Basic Elex Notes

8/16/2019 Basic Elex Notes

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By: Mr. Navneet Pal Email: [email protected] Page 1

Valance Electrons- The electrons in the outermost shell of an atom are called Valence electrons. Energy Levels- Electrons revolve around the nucleolus in different shells. These electrons are bound tothe nucleus with some specific energy. Each electron shell exhibits a particular energy value called thediscrete energy level.The discrete energy levels depend on the following parameters

Momentum of electrons in the orbits. Distance of the orbit from the nucleus.

The energy levels of various shells are calculated using a simplified general formula:

En = - eV

eV (Electron volt) - It is the energy gained by an electron when it passes through a potential difference ofone volt. 1eV= 1.6 x 10 -19 Joule.

S.No. Shell N Energy Level

1 K-Shell 1 -13.6 eV

2 L-Shell 2 -3.41 eV

3 M-Shell 3 -1.51 eV

4 N-Shell 4 -0.87 eV

5 O-Shell 5 -0.56 eVThe Negative sign in equation indicates that the electrons are bounded to the nucleus with an

attractive force. The energy gap between two consecutive shell is called the forbidden gap : an electron inan isolated atom cannot have an energy in the gap.

Energy Band- The energy bands in solid can be as a set of energy levels closely placed such that theenergy band are considered to continuous ranges of permissible electron energy.

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Valence Band- The electrons in the outermost shell are called valence electrons, Consequently, Theenergy band associated with the outermost shell is known as the valence band.

The valence electrons are loosely bonded with the nucleus: therefore, a small amount ofexternal energy is sufficient to free them from their atom.Conduction Band- When the valence electrons are freed from the atom, these free electrons areresponsible for the conduction of current in solids and are therefore called conduction electrons / freeelectrons. The bond occupied by these electrons is called the conduction band .

Therefore, the loosely bound valence electrons jump from the valence band to the conduction bandwhen sufficient external energy is supplied to the atoms.

The gap between valence band and conduction band is called the forbidden energy gap(Eg).

Eg > 5 eV Eg < 3 eV Eg = 0 eV Examples Eg for Glass= 10 eV Eg for silicon = 0.72 eV All Metals

Eg for Diamond = 6 eV Eg for Germanium = 1.1 eVEg for GaAs = 1.41 eV

Semiconductors

A semiconductor is a material which has electrical conductivity between that of a conductor such as copper and that of an insulator such as glass.

The conductivity of a semiconductor material increases with increasing temperature. All the elements of IV group of periodic table are semiconductors (C,Si,Ge) etc. The energy band gap between valence band and conduction band is less than 3 eV There are two types of semiconductor material

Intrinsic or Pure semiconductors.Extrinsic or Doped Semiconductor.


http://en.wikipedia.org/wiki/Electrical_conductivityhttp://en.wikipedia.org/wiki/Electrical_conductivityhttp://en.wikipedia.org/wiki/Electrical_conductivityhttp://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Insulator_%28electrical%29http://en.wikipedia.org/wiki/Insulator_%28electrical%29http://en.wikipedia.org/wiki/Insulator_%28electrical%29http://en.wikipedia.org/wiki/Insulator_%28electrical%29http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Electrical_conductivity


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Intrinsic Semiconductors -An intrinsic semiconductor material is chemically very pure and possesses poor conductivity. It has equal numbers of negative carriers (electrons) and positive carriers (holes).

Fig: Structure of intrinsic semiconductor Fig:Generation of electron-hole pair in an atabsolute zero temperature Intrinsic semiconductor

Extrinsic Semiconductor- The conductivity of intrinsic semi-conductor can be increased by adding somesuitable impurity atoms to the semiconductor. When a small amount of impurity atom is added to theintrinsic semiconductor, it is called extrinsic or impurity semi-conductor and the process of addingimpurities to the semiconductor is known as doping.

P-Type- When a trivalent impurity atom is added to an intrinsic semiconductor, it is called P-typesemiconductor. trivalent impurities are Boron, Aluminum, Gallium and Indium. They are known asacceptor impurities because the holes created can accept the electrons.

P-Type Semiconductor: - a) Boron added to Silicon, b) Creation of a hole.

N-Type- When a pentavalent impurity atom is added toan intrinsic semiconductor, it is called N-typesemiconductor. pentavalent atoms are phosphorus,arsenic and antimony. They are called donor impurities

because they donate one valence electron to thesemiconductor crystal.

