenic and phosphorus

LED that produce visible radiation are used in instrument display

calculator, digital watches, panel indicators etc.

The infrared LED can be used in burglar-alarm system.

Advantages:

Small in size and light in weight

Not affected by mechanical vibrations.

Long life.

Amplifiers

Amplification is the process of raising the strength of weak signal without any

changes in its general shape. The device used for such purpose is called an

amplifier. A small ac signal is applied to a transistor and a large ac signal is

obtained at the output.

Q) What do you mean by baising? Explain the need for bias stabilization.

1. Before applying ac input signal to te transistor, we have to set up a operating

point typically near the middle of the dc load line.

2. The process of obtaining certain dc collector current at a certain dc collector

voltage by setting up a operating point is called biasing.

NEED FOR BIAS STABILIZATION

1. The parameters of the transistor depends upon temperature. As temperature

increases, the leakage current due to minority carriers increases this cause

increases in the collector current. This produces heat at the collector junction

and thus operating point may be shifted into the saturation region.

2. When a transistor is replaced by another of the same type, the operating may

shift. This is due to the change in parameters such as which change from unit

to unit.

3. To avoid shifting of operating point bias stabilization is necessary. Biasing

circuits can be used for this purpose.

Requirements of a biasing circuit:

1. Operating point is established in the center of rhe active region of the

characteristics. It should not move either to the saturation region or to the cut-

off region, when input signal is applied.

2. The operating point should be independent of the transistor parameters. Then

it does not shift when the transistor is replaced by another of the same type.

3. The collector current should be stabilized against changes in temperature.

Types Of Biasing Circuits :

There are five types of biasing circuits.

1. Fixed bias

2. Collector-to-base bias

3. Fixed bias with emitter resistor

4. Voltage divider bias.

Fixed bias(Base bias)-In this type, two batteries VCC and VBB can be used or a

single battery Vcc can be used for collector and base of the transistor.

As Vcc is of fixed value, When RB is selected the base current IB is also fixed.

Therefore this type is called fixed-bias circuit.

IC=β IB

Vcc=Ic Rc + VCE

.. VCE=VCC-ICRC…………(2)

As co-ordinates of operating point are(VCE, Ic)by using equations (1)and(2),the

operating point may be fixed.

Merits-

1. It is very easy to fix the operating point anywhere in the active region by

changing RB.

2. Very few components (only two resistors and a battery) are required.

Demerits-

1. The collector current does not remain constant, When the temperature

increases. Therefore operating point is not stable.

2. When the transistor is replaced by another with different value of β the

operating point will shift.

Due to these drawback, fixed-bias circuit is rarely used. This method can not be

used in linear circuits, i.e , the circuits those use a transistor as a current source.

This circuit can be used in digital circuits in which the transistor is used as a switch.

Collector-to-base-bias circuits

In this type , the base resistor RB is connected to the collector instead of

connecting it to the battery VCC.

VCE ~VC ICRC since IB<<IC

Collector current tends to increase either because of increase in temperature or

due to the replacement of the transistor. Then the voltage drop across RC

increase. This causes decrease in VCE. Therefore, base current reduces which

compensates for the increase in collector current.

CB

CC

BRR

VI

CCCCCE RIVV

Therefore, there is a tendency to stabilize the operating point.

Merit: - This circuit has a tendency to stabilize the operating point against

temperature and variations.

Demerit- The resistor BR causes an ac feedback. This reduces the voltage-gain of

the amplifier. Because of this drawback, this type is not commonly used.

Fixed bias with emitter resistor:

The fixed bias circuit is modified by connecting a resistor to the emitter terminal.

Since, VBE is very small,

B

EECC

BR

RIVI

When the temperature increases, the leakage current increases. Therefore,

there is increase in IC and IE. This increases the emitter voltage, which reduces the

voltage across the base resistor. This reduces the base current which results in less

collector current. Thus, the collector current is not allowed to increase and

operating point is kept stable.

Similarly, if the transistor is replaced by another, which may have different

value of , there may be change in IC. But by the similar process, this change in

IC is compensated and operating point is kept stable.

.....CECCCCE IRRVV (since EC II )

Merit- This circuit has a tendency to stabilize the Q point against the changes in

temperature or of the transistor.

Demerit- In this type following condition must be satisfied.

