Semi Conductor Electronics

Second Year Physics study material

Modern physics possess five important chapters

1. Dual nature of Radiation and Matter

2. Atoms

3. Nuclei

4. Semi conductor electronics

5. Communication systems

Blue print of marks for PUC and CBSE board are as follows

CBSE Board:

For PUC board Karnataka

Semi conductor electronics

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Dual nature of

radiation and matter

Atoms and nuclie

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4

6

8

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Dual nature of

radiation and

matter

Atoms and nuclie


Modern physics possess five important chapters

1. Dual nature of Radiation and Matter

4. Semi conductor electronics

and CBSE board are as follows


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3

Atoms and nuclie Electronic devices Communication

systems

Marks weightage

1110

3

Atoms and nuclie Electronic devices Communication

systems

Marks weightage

Second Year Physics study material 2015

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Marks weightage

Marks weightage


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1. Explain briefly classification of materials based on conductivity?

A) Materials are broadly classified in to three categories based on conductivity.

1. Metals 2. Semi conductor 3. Insulators

Metals : The substances which easily allow the electric current through them are

called conductors. Ex: Gold , silver, Aluminium etc.,

Semi conductors : The substances whose conductivity lies in between conductors

and insulators are called Semi conductors. Ex: Germanium , silicon

Insulator : The substances which do not allow the electric current through them are

called Insulators. Insulators have high resistivity and low electrical conductivity .

example : Glass , wood , mica etc.,

2. Write a short note on Band theory of Solids?

A)

In solids they are many atoms. Energy levels of inner orbit electrons of an atom are

not influenced by neighboring atoms as they are tightly bound to parent nucleus.

But energy levels of outermost orbit electrons are influenced by neighboring atoms.

The outermost electrons of an atom are common to several neighboring atoms.

Therefore the energy levels corresponding to the outer shell electrons spread up to

form a band of energy.

Energy band: A collection of many closely spaced energy levels is known as energy

band.

Valency band: Energy band occupied by valency electrons of all the atoms of a solid is

known as Valency band. This may be either completely filled or partially filled.

Conduction band: The energy band occupied by the free electrons in a solid is known

as conduction band. They may be partially filled or empty.


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Forbidden energy gap: It is the energy difference between lowest level of conduction

band and the highest level of valency band.

3. Explain briefly classification of solids on the basis of energy bands?

A) Solids can be classified in to various categories on the basis of energy band.

Insulators : The forbidden energy gap between valence band and conduction band is

large. In insulators , valence band is filled with electrons and they are bound very

tightly to their parent atoms so the conduction band is empty. Due to large forbidden

energy gap electrons cannot jump from valence band to conduction band.

Semiconductors : The forbidden energy gap between valence band and conduction

band is very small. In Germanium forbidden energy gap is 0.7 eV and in silicon

forbidden energy gap is 1.1 eV

At 0K , there are no electrons in conduction band and the valence band is completely

filled with electrons so they behave as insulators . When small amount of energy is

supplied , the electrons can easily jump from valence band to conduction band.

Conductors : The valence band and conduction band overlap each other. In

conductors the forbidden energy gap is 0 eV. The electrons in valence band can easily

enter in to conduction band . A slight potential difference across the conductor cause

the electrons to constitute the electric current.

4. Write a short note on Intrinsic semiconductor

and Extrinsic semiconductor?

A) Intrinsic semiconductor: A semiconductor in

an extremely pure form is known as intrinsic

semiconductor.

Ex : Silicon , Germanium

The semiconductor in which the current carriers

(Holes and electrons) are created due to thermal

excitation only across the forbidden energy gap is

called intrinsic semiconductor.

At oK all covalent bonds are complete, so no free

electron available in crystal for conduction of current. Thus silicon or Germanium

behaves as insulator at 0K. At room temperature some electrons move from valence

band to conduction bond due to thermal energy. The electron which leaves the

valence bond is called free electron and vacancy created in the valency bond due to

release of electron is called a hole. Hole is equivalent to positive charge.


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Thermally generated electrons keep on occupying the positions of nearby holes. They

collide with holes and recombine. At steady state equilibrium , rate of combination of

holes and electrons is just equal to the rate of production of holes and electrons.