N-Type Semiconductor



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N-Type P-Type

Doping Pentavalent Atoms(P, As,Sb,Bi) Trivalent Atoms (B,Al,Ga,In)MajorityCarriers

Electrons Holes

MinorityCarriers

Holes Electrons

Ions Positive / Donor Ions Negative / Acceptor Ions

P-N Junction Diode

Made by joining P-Type and N-Type Semiconductors Two terminal device Unidirectional Device Non Linear Device Basic Operation- as a SWITCH in electronic devices Applications: Rectifier, Zener Diode, Clipper , Clamper etc

Unbiased P-N Junction Diode- (V= 0 V) A P-NJunction formed by injecting P-Type and N-Typeimpurity atoms into the two halves of a single intrinsicsemiconductor crystal is called unbiased P-N-Junction .The concentrations of the electrons and holes aredifferent across the junction. This is known asconcentration gradient ; due to the concentrationgradient across the junction, some changes start taking

place as the P-N junction is formed.

The holes from the P-region cross the junction & the N-region where there is a scarcity of holes.This is because the P-N junction tries to have an equal number of holes on both sides of the junction.This process is called diffusion.

As soon as a hole from the P-region crosses the junction into the N-region, it confronts a freeelectron generated by a donor ion. This hole recombines with the free electron, and the free electronvanishes. The corresponding donor ion is now left without any electron associated with it. This donor ionis called an uncovered charge. In this manner a large number of uncovered positive charges are producednear the junction in the N-region.

The electrons from the N-region diffuse into the P-region due to the concentration gradient across the junction.These electrons are recombined with the free holesassociated with immobile acceptor ions near the junction,uncovered negative charges are produced in the P-regionclose to the junction.

The uncovered negative charges create a negativeelectric field in the P-region & the uncovered positivecharges create a positive electric field in the N-region .

Now, if positively charged holes try to cross the junctioninto the N-region, the positive field repels them back intothe P-region. Similarly the electrons are prevented fromcrossing the junction from N-region to the P-region, due toa repelling force exerted from the uncovered negativecharges in the P-region.

UNBIASED PN JUNCTION



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After some time, the transportation of mobile charges, electrons & holes stops due to a negativeelectric field in the P-region and a positive electric field in the N-region.

Depletion Region- The region of uncovered positive and negative ions is called the depletion region dueto the “ depletion” of free carriers in the region.“ In the absence of an applied bias across a semiconductordiode, the net flow of charge in one direction is zero.”

VB = 0.3V for GeVB = 0.7V for Si

Reverse Bias Condition (V< 0V) – when the negativeterminal of the DC source is connected to the P-side & thepositive terminal of the DC source is connected to N-sideof the P-N junction. The P-N junction is known as areverse biased junction .

1- The Current does not flow because of majority carrier.2- Due to an increase in positive & negative ions around the junction, the electric field around the

junction is increases, and as a result, the height of the potential barrier is increased. 3- The current that exist under reverse bias condition is called the reverse saturation current & is

represented by I s. (Because of minority carriers only )

Forward Bias Junction (V>0V)- If the positive terminal ofthe voltage source is connected to the P-side & thenegative terminal of the source is connected to the N-sideof the diode, then the diode is said to be biased in theforward direction or forward biased .

1- The current Flows because of majority carriers only.

2-

Depletion layer decreases with the increase of voltage because effect of external electric fieldis higher than internal potential barrier.

Forward Bias Reverse Bias1 P-Type Positive Terminal

N- Type Negative Terminal

P-Type Negative Terminal

N- Type Positive Terminal2 Current Flows because of majority

carriers.Current Flows because of Minority carriers.

Reverse Saturation Current

3 Depletion Layer decreases with increasingvoltage.

Depletion Layer increases with increasingvoltage.

4 Less voltage required High Voltage required

V-I Characteristics of P-N Junction DiodeV-I characteristics of P-N junction diode shows the graphical relationship between voltage and flowingcurrent through diode



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Forward Bias- When a forward voltage is appliedat the terminals of a diode, the diode begins toconduct. During conduction, the cut in orthreshold voltage exceeds the applied forwardvoltage. The threshold voltage for a germanium

diode is 0.3V and for silicon diode is 0.7V. Theforward current (miliampere range) initiallyincreases linearly and then increasesexponentially for high currents.

Reverse Bias- When a a reverse voltage is applied,a reverse saturation current flows through thediode. The diode continues to be in the nonconducting state until the reverse voltage dropsbelow the zener voltage. As the reverse voltageapproximates the peak inverse voltage abreakdown called as the ’Avalanche breakdown ’occurs. During the breakdown, the minoritycharge carriers ionize the stable atoms which arefollowed by a chain ionization to generate a largenumber of free charge carriers. Thus the diodebecomes short circuited and gets damaged.