B

E

RR . If RE is of a large value, high VCC is necessary. If RB is low, a

separate low voltage supply should be used in the base circuit. As this is

impractical and ac feedback through RE reduces the voltage gain, this type is

generally not used.

Voltage divider bias:

This type is most widely used in linear circuits. The voltage divider is formed

by R1 and R2. The voltage across R2 forward-biases the emitter junction. By

properly selecting the values of R1 and R2, the operating point of transistor can

be made independent of (beta).

CCRB VRR

RVV *

21

2

2

EEBE RIV

EEBBE RIVV

When the temperature increases, Ic increases. As Ic = IE , IE increases.

When IE increases, VBE decreases. This causes decrease in IC and thus

operating point remains stable.

CCCCC RIVV

Since EC II , CCECCCE IRRVV

In these equations is not present. Therefore, if the transistor is replaced

by other transistor having different , the operating point is not affected.

An ac feedback is provided through RE which reduces the voltage gain of

the amplifier. To avoid this capacitor CE is connected in parallel with RE. This

capacitor offers very low impedance to the ac. The emitter is placed at ground

potential for ac signal. Only dc feed back is provided for the stabilization of Q

point.

Merits –

1. Only one dc supply is necessary.

2. The operating point is almost independent of β (beta).

3. The operating point is not affected by the temperature variations.

Demerit-

AC feedback is provided by RE which reduces the voltage gain. This can be

avoided by connecting CE in parallel with RE. Voltage divider bias circuit is most

widely used.

Emitter Bias:

When a split supply ( dual power supply) is available, this type can be

used. The negative supply VEE is used to forward bias emitter junction through

resistor RE. the VCC supply reverse biases the collector junction. Only three

resistors are necessary. If RB is small enough, the base voltage is approximately

zero. The emitter voltage is less by VBE than this. Therefore, emitter current is,

IE = 𝑉𝐸𝐸−𝑉𝐵𝐸

𝑅𝐸 the operating point is independent of β, if RE ˃˃

𝑅𝐵

𝛽.

Merit – this type provides good stability of operating point similar to voltage

divider bias.

Problem : For the voltage divider bias circuit, R1 = 30kΩ, R2 = 4kΩ, RC = 4kΩ, β=

50, RE = 1.2kΩ, calculate IE and VCE. Assume VCC = 10V and VBE = 0.3V

Solution : The base voltage VB = 𝑹𝟐

𝑹𝟏+𝑹𝟐 x VCC

∴ VB = 4 𝑥 103

30+4 𝑥 103 x 10

VE = VB – VBE = 1.176-0.3 = 0.876 V

IE = 𝑉𝐸

𝑅𝐸 =

0.876

1.2 𝑋 103 = 0.73 mA

IC ≃ IE = 0.73Ma.

Vc = VCC- ICRC

= 10-0.73 X 10−3 X 4X 103 = 7.08V

VCE = VC – VE = 7.08 – 0.876 = 6.204 V

Single stage CE Amplifier

After a transistor has been biased in the active region, it can work as an

amplifier. We can apply a small ac signal to the base. This produces fluctuations

in the collector current. But there is no change in the shape and frequency of the

signal. When the input signal is so weak that it produces small fluctuations in the

collector current. But there is no change in the shape and the frequency of the

signal. When the input signal is so weak that it produces small fluctuations in the

collector current compared to the (quiescent value), the amplifier is called

“small – signal amplifier “or a linear amplifier.

Small signal amplifier is used as the first stage of the amplifier used in radio

receivers and TV receivers, tape recorders and measuring instruments etc.

Common emitter amplifier is show. Voltage divider biasing is used in this

circuit, as it provides good stabilization of the operating point. The capacitors Cin

and Co are coupling capacitors. These capacitors pass an ac signal; from one

side to the other. As they allow only ac signals and block dc signals, these

capacitors are also called blocking capacitors. Capacitor CE acts as a bypass

capacitor. It bypasses ac currents from the emitter to the ground. If CE is not

present, the ac voltage drop across RE produces negative feedback which

causes decrease in voltage gain.

RC is collector load resistance. Output signal is coupled through Co to any

device having the resistance Ro.

In common emitter amplifier, during positive half cycle of the input

voltage, the base current increases. As forward bias increases, there is increase in

IE and IC. Therefore, voltage drop across RC increases. As VC = VCC – ICRC,

there is decrease in VC. Thus for a positive half cycle at the input, negative half

cycle is obtained at the output. Thus in CE amplifier, output is 180° out of phase

with input.