In an intrinsic semiconductor , number density of free electrons ne , is equal to

number density of holes nh and is known as number density of intrinsic carriers ni .

ne = nh = ni

When an intrinsic semiconductor , Holes and electrons move randomly. When it is

connected to electrical source , then electrical field is setup across the conductor.

Holes in valence bond drift towards negative terminal of battery and conduction

electrons move in opposite direction to hole. The motion of charge carriers gives rise to

electric current.

The total electric current I is thus sum of electron current Ie and hole current Ih .

I = Ie + Ih

Extrinsic semiconductor :

The semiconductors with impurities are called Extrinsic semiconductors. Extrinsic

semiconductors are of two types

1. P- type extrinsic semiconductor

2. N- type extrinsic semiconductor

P Type semiconductor:

When suitable trivalent impurity added to pure Intrinsic semiconductor we get

extrinsic semiconductor known as P- type semiconductor.

Majority charge carriers in this type of semiconductor are positively charged holes. So

this type of doped semiconductor is called P- type semiconductor.

Trivalent impurity has three valance electrons . When an atom of Indium is added in

to semiconductor (say silicon) , atom replaces one of semiconductor atom and settles

in the lattice site of replaced semiconductor atom.

Indium atom forms three covalent bonds with neighbouring atoms , fourth bond

remains incomplete . This deficiency of electron is called Hole.

Energy band diagram of P- type semiconductor is as shown in figure. The energy level

corresponding to the holes in the P- type semiconductor lies just above the valence

bond. This energy level is known as acceptor level.


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The energy difference between the acceptor level and the highest energy level of

valence band is muss less than forbidden energy gap.

At room temperature, thermally generated electrons in the valence band are easily

transferred to acceptor level and hence large number of holes are created in the

valence bond. These holes act as current carriers when P type semiconductor is

connected across the battery.

N Type Semiconductor:

When a suitable pentavalent impurity added to a pure intrinsic semiconductor , we

will get Extrinsic semiconductor of N- type.

Majority charged carriers in this type of semiconductor are electrons. Since each

pentavalent impurity atom donates one electron to the crystal , so it is known as

Donar impurity.

Pentavalent impurity say Arsenic (As) have five valency electrons. When Arsenic atom

is added to semi-conductor crystal, it replaces semiconductor atom and settles in the

lattice site of replaced silicon atom. This arsenic atom forms four covalent bonds by

sharing its four electrons with the neighboring four semiconductor atoms. The fifth

valence electron are free. This fifth electron is loosely attached and can move randomly

throughout the crystal. In this way a large number of free electrons are available

when Arsenic is added to silicon crystal.

The fifth electron in N-type semiconductor occupies a discrete energy level known as

donar level just below conduction band of semiconductor crystal. Energy gap between

donor level and conduction band is very small. Even room temperature provides

sufficient thermal energy to the free electrons at donor level to jump to conduction

band. These electrons in conduction band are mainly responsible for conduction of

current in the N- type semiconductor.


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5. Define Doping?

A) The process of adding impurities to a pure semiconductor crystal so as to increase

its conductivity is called doping and the impurity atoms are called dopants.

6. Explain briefly P-N junction diode? Explain the formation of depletion region

in pn junction? How does width of depletion region changes in forward and

reverse bias?

A) A junction formed when P type semiconductor joined with N type semiconductor

and the crystal structure remains continuous at the boundary is termed as P-N

junction diode.

Formation of P-N junction:

In P type semiconductor major charge carriers are holes and in N type semiconductor

major charge carriers are electrons. When P N junction is formed , majority charge

carriers diffuses across the junction giving rise to diffusion current.

Holes diffuse from P semiconductor to N semiconductor and electrons diffuse from N

semiconductor to P semiconductor.

When electron diffuses from N to P region , it leaves behind immobile position ion on N

side. Similarly when a hole diffuses from P to N region it leaves behind immobile

negative ion on the P side.

The space charge region at the P N junction which consists only of immobile ions and

is depleted of mobile charge carriers is called depletion region. The depletion region

prevents further diffusion of majority charge carriers.

Due to this space charge region in electric field E is developed which is directed from

N region to P region. The depletion region has a layer of positive charge on N side and

layer of negative charge on P side.