Temperature Effect on V-I Characteristics

As the temperature increases, the electron pairs generated thermally also increases thereby increasingthe conductivity in both directions. The reverse saturation current also increases with the increase in

temperature. The change is 11% per °C for a germanium diode and 8% per °C for a silicon diode. On theother hand the diode current is doubled for every 10°C rise. With increase in voltage, the firing voltagein forward characteristics is reduced while peak reverse voltage is increased.



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Diode Equation

The current that flows through a diode is given by the equation:

where I D - diode current. (Positive for forward and negative for reverse)IS - constant reverse saturation currentV - Applied voltage. (Positive for forward and negative for reverse)

- Factor dependent upon the nature of semiconductor.(1 for germanium and 2 for silicon)

VT - volt equivalent of temperature which is given by T/11600. (T isTemperature in Kelvin)

Diode CapacitanceStorage or Diffusion Capacitance- This capacitance originates due to diffusion of charge carriers in theopposite regions. The capacitance which exist in a forward-biased junction is called a diffusion orstorage capacitance. It is called diffusion capacitance to account for the time delay in moving chargesacross the junction by diffusion process.

CD = Where C D = Diffusion capacitance

η = Constant *1 for Si and 2 for Ge+ VT = volt equivalent of temperatureτ = mean life time of carrierIF = Forward current

Hence CD is proportional to I D .

Depletion or Transition Capacitance - The capacitance which appears between positive ion layer in n-region and negative ion layer in p-region. The transition capacitance is very small as compared to thediffusion capacitance.

When a PN junction is formed, there exists a depletion region at the junction. thisdepletion region or layer consist of positive and negative immobile ions. This depletion layer is nonconductive and hence acts as a dielectric medium between P-region and N-region.In reverse bias transition, the capacitance is the dominant and is given by:

where C T - transition capacitanceA - diode cross sectional area

W - depletion region width

Since the depletion layer width (d) increases withincrease in reverse bias voltage, the depletioncapacitance should decrease with the increase inreverse bias.

Diode capacitance is given by

CT = where K is constant Diode Capacitance Curve



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Diode ResistanceStatic Resistance or DC resistance- Static Resistance is basically the DC resistance offered by a pn junction diode and originates when it is connected in a DC circuit. It is the resistance offered by thediode to the flow of DC current.

Mathematically, it is given as the ratio of the DC voltage across the terminals of

the diode to the DC current flowing through it.Rf = =

Dynamic Resistance or AC resistance- Dynamic Resistance is the AC resistance offered by a pn junctiondiode and originates when it is connected in an AC circuit. It is the resistance offered by the diode to theflow of AC current.

Mathematically, it given as the ratio of the change in AC voltage across theterminals of the diode to the resulting change in AC current flowing through it.

Rf = =

Breakdown Mechanism

If the reverse-bias applied to a P-N junction is increased; a point will reach when the junction breaks

down and reverse current rises sharply to a value limited only by the external resistance connected inseries. This specific value of the reverse bias voltage breakdown voltage (V Z). After breakdown voltagedepends upon the width of depletion layer. The width of depletion layer depends upon the dopinglevel.The following two processes cause junction breakdown due to the increase in reverse bias voltage.

1- Zener Breakdown-

Occurred when a heavily doped junction is reverse biased.This is observed at V< 6 V.Field Ionization (E= 3 x 10 6 V/cm) takes place in this mechanism.Tunneling of electrons [ the valence electrons are pulled into conduction band]V-I characteristics with zener breakdown is very sharp.

Negative temperature coefficient. (When T increases V Z decreases)

Tunneling Process- Due to intense electric field (E= 3 x 10 6 V/cm), the valence electrons are pulled intoconduction band by breaking covalent bonds. These electrons become free electrons which are availablefor conduction. A large no. of such free electrons will constitute a large reverse current through the zenerdiode and breakdown is said to have occurred due to the zener effect. A current limiting resistance should

be connected in series with the zener diode to protect it against the damage due to excessive heating.



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2- Avalanche Breakdown-

Occurred when a lightly doped junction is reverse biased.Multiplication of electrons takes place in avalanche breakdown.This is observed at V> 8 V

Impact Ionization.This V-I characteristics with the avalanche breakdown increases gradually.Positive temperature coefficient of voltage. (When T increases V Z increases).