Voltage gain = AV = 𝑜𝑢𝑡𝑝𝑢𝑡 𝑎𝑐 𝑣𝑜𝑙𝑡𝑎𝑔𝑒

𝑖𝑛𝑝𝑢𝑡 𝑎𝑐 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 =

𝑉0

𝑉𝑖

Current gain = AI – 𝐼𝑜𝑢𝑡

𝐼𝑖𝑛

Power gain = Ap = Av AI

Multistage amplifiers

Necessity of coupling: The voltage gain or current gain obtainable from a

single transistor amplifier stage is usually not sufficient for most of the applications.

Hence several amplifier stages are connected in cascade, i.e. such that the

output of one stage forms the input of the next stage. Such multistage or

cascade amplifiers provide desired voltage or current gain. In multistage or

cascade amplifiers, total gain is the product.

The logarithmic scale is used instead of linear scale due to the following reasons.

1. The decibel gains can be directly added.

2. Very small as well as vey large gains can be denoted by small figures. A

negative value of dB indicates that power P2 is less than the reference

power P1.

3. The output of amplifier is mostly converted into sound. The sound is

received by human ear. The ear responds to the sound intensities on a

proportional or logarithmic scale.

Types of multistage amplifiers : RC coupled amplifier

In multistage amplifiers output of first stage is coupled to the base of the

next stage. In RC coupled amplifier a capacitor CC is used for this purpose. Cin is

the input capacitor. If this capacitor is not connected, the source resistance gets

connected in parallel with R2, which causes change in biasing of the transistor.

Resistors R1, R2 along with RE provide voltage divider bias. If CE is not connected,

negative feedback is provided for the ac component which causes decrease in

the voltage gain. CE should be sufficiently high value, so that its reactance

becomes low and bypasses the ac component from emitter to ground. RC is the

collector load resistance. Transistors Q1 and Q2 provide the amplification.

Coupling capacitors CC and CO block the dc components from reaching

the next stage. Therefore, dc biasing of the next styage is not affected.

In fig. two stages of RC coupled amplifiers are shown. If the voltage gains are

AV1 and AV2 respectivly for two stages, total gain is AV1 x AV2.

As CE configuration is used, each stage provides a phase shift of 180° RC

coupled amplifier is the most widely used type. This type can be used in radio

recivers, TV receivers, record players, public address (PA) systems etc.

Frequency respose of RC coupled amplifier:

Fig shows the frequency response curve of a RC coupled amplifier. it shows

the change in voltage gain or output voltage with change in frequency. The

curve is usually plotted on a semi log graph paper with frequency range on

logarithmic scale. For testing, input signals at different frequencies can be

obtained from the signal generator. Effect of frequency can be explained for

three different ranges.

Mid frequency range:

In this range maximum and uniform gain can be obtained. The coupling

and bypass capacitors are as good as short circuits. Therefore, gain remains

nearly constant. It is denoted by AVm.

Low frequency range:

The reactance of capacitor is given by 𝑋𝑐 = 1

2𝜋𝑓𝑐. At low frequencies the

reactance of coupling capaciors is high and they act as an open circuit at zero

frequency (dc signals). Therefore, for dc output voltage falls to zero. The

bypassing action is not good in low frequency range.

High frequency range:

The current gain ( ) of the transistor depends upon the frequency. Its value

decreases at high frequencies. Therefore, the voltage gain of the amplifier

reduces as the frequency increases.

At high frequency, interelectrode capacitances are effective. The

capacitance Cbc between the base and collector connects the output with the

winding. AC voltage across the primary is transferred to the secondary. But there

is no dc path between the primary and secondary windings of a transformer.

Therefore, blocking capacitor is not necessary. Usual voltage divider biasing is

used. Transistors Q1 and Q2 provide amplification. Input transformer is used for

coupling input signal to the base of the transistor.

Advantages:

1. The dc resistance of primary winding is very low. Therefore, voltage drop across

it is negligible and all dc voltage supplied by Vcc is available at the collector.

2. It provides a higher voltage gain.

3. The absence of Rc eliminates the unnecessary power loss in the resistor.

4. If the interstage transformer is a step-up transformer, the ouput voltage can be

increased.