Under Forward bias :

PN junction diode is forward biased when external voltage is applied such that P side

is connected to positive terminal and N side is connected to Negative terminal of

battery.

Due to forward bias, the depletion region width decreases and potential barrier is

reduced. As potential of battery increases the majority charge carriers , electrons from

N side and Holes from P side diffuse across the junction since the potential across the

depletion region decreases. Effective resistance of PN junction decreases.

Under Reverse bias :


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PN junction diode is reverse biased when external voltage is applied such that P side is

connected to Negative terminal and N side is connected to positive terminal of battery.

In reverse bias , the width of depletion region increases and the potential barrier

height also increases.

Majority charge carriers move away from the junction increasing the width of depletion

layer . The resistance of the PN junction diode becomes very large.

7. What is the symbol of PN junction diode?

A)

8. Draw and explain current voltage (I-V) characteristics curves of junction diode

in forward and reverse bias?

A) A graph drawn between Voltage applied across the terminals of P-N junction diode

and current flow in the circuit Is called I V characteristics of junction diode.

Usually Voltage is taken along X axis and Current along Y axis.

Forward bias characteristics:

1. As forward voltage is zero, current in the circuit is zero. This is indicated by a point

at origin.

2. From origin to a point A, when forward voltage increases , increases in the current

increases is small because forward voltage is less than barrier voltage.

3. At some forward voltage the potential barrier is eliminated and current starts

flowing. This is known as Threshold voltage or cut in voltage or Knee voltage.

4. As forward applied voltage increases beyond threshold voltage , the forward current

rises exponentially . This forward current is due to majority charge carriers in P-N

junction diode.

Reverse bias characteristics:

1. When reverse voltage is increased from origin to C a small reverse current flows

due to minority carriers crossing the junction.


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2. When reverse voltage reaches the point C the reverse current suddenly increases to

large value, because of breaking of covalent bonds and releasing large no of electron

hole pairs. This voltage is called break down voltage.

3. At break down voltage, there is sudden rise of reverse current and diode is

destroyed permanently

9. Define Dynamic Resistance?

A) The ratio of small change in voltage (dV) to the small change in current (dI) is called

dynamic resistance of junction diode.

rd = dV/dI

10. How P-N junction diode is used as a Rectifier?

A) Rectification : The process of converting an Alternating current into a direct

current is called Rectification.

Rectifier : The device used to convert an alternating current into a direct current is

called Rectifier. PN junction diode acts as a rectifier because it permits current in one

direction only.

They are two types of Rectifiers

1. Half wave Rectifier

2. Full wave Rectifier

Half wave Rectifier :

A Half wave rectifier can be constructed with single diode as shown in figure.

1. During positive half cycle of A.C. input , end A becomes Positive and end B becomes

negative. This makes diode forward bias and conducts current. So output is obtained

across load resistance.

2. During negative half cycle of A.C. end A becomes Negative and end B becomes

positive. This makes diode reverse bias and does not conduct current. So no output is

obtained across load resistance. Thus a half wave rectifier gives discontinuous and

pulsating d.c. output across load resistance.

3. The efficiency of a rectifier is the ratio of dc power output to the ac power input

Efficiency of rectifier = dc power output/ac power output


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For half wave rectifier = (0.406 RL)/(rf + RL)

In halfwave rectifier , a maximum of 40.6 % of ac power is converted into D.C. power.

Full wave Rectifier:

A Rectifier which rectifies both halves of ac input is called Full wave rectifier. A full

wave rectification can be achieved by two diodes as shown figure. This circuit has two

diodes D1 and D2 and a transformer known as center tap transformer.

A full wave rectifier with two diodes as shown in figure.

1. During Positive half cycle , end A becomes positive and end B becomes Negative.

This makes diode D1 forward biased and diode D2 reverse biased. So D1 conducts

and D2 does not. So output voltage obtained through load resistance due to diode D1.

2. During Negative half cycle, end A becomes negative and end B becomes positive.

This makes diode D1 reverse bias and D2 forward bias. So D2 conducts and D1 does

not. So output voltage obtained across load resistance due to diode D2.

3. This full wave rectifier gives continuous and pulsating out D.C. output.

4. for full wave rectifier = (0.812 RL)/(rf + RL)

In full wave rectifier 81.2% of A.C. converted in to D.C.