Impact Ionization - In reverse biased condition, the conduction will take place only due to minoritycarriers. As we increase the reverse voltage applied to the zener diode, these minority carriers tends toaccelerate. Therefore, the kinetic energy ( ) associated with them increases. While travelling, theseaccelerated minority carriers will collide with the stationary atoms and impart some of the kinetic energyto the valence electrons present energy these valence electrons will break their covalent bonds and jumpinto the conduction bond to become free for conduction. Now these newly generated free electrons getaccelerated. They will knock out some more valence electrons by means of collision. This phenomenon iscalled as carrier multiplication. In a very short time, a large number of free minority electrons and holeswill be available for conduction and carrier multiplication process become self sustained. This selfsustained multiplication is called “ avalanche effect” .a large reverse current starts flowing through thezener diode and the avalanche breakdown is said to have occurred.

Zener Diode

Zener diode is a reverse biased heavily doped P-N junction diode which operates in the breakdownregion. The reverse breakdown of a P-N junction may occur either due to zener effect or avalanche effect.

Conduction direction of zener diode - For the zener diode thedirection of conduction is opposite to that of the arrow in thesymbol. Zener diode equivalent circuit-

Complete Approximate

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The complete equivalent circuit of the zener diode in the zener region includes a small dynamicresistance and dc battery equal to the zener potential. Generally, the zener resistance value is quite smallthan external resistance of the circuit in which zener diode is connected. Therefore zener resistance may

be neglected generally.

On State (V>V Z)

If the reverse bias voltage across a zener diode is equal to or more than breakdownvoltage V Z, the current will increase sharply. Hence, in this region the current will be almost vertical.

V>V Z Equivalent zener diode at „ON‟ state

Off State (0


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Voltage Multiplier CircuitsA voltage multiplier is that of circuit which produces an output d.c. voltage whose value is multiple of

peak a.c. input voltage (i.e 2V m , 3 Vm , 4Vm….). a voltage multiplier circuit is a combination of two or morepeak rectifier circuits. Each peak rectifier contains a diode and a capactor.Voltage Doubler- A voltage multiplier circuit, whose output d.c. voltage is double of the peak a.c. input

voltage, is known as voltage doubler.(i) Half –Wave voltage doubler(ii) Full- Wave voltage doubler

(i) Half –Wave voltage doubler- During the positive half cycle of the input signal the diode D 1 conducts (and diode D 2 is cut-off), charging the capacitor C 1 upto the peak rectified voltage(Vm). During negative half cycle, the diode D 1 is cut-off and diode D 2 conducts chargingcapacitor C 2

-Vm - VC1 + VC2 = 0-Vm - Vm + VC2 = 0 [VC1 = Vm]

VC2 = 2 VmOn the next positive half cycle, diode D 2 is non conducting and capacitor C 2 will dischargethrough the load. If no load is connected across capacitor C 2 , both capacitors stay charged staycharged - C 1 to Vm and C 2 to 2 V m .

(ii) Full- Wave voltage doubler - During the positive half cycle of a.c. input voltage, the D 1 conducts charging capacitor C 1 to a peak voltage V m , the diode D 2 is cut-off at this time.

During the negative half-cycle, the diode D 2 conducts (while D 1 is at cut-off)charging capacitor C 2 to Vm. If there is no load is connected across the output then theoutput voltage is equal to 2V m . However, if the load is connected then the voltage will beless than 2V m . the peak inverse voltage ( PIV) across each diode , in a full wave voltagedoubler is equal to 2V m.

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Rectifier- A rectifier is a circuit which is used to convert A.C. voltage into the pulsating D.C. voltage.1- Half-wave rectifier(HWR)2- Full- wave rectifier (FWR)

1- Half-wave Rectifier-

During the interval t=0 T/2 - The Diode is in state of forward bias, Diode will behave as a shortcircuit.

Vo = Vi During the interval t=T/2 T - The diode is in state of reverse bias, diode will behave as open circuit.

Vo = 0

2- Full Wave Rectification – Full wave rectifier is that type of rectifier which utilizes both the halfcycle of a.c. input voltage.

(i) Centre-tap full wave rectifier(ii) Full wave Bridge rectifier

(i) Centre-tap full wave rectifier - it contains two diodes with a centre- tapped transformer toestablish the input signal across each section of the secondary of the transformer.

During positive portion of V i applied to the primary of the transformer, D 1 is short circuit & D 2 is opencircuit.During the negative portion of the input , diode D 2 is forward bias & D 1 is reverse bias.