5. By selecting suitable turns ratio of the transformer, impedence matching is

possible. Therefore, maximum power can be transferred from the amplifier to

the load.

Drawbacks:

1. The transformer is bulky and costly.

2. Frequency response curve is not uniform. This is due to leakage inductance

and interelectrode capacitance.

3. At a high frequency, the gain of amplifier increases to maximum value due to

interelectrode capacitance. This is called ‘resonance’.

4. At low frequencies gain is very low.

5. The transformer tends to produce hum.

Uses –

1. Transformer coupling can be used in power amplifiers.

2. By connecting shunt capacitor across the winding of a transformer.

Advantage:

DC voltage drop across the coil is very small. Therefore, collector supply

voltage increases. This causes increase in the amplification.

Drawbacks –

1. Coils are larger, heavier and costlier than the resistors.

2. In order to prevent the magnetic field of the coil from affecting the signal,

shielding is necessary.

3. Frequency response curve is not uniform. The gain increases with increase

of frequency. But at higher frequencies gain falls due to interelectrode

capacitances. This may produce resonance at a frequency showing

sudden increase in the gain.

Transformer coupled amplifier:

In this type, a transformer is used for coupling ac output voltage of the first

stage to the input of the next stage. RC is replaced by the primary the resonance

can be obtained at any desired radio frequency. Such tuned voltage amplifiers

are used in radio and TV receivers.

Direct coupled Amplifiers (D.C. amplifier):

In this type, the output of one stage of the amplifier is connected to the input

of the next stage directly without using any reactive components like capacitors,

inductors, transformers, etc. Therefore, the frequency response of this amplifier is

independent of frequency. D.C. amplifier does not mean direct current amplifier

as this amplifier can be used for both d.c. and a.c. signals. The amplification of

D.C. (zero frequency) signals is possible only by this amplifier. Low frequency

signals (below 10 Hz) can be amplified by d.c. amplifier only. The use of coupling

and bypass capacitors is not possible at very low frequencies as they provide

very high resistance. This is avoided in d.c. amplifiers.

Advantages:

1. The circuit arrangement is very simple since it uses minimum number of

components.

2. It is quite inexpensive.

3. D.C. amplifier can be used to amplify d.c.(zero frequency) and low frequency

(slowly varying) a.c. signals which is not possible in any other type.

4. It provides uniform frequency response upto a high frequency. Lower cut-off

frequency is zero and upper-cut-off frequency is f2.

Drawbacks:

1. An unwanted change in output voltage due to changes in temperature is

observed without change in input. This is called drift. This drawback can be

removed by using differential amplifier.

2. At high frequencies gain decreases.

3. The transistor parameters like VBE and change with temperature. This

causes change in collector current and voltage. Therefore, the output

voltage changes. Even if input is ac, a dc component is present in the

output.

4. Any noise or stray pickup appearing at the input of the amplifier increases at

the output, due to high gain.

Applications

D.C. amplifiers are used in TV receivers, computers, in the regulator circuits

and other electronic instruments. Differential amplifiers and operational amplifiers

are direct coupled amplifiers.

Classification of Amplifiers

Amplifiers can be classified in accordance with one of the following ways:

(I) In accordance with the frequency range

(i) D.C (direct coupled) amplifiers capable of amplifying even zero

frequency (dc signals).

(ii) Audio frequency (AF) amplifiers – 20 Hz to 20 kHz.

(iii) Video amplifiers – upto few MHz.

(iv) Ultra-high frequency (UHF) amplifiers – upto thousands of MHz.

(II) In accordance with the type of load

(i) Untuned amplifiers – (a) audio amplifiers (b) video amplifiers

(ii) Tuned amplifiers (RF amplifiers) –These are used for amplifying a

single radio frequency or a band of frequencies.

(III) In accordance with the number of stages and method of coupling

(i) Single stage amplifiers

(ii) Multi-stage or cascade amplifiers – (a) R-C coupled amplifier, (b)

L-C coupled amplifiers, (c) transformer coupled amplifier and (d)

direct

CHAPTER 3

Oscillators

Q. What is an oscillator?

An oscillator is an electronic circuit which generates an ac output signal without

requiring any externally applied input signal. The frequency, waveform and

magnitude of output are controlled by the circuit itself. The oscillator does not

required an external signal either to start or maintain energy conversion process.

It receives energy from dc power source and changes it into ac energy of

desired frequency. Thus it acts as a source of energy at a specific frequency

which may range from a few Hz to several MHz.