12. How Zenar diode acts as a voltage regulator?

A) A heavily doped P-N junction diode which has sharp breakdown voltage when

operated in reverse bias condition is called Zenar diode.

The circuit symbol of Zenar diode as shown in figure

Zenar diode as Voltage regulator :


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A device used to give constant output voltage even when the input voltage to it varies

or load resistance to it change is called voltage regulator or voltage stabilizer.

The unregulated input voltage is applied across Zenar diode and Resistance R which

are connected in series , in such a way that Zenar diode is reverse biased. The load

resistance is connected across Zenar diode output as shown in figure.

If I is the input current , Iz and IL are Zenar and load currents respectively then

I = Iz + IL

Vin = IR +Vz , But Vout = Vz

Therefore Vout = Vin IR

The value of R is selected in such a way that in the absence of load resistance ,RL

maximum safe current flows through the diode. During the fluctuations even through

the current in the source changes , the voltage across zenar diode remains constant.

The voltage across zenar diode remains constant even if the load resistance RL varies.

When RL is increased but applied input voltage (Vin) is fixed, the current IL decreases

and current Iz increases by an equal amount. So that the total current I remains

constant. Hence output voltage remains constant. Thus zenar diode works as voltage

regulator.

13) What are the I-V characteristics of a Zenar diode?

A) The voltage current characteristics is as shown in figure. In forward bias, its

characteristics are similar to that of an P-N junction diode.

When reverse biased , a small reverse saturation current flows through it which

remains approximately constant until a certain critical voltage called breakdown

voltage is reached.

At this voltage , the reverse current increases sharply to a high value. This break down

voltage is called zenar voltage and the reverse current is called Zenar current.

The zenar voltage depends on the amount of doping. A heavily doped diode has a

narrow depletion layer and consequently a lower breakdown or zenar voltage.

On the other hand, if the diode is lightly doped the breakdown of junction will occur at

higher voltage.

14) Explain briefly various types of Opto electronic devices ?


A) The junction diode which conducts when charge carriers are generated by photons

that is light incident on it is known as optoelectronic junction devices.

The various types of opto electronics junction devices are

1. Photo diode

2. Light emitting diode

3.Solar cells or photo voltaic device

Photo diode:

A photo diode is essentially a

respond to light absorbed

Symbol :

Working:

When light photons each of energy E = h fall on semiconductor , the valence electron

absorb this energy and jump to conduction band leaving a hole in the

Thus electron hole pairs are produced. This electron hole pairs constitute a photo

current which flows in a circuit. As intensity of light increases photo current also

increases.

I V characteristics of Photodiode

1. When no light falls on diode , a small reverse current flow due to minority carriers.

This current is called dark current.

2. With increase in intensity of incident light the value of reverse current also

increases.

3. Measurement of change in reverse current on illumination ca

light intensity.

Light emitting diodes:

A light emitting diode (LED) is a forward biased P

visible light when energized.

Symbol :


The junction diode which conducts when charge carriers are generated by photons

that is light incident on it is known as optoelectronic junction devices.

The various types of opto electronics junction devices are

3.Solar cells or photo voltaic device

A photo diode is essentially a reverse biased PN junction diode which is designed to


absorb this energy and jump to conduction band leaving a hole in the valence bond.



V characteristics of Photodiode:

diode , a small reverse current flow due to minority carriers.

This current is called dark current.


Measurement of change in reverse current on illumination can give the values of

A light emitting diode (LED) is a forward biased P-N junction diode , which emits


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The junction diode which conducts when charge carriers are generated by photons

reverse biased PN junction diode which is designed to


valence bond.



diode , a small reverse current flow due to minority carriers.


n give the values of

N junction diode , which emits


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Working:

When a junction diode is forward biased , electrons from N side and holes from P

side move towards the depletion region and recombination takes place. When an

electron in the conduction band recombines with a hole in the valence band energy is

released.

In case of semiconducting materials like Gallium arsenide (GaAs) , Gallium phosphide

(GaP) amd gallium arsenide phosphide (GaAsP) a greater percentage of energy is given

out in the form of light. If the semiconductor material is translucent , light is emitted

and junction becomes a light source.