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(ii) Bridge Rectifier- The dc level obtained from a sinusoidal input can be improved 100% using

a process called full wave rectification.

In +ve half cycle D 2 & D3 diodes are conducting while D 1 & D4 are in “off” state.

In -ve half cycle D 1 & D4 diodes are conducting while D 2 & D3 are in “off” state.

S.No. Parameter Half WaveFull Wave

Centre-Tap Bridge

1 No. of diodes 1 2 4

2 TransformerNecessary No YES NO

3 Efficiency 40.6 % 81.2 % 81.2 %

4 Ripple Factor 1.21 0.482 0.482

5 Peak Inverse Voltage Vm 2Vm Vm

6 Output Frequency f i 2 f i 2 f i

7 RMS Current Im /2 Im / Im /

8 DC Current Im/π 2Im/π 2Im/π

Ideal Diode- : VON= 0, Rr=∞and Rf= 0. In other words, the ideal diode is a short in the forward biasregion and an open in the reverse bias region.

Practical diode(silicon): VON= 0.7V,Rr< ∞(typically several MΩ), Rf≈rd(typically < 50Ω).



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Diode Equivalent Models

Clipper

A circuit which cutoff voltage above or below are both specified level is called clipper. A clipper whichremoves a portion of positive half cycle of the input signal is called positive clipper. A clipper circuit thatremoves the negative half cycle is called negative clipper.



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Figure (a) shows the circuit of a positive clipper. Itconsists of a diode D and a resistor R with outputtaken across the resistor. During positive half cycle the input voltage, the terminal A is positive withrespect to B. This reverse biases the diode and it acts as an open switch. Therefore all the appliedvoltage drops across the diode and none across the resistor. As a result of this, there is no outputvoltage during the positive half cycle of the input voltage.

During the negative half cycle of the input voltage, the terminal B is positive with respect to A. Thereforeit forward biases the diode and it acts as a closed switch. Thus, there is no voltage drop across diode.During negative half cycle of the input voltage. All the input voltage is drop across the resistor as shownin the output waveform.Figure (b) shows the waveform of the input voltage. During the positive half cycle of the voltage, theterminal A is positive with respect to the terminal B. Therefore the diode is forward biased; as a result allthe input voltage appears across the resistor. During negative half cycle of the input voltage, theterminal B is positive with respect to the terminal A. Therefore the diode is reverse biased and hencethere is no voltage drop across the resistor during negative half cycle.

Clamping

A clamper is a network constructed of a diode, a resistor and a capacitor that shifts a waveform to adifferent dc level without changing the appearance of the applied signal.

Analysis- During reverse biased, the diode is open circuited (i.e“off”state). The voltage will be Vo=0since the current is shorted through diode. The voltage across R will be V DC+ VC= -V+(-V)=-2V .



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Example Draw the output waveform for the following circuit.

Solution:

Step-1- Start the analysis when diode is in forward bias andfind V C and V O .In interval t 1 to t 2 , diode is in forward bias

At 2nd interval (t1

t2), the diode is short circuited, the

voltage across R will be the same as across the battery(parallel) V o= 5VThe voltage that charge up the capacitor, Applying KVL

-20V +Vc-5V =0 , then VC=25V

Step-2

The third interval will make the diode open circuited againand current start to flow in the resistor (discharged thecapacitor). Applying the KVL

+10V +25V –V o=0Vo= 35V



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Diode Ratings

1- Average Current:- it is defined as average value of periodic function given by area under onecycle of the function divided by the base.

2- Maximum Forward Current:- The maximum value of diode forward current, which a PN junction diode can carry without damaging itself, is known as maximum forward current .

3- Peak Inverse Voltage(PIV):- The maximum value of reverse bias that a PN junction canwithstand without damaging it is called its Peak Inverse Voltage (PIV).

4- Maximum Power Rating:- The maximum power that a P-N junction can dissipate withoutdamaging . it is called as maximum power rating .

5- Reverse Saturation Current:- The amount of current through the diode in reverse-bias operation, with the maximum rated inverse voltage applied (V DC). Sometimes referred to asleakage current .