Q. What are the uses of oscillators?

Uses of Oscillators:-

1. Oscillators are used in sine wave and square wave signal generators,

which are useful as testing instruments.

2. Sine wave oscillators are useful as local oscillators in the tuner circuits of

radio and TV receivers.

3. High frequency carrier signal required is Am and EM (amplitude

modulation and frequency modulation) can be produced by using

oscillators.

4. High frequency oscillators are used in induction and dielectric heating

5. Crystal oscillators are used in transmitter and receiver systems used in

aircrafts.

6. Relaxation oscillators using UJT is used in CRO for the generation of time

base (sawtooth) signal.

Q. Give classification of oscillators?

Classification of Oscillators:-

The oscillators may be classified in the following different ways :-

I. According to the frequency generated

i. Audio frequency (AF) oscillators : Frequency range 20Hz to 20 KHz

ii. Radio frequency (RF) oscillators : 30 kHz to 30MHZ

iii. Video oscillators : upto 5 MHz

iv. High frequency (HF) oscillators : 3 MHz to 30MHz

v. Very high frequency oscillators ; 30 MHz to 300MHz

vi. Ultra high frequency (UHF) and microwave oscillators – above 300 MHz

II. According to the design principle

i. Negative resistance oscillators using UJT or tunnel diode etc.

ii. Positive feedback oscillators: e.g. LC or RC oscillators.

III. According to associated circuit or components

i. L – C oscillators

ii. R – C oscillators

iii. Crystal oscillators

IV. According to the wave generated

i. Sine wave (sinusoidal) oscillators ; RC oscillators , LC oscillators,

ii. Relaxation (non – sinusoidal) oscillators:

Triangular, rectangular, sawtooth waveforms are produced using UJT and other

components.

Q. What are the requirements of oscillators?

Requirements of an oscillator

Every oscillator consists of following three basic sections:-

(1) Internal or basic amplifier, (2) Positive feedback network or negative

resistance effect and (3) Amplitude limiting device.

In the block diagram of oscillator basic amplifier with positive feedback is shown.

The components used in the feedback network determine the frequency of

oscillations. In practical oscillators, the amplitude of oscillations is limited by the

non – linearity in ioperation of the active device (transistor) used as amplifier.

Barkhausen criterion of oscillations :-

This gives the condition under which a feedback amplifier can work as an

oscillation. For an amplifier with positive feedback, the voltage gain is given by,

Af = A

1−Aβ=

Vo

V i

Q. Give in brief the concept of phase – shift?

Concept of phase – shift

When a sinusoidal voltage Vi = vm sin wt is applied to the circuit consisting of R1

and C1 in series. The alternating current in the circuit leads the applied voltage

by an angle Φ1 which is give by

𝑡𝑎𝑛 Φ1 = 1

ωR1 C1=

1

2πfR1 C1

Alternating voltage across R1 leads V1 by an angle Φ1. This is the phase – shift of

ladder network consisting of R1 and C1. The values of R1 and C1 may be selected

so that for a frequency Fo, Φ1 = 60°

If a ladder network consisting of 3 R – C sections is built as shown in fig (5.2), total

phase shift is given by

Φ = tan −1 1

2πfR1 C1 + tan−1

1

2πfR2 C2 + tan −1

1

2πfR3 C3

If R1 = R2 = R3 = R and C1 = C2 = C3 = C and if the value of C and R are so selected

that for frequency Fo, phase – shift of each R – C section is 60°, than phase – shift

is 180°.

Q. Draw and explain the RC phase – shift oscillator?

RC phase – shift oscillator

In this oscillator transistor works as common emitter (CE) amplifier, CE

amplifier provides 180° phase – shift. Three R – C sections are used which are

called ladder networks. The phase – shift provides by each R – C network

Φ = tan −1 1

2πfR1 C1

All capacitors are of equal values and all resistors are also of equal values. For a

particular frequency, phase – shift provided by each R – C section is 60°. Total

phase – shift provided by 3 R – C networks is 180°. Thus, the total phase – shift in

the circuit is 360° or zero. The gain of the amplifier must be sufficient so that

AB ≥ 1

Then Barkhausen criterion is satisfied and sustained (continuous) oscillation are

produced.

The frequency of oscillation fo = 1

2πf RC 6

At this frequency, the feedback factor = 1

29 . This means the gain of the amplifier

must be greater than 29.