Advantages of LED over conventional incandescent lamps:

1. Low operational voltage and less power consumption

2. Fast action and no warm up time required

3. Band width of emitted light is 100 A to 500 A that is light is nearly monochromatic

4. long life and raggedness

5. Fast on/off switching capability.

Solar cell :

Solar cell is a junction diode which converts solar energy into electricity and is based

on photo voltaic effect.

Working :

When light photons reach the junction , electron hole pairs generated in the depletion

region and move in opposite direction due to barrier field.

Now electrons move to N side and hole move towards P side. Thus P side becomes

positive and N side becomes negative giving rise to photo voltage.

When load resistance is connected in external circuit, a photo current flows. The

current is proportional to intensity of light.

I-V characteristics of solar cell :

Here V0 is open circuit voltage of a solar cell. And Is is the maximum current , that is

short circuit current drawn from the cell. The curve is available in fourth quardrant

because current I is supplied by the cell and not drawn by the cell.


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Uses of solar cell :

1. They are used in street lights

2. They are used in solar heaters

3. They are used in power supply of satellites and space vehicles

4. They are used in calculators.

15. Define Transistor and Explain the working of Transistor?

A) A junction transistor consists of a thin layer of one type of extrinsic semiconductor

sandwiched between two thick layers of other type of extrinsic semiconductor.

The word Transistor means Transfer of resistance. A transistor has three regions they

are

1. Emitter : Section at one end of transistor is called emitter. It is a heavily doped

region. It consists of large number of charge carriers.

2. Base : The middle section of Transistor is called Base. This is lightly doped and

very thin. Most of charge carriers flow through it in to collector with our neutralized.

3. Collector : Section at other end is called collector, it is moderately doped. Physically

it is largest and collect charge carriers from the base.

They are two types of junction transistors

1. N-P-N Transistor

2. PN-P transistor

N P-N Transistor :

In N-P-N transistor the emitter junction is forward biased with negative terminal of

battery connected to the emitter and positive to the base. The collector junction is

reverse biased with positive terminals to the collector and negative to the base.


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In N-P-N transistor, the electrons in emitter are repelled by negative terminal of

battery and cross the base region causing the emitter current IE. As base is lightly

doped few electrons combine with holes causing base current IB and the majority of

electrons enters the collector. These electrons rapidly swept out by the positive

terminal of the battery, causing collector current IC.

IE = IB + IC

A continuous supply of electrons injected into the emitter flows across the base to the

collector.

In N-P-N transistor charge carriers inside and outside the circuit are electrons.

P N-P Transistor:

In a P-N-P transistor emitter junction is forward biased by connecting positive

terminal of a battery to the emitter (E) and negative to the base. The collector junction

is reverse biased by connecting battery positive to the base and negative to the

collector.

The holes in the emitter are repelled by positive terminal of battery and cross the

emitter junction enters into the base causing emitter current IE. A base is lightly doped

, a few number of holes combined with electrons causing a base current IB. Majority of

holes enters the collector region. The collector terminal connected to the negative of

battery. It rapidly sweep the holes in collector causing a collector current IC.

IE = IB + IC

A continuous supply of holes injected into the emitter flows across the base to the

collector. In P-N-P transistor majority charge carriers inside the circuit are holes and

outside the circuit change carriers are electrons.

16. Explain briefly the configurations of Transistors?


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A) For any electronic circuit, there has to be two terminals for input and two

terminals for output. But in a transistor only three terminals are available Emitter,

Base , Collector. So, in a circuit the input and output connection have to be such that

one of these (E, B and C) are common to both input and output

Transistor should be connected in any of the three following configurations

1. Common base configuration (CB)

2. Common Emitter configuration (CE)

3. Common collector configuration (CC)

17) Explain briefly the Input and output characteristics of Transistor in common

emitter configuration?

A) The Transistor is connected in common emitter mode. The input is between base

and emitter terminals while the output is between collector and emitter terminals

Input characteristics :

The graphical representation of variation of base current IB with the base emitter

voltage VBE for a fixed value of collector emitter voltage VCE is called input

characteristics

VCE is kept fixed. VBE is varied and the variation in IB noted in regular intervals. For

small values of VBE the base current IB is negligible. When VBE exceeds barrier voltage

IB increases sharply even with small increase in VBE.

A set of such curves can be plotted at different fixed values of collector emitter voltage

(VCE)

Conclusions:

1. The input characteristics are similar to forward bias characteristics of junction

diode.

2. For a given value of emitter base voltage (VBE) the base current decreases with the

increase in collector emitter voltage.

Input dynamic resistance :


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It is the ratio of small change in VBE ( VBE) to a small change in IB ( IB) at constant

collector emitter voltage (VCE)

ri = VBE/ IB |VCE =constant

It is clear that ri changes continuously

Output characteristics :

The graphical representation of the variation of collector current IC with the collector

emitter voltage VCE for a fixed value of base current IB is called output characteristics.

Keep the Base current (IB) fixed. Now change the collector emitter voltage and note the

corresponding values of collector current (IC).

Graph between various values of VCE and IC is plotted which is output characteristics

of transistor.

A set of such curves can be plotted for various values of Base current (IB)

Conclusion :

1. For a given value of base current, collector current increases rapidly with the

collector emitter voltage in the beginning but at high value of VCE , collector current

becomes constant.

2. For a given value of VCE , the collector current (IC) is high value of Base current (IB)

Output characteristics of a transistor in common emitter configuration are divided into

three regions

1. Active region 2. Cut off region 3. Saturation region

Active region :

Active region lies above IB =0 , In this region collector junction is reverse biased and

emitter junction is forward biased. For a given value of IB, collector current increases

as VCE increases. A transistor is operated in active region if it is used as an amplifier.

Cut off region:

Cut off region lies below IB = 0. The collector current has finite value under this

condition. In order to cut off the transistor, the emitter junction has to be made

slightly reverse biased in addition to IB = 0.

Saturation region :

Saturation region lies close to zero voltage axis where all the curves coincide. In this

region collector current is independent of base current.


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Output dynamic resistance:

It is defined as the ratio of small change in VCE to the small change in collector current

at constant base current (IB).

18) Explain how transistor can be used as a switch?

A) Transistor as a switch: A transistor can be used as a switch if it is operated in cut

off and saturation regions.

Working: Applying Kirchoffs second rule to input and output circuits separately

VBB=IBRB+VBE and VCE=VCC-ICRC

The voltage VBB can be regarded as the d.c input voltage Vi and VCE as the d.c output

voltage Vo so we can write

Vi=IBRB+VBE (1)

V0=VCC-ICRC (2)

1) Cut off region: When Vi is increases from zero to low value less than 0.6 V, it is

unable to forward bias the input of a transistor. i.e., IB=0 and hence IC=0. The

transistor is said to be off state from equation (2) V0=VCC (high).

2) Saturation region: When Vi is very high i.e., the emitter-base junction is heavily

forward biased a large collector current IC flows which produces a large potential drop

across load resistance RC that the emitter-collector junction also get forward biased

from equation (2) output voltage decrease almost to zero. Now the transistor is said to

switch on.

It should be kept in view that transistor switching circuit is so designed that It never

remains in active region.

19) Describe with a circuit diagram the working of an amplifier using an npn

transistor in CE configuration. Draw relevant waveforms and obtain an

expression for the voltage gain.

A) The transistor of CE amplifier employing npn transistor is as shown. Here C1 and

C2 are coupling capacitors which block DC and allow only AC.


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The transistor operating point is fixes on the middle of the active region. This fixes DC

base current IB and the corresponding collector current Ic while DC voltage VCE would

remain constant. The operating values VCE and IB determine the operating point of the

amplifier.

A small sinusoidal voltage of amplitude Vi is superposed on the DC base bias. The base

current will have sinusoidal variations superimposed on IB. The collector current also

will have AC variations superimposed Ic. This produces corresponding change in the

value of output voltage V0.

During the positive half cycle of the input AC signal the emitter-base voltage increases.

As a result the input current IB and hence the output current Ic also increases.

Consequently the voltage drop across RL increases. The output voltage V0 taken across

collector and emitter becomes less positive (or more negative) i.e., the amplified

output signal goes through a negative half cycle.