Advantage

1) The phase – shift oscillators is useful over a wide frequency range, from a

few Hz to several hundred kHz.

2) Large size inductors or transformers are not required

3) In the low – frequency range large inductors required for LC oscillator

would be impractical. But frequencies as low as 1 Hz also can be easily

obtained by using R – C phase – shift oscillator.

Disadvantage

1) Grain of transistor must be high to overcome losses in the RC network

2) The upper frequency is limited upto about 10 kHz because the impedance

of the RC network becomes so small that it loads the amplifier heavily.

3) Frequency of oscillation cannot be varied easily since it is difficult to vary

three capacitors simultaneously.

Q. Explain the concept of Tank Circuit?

Tank Circuit

The parallel combination of a charged capacitor (C) with an inductor (L) is

called ‘tank circuit’. Tank circuit is an essential part of LC oscillators.

A fully charged capacitor c is connected in parallel with inductance L.

The energy is stored in electrostatic field of capacitor.

Now the capacitor discharges through L . After 1

4th cycle (90°), energy is stored in

magnetic field of L . After next 1

4th cycle, the induced magnetic field begins to

collapse sending electric current and C gets charged in opposite polarity. After

next 1

4th cycle again C discharged in the reverse direction. Therefore, energy is

stored in magnetic field of L in opposite direction. After next 90° energy is again

stored in electrostatic field of capacitor.

Thus, charging and discharging of capacitor produces oscillation the frequency

of oscillations is given by,

fo = 1

2π LC

If there is no power loss in the circuit, sustained oscillations are produced at the

frequency given by above equation. But in practice (1) there is some power loss

during each oscillation as some resistance is always associated with LC circuit.

Hence energy is dissipated as heat. (2) some part of energy is used in the

generation of electromagnetic waves, if the frequency is high. Due to such

power losses, damped oscillations are produced.

To obtain sustained oscillations, energy must be supplied to the LC circuit at the

same rate at which it is dissipated. In an oscillator, the transistor and the power

supply feed energy to the circuit to overcome the losses.

L – C Oscillators

Types of L – C oscillators are (1) Hartley oscillator and (2) Colpitt’s oscillator.

Q. Draw and explain Hartley oscillator. And state the formula of frequency?

Hartley oscillator

In Hartley oscillator L-C tank circuit is used. Instead of using two separate

inductors L1 and L2 a signal coil may be used and any desired point on the coil

may be grounded. The function of each component is described below. The

resistors R1, R2 and RE provide self – bias with a voltage divider. CE acts as bypass

capacitor. The tank circuit consisting of L1 L2 and C determines to the tank circuit.

RFC is a radio frequency choke. It prevents the RF output from reaching the

supply voltage VCC and it also prevents the supply voltage from short circuiting

ac output voltage C2 and R2 self – bias voltage so that when amplitude of input

voltage is large, transistor operates under class C condition.

Frequency of oscillations is F0 = 1

2π LC

Advantage

1. The frequency of oscillations may be varied easily by using variable

capacitor.

2. Hartley oscillator may be used as local oscillator in radio receiver to

provide R.F output

3. Hartley oscillator may be used in high frequency heaters.

Q. Draw and Explain Colpitt’s oscillator. State the formula of frequency.

Colpitt’s oscillator:

The circuit of colpitt’s oscillator is similar to that of Hartley oscillator. In the

tank circuit instead of using two coils L1 and L2 two variable capacitors C1 and C2

are connected in series. These capacitors are connected in parallel with coil L.

frequency of oscillation may be varied by gang – tuning the two capacitors C1

and C2.

The functions of different component are similar to those in Hartley

oscillator. Common emitter amplifier provides a phase – shift of 180°. Another

phase – shift of 180° is provided by the tank circuit. Thus, total phase shift is 360° or

zero between signal developed across C2 and the input signal. The input signal is

initially developed due to the transient current produced, when the power

supply is switched on.

The frequency of oscillation is given by

fo = 1

2π LC

Where C = C1 C2

C1+ C2

The oscillations are sustained if 𝐴𝛽 ≥ 1

Advantage

1. Colpitt’s oscillator can be used in commercial signal generators for

producing frequency above 1 MHz.

2. It can be used in radio and TV receivers as local oscillator.

3. It can be used in high frequency heaters.