Similarly during negative half cycle of the input AC, input voltage decreases, IB and IC

decreases. As a result voltage across RL also decreases. But the output voltage V0

goes through a positive half cycle. Thus the output voltage V0=VCE is out of phase by

1800 with the input voltage Vi.

In the absence of input AC signal Vi, applying Kirchoffs voltage law to the input loop.

VBB=VBE+IBRB.

When the signal Vi is superimposed, VBB+Vi=VBE+IBRB+(RB+ri)

Vi= IB(RB+ri)

Vi= IBr . (1)

Where ri is input resistance and r=(RB+ri)

Applying Kirchoffs voltage law to the output part, Vcc=VCE+ICRC

The change in IB causes change in Ic which in turn causes change in VCE. Voltage drop

cross Rc also changed, since Vcc is fixes.


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VCC=0

O= VCE+RC IC

VCE=-RC IC

This is the output voltage Vo, which is taken between collector and the ground.

Vo= RC IC . (2)

The voltage gain of the amplifier, Av=Vo/Vi=-(RCIC/rIB)=-(Ic/IB)Rc/r

Av=-ac(Rc/r)

Where r=RB+ri

The negative sign indicates that the output voltage is out of phase input voltage.

20) Explain how transistor can be used as an oscillator?

A) Oscillator: An electronic device which produces electrical oscillations of constant

frequency without requiring any external input signal is called oscillator.

Transistor is an oscillator circuit:

Working: 1) When key k is closed, collector current starts growing through L. Since L

is inductively coupled to L, increasing collector current through L induces voltage

across L in such a way that base-emitter junction becomes forward biased.

2) This causes increase in collector current at a faster rate and induced voltage

increases further across L.

3) As a result capacitor gets charged.

4) When transistor reaches to saturation state, the collector current increases at lesser

rate and thus decreases induced voltage across L.

5) Now the capacitor starts discharging making the base of the transistor negative.

6) Discharging of capacitor drives the transistor in cut off so that collector current

becomes zero.

7) This process repeated again and again produced, sustained oscillations in the

output.


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Basic Logic Gates A logic gate is an physical device implementing a Boolean function, that is, it performs

a logical operation on one or more logic inputs and produces a single logic output.

Gates also called logic circuits.

Or

A gate is simply an electronic circuit which operates on one or more signals to produce

an output signal.

Truth table:

A table which gives the output states for all possible inputs.

NOT gate (inverter): The output Q is true when the input A is NOT true, the output is

the inverse of the input: Q = NOT A . A NOT gate can only have one input. A NOT gate

is also called an inverter.

AND gate

The output Q is true if input A AND input B are both true: Q = A AND B An AND gate

can have two or more inputs, its output is true if all inputs are true.

OR gate The output Q is true if input A OR input B is true (or both of them are true): Q = A OR B An OR gate can have two or more inputs, its output is true if at least one input is true. NAND gate (NAND = Not AND)

This is an AND gate with the output inverted, as shown by the 'o' on the output. The

output is true if input A AND input B are NOT both true: Q = NOT (A AND B) A NAND

gate can have two or more inputs, its output is true if NOT all inputs are true.

NOR gate (NOR = Not OR)


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This is an OR gate with the output inverted, as shown by the 'o' on the output. The

output Q is true if NOT inputs A OR B are true: Q = NOT (A OR B) A NOR gate can

have two or more inputs, its output is true if no inputs are true.

Write a short note on Integrated circuits?

A) Integrated circuits is a collection of interconnected transistors, diodes (active

devices), resistors and capacitors (passive components) fabricated onto a single piece

of silicon, known as chip.

Integrated circuit is assembly large number of transistors , capacitors and resistors

are joined together on a single piece of silicon which may be very small in size.

Integrated circuits are two types

1. Monolithic

2. Hybrid

Monolithic integrated circuits:

It consists of a silicon wafer called a chip on which a large number of components

(transistors , diodes , resistors , capacitors and their interconnections)

Hybrid integrated circuits:

It consists of large number of monolithic integrated circuits (IC) . In this circuit , the

components that is transistors , diodes , capacitors , resistors etc are mounted on

ceramic substrate and they are interconnected by wires.

Uses :

IC technology is widely used in televisions , computers , amplifiers, radios , video

recorders , telecommunication components etc.,

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Semi Conductor Electronics