Applications of Diode:

1- Signal rectifier 5- As a Clipper 9- Freewheeling Diodes2- Diode gate 6- As a Clamper 10- Precision rectifier using Op-Amp3- Diode clamps 7- AM Detection4- Limiter 8- Voltage Multiplier



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B y : N a v n e e t P a l E m a i l : e r . n a v n e e t p a l @ g m a i l . c o m Page 1

Light Emitting Diode is a photo electronic device which converts electrical energy to light energy.It is heavily doped P-N junction diode which under forward biased emits spontaneous

radiation. The diode is covered with a transparent cover so that the emitted light may come out.Construction- In an LED, the upper layer of p-type semiconductor is deposited by diffusion on n-typelayer of semiconductor. The metalized contacts are provided for applying the forward bias to the P-N

junction diode from battery B through resistance R which controls the brightness of light emitted.

Fig: LED construction Working of LED

The LED works on the principle of electroluminescence.When a P-N junction is forward biases, the electrons in N- region cross the junction &recombines with holes in P-region.When electrons recombine with the holes, they move from conduction band to valence bandwhich is at lower energy level.While moving, the additional energy is released by the free electrons which appear in the formof light due to the special material used in the LED.The energy released depends on the forbidden gap energy which determines the wavelength &the colour of the emitted light.

Materials of LEDThe materials which are the mixtures of Gallium, Arsenic & Phosphorus are used in LED to obtaindifferent coliour of light.S.No. Material Symbol Colour

1 Gallium Arsenide GaAs Infrared(Invisible)2 Gallium Phosphide GaP Red or Green3 Gallium Arsenide Phosphide GaAsP Red or Yellow

1- Light Emitting Diode (LED)

Content: - Special Purpose two terminal Devices: Light-Emitting Diodes, Varactor (Varicap)Diodes, Tunnel Diodes,Liquid-Crystal Displays.



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Output Characteristics- The amount of power output translated into light is directly propotional tothe forward current I f , more forward current I f , the greater the output light.

Fig: Typical output characteristics of LED

Advantages of LEDs Applications of LEDs LED has less power & low operational voltage. In Burglar- Alarm System.

LED has fast action & requires no warm uptime.

In calculators & Digital watches.

LED is cheap & easy to handle In the field of communication.The LEDs are light in weight. In computers & remote control.The LEDs are available in various colours. In picture phones & video displays.The LEDs have long life. In traffic light.

Varactor Diode can be used as variable capacitors in microwave circuits. In reverse bias , the P-region & N-region act like the plates of capacitor while the depletion

region acts like dielectric. Thus, there exists a capacitance, space charge capacitance ordepletion region capacitance.

Mathematically ε = permittivity of semiconductorA= area of cross sectionW= width of depletion region.

As the reverse biased applied to the diode increases, the width of the depletion region (W) increases.Thus the transition capacitance C T decreases.

In short, the capacitance can be controlled by the applied voltage.

Fig: Variation of C T according to V R

CT=

2- Varactor Diode (VARICAP)

LED Symbol



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Expression for varactor diode- The diode capacitance (C T) as a function of applied reverse-bias potential described as.

Where, C T= diode capacitance C= diode capacitance at unbiased condition V= applied potentialVb=barrier potential m= constant that is material dependent K= a constant (often 1)Equivalent Circuit

Fig: Equivalent Circuit of tunnel diode

The series resistance & diode capacitance determine the maximum operating frequency according tothe relationship.

Where K= cut-ff freq in hertzRs= series resistance in Ώ

CT= diode capacitance in faradsApplications of Varactor Diode

A varactor diode is used as an automatic control device in a receiver.Varactor diodes are also used as frequency modulator.

Varactor diodes can also be used as frequency multiplier.It can be used as phase shifter.

A tunnel diode is a semiconductor P-N junction device which is extremely heavy doping on both the sideof junction & abrupt transition from p-side to n-side .i.e. the depletion width is very small. Hence thecarriers are tunnel through the potential barriers, even if they do not have sufficient kinetic energy tocross it.

F=

3- Tunnel Diode

Reverse Biased P-N Junction



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Due to the depletion region, an electron penetrates through the barrier. This is called tunneling &hence such high impurity density P-N junction devices are called tunnel diodes.

Characteristics of tunnel diodeCharacteristics shows that at first forward current rises sharply as voltage is applied up to point P. Thevoltage associated with this point P is called as peak voltage V P & current associated with this point is

called as peak current I P.As the forward bias is increased further, the forward current drops continuously up to point V. Thevoltage & current associated with this point is called as valley voltage(V V) & Valley Current (I V)respectively. If voltage is increased further, the current through diode increases at shown in figure.When tunnel diode is reverse biased. It acts as a good conductor.

Fig: Tunnel Diode Characteristics

Tunnel Diode Construction- Semiconductor material used in tunnel diode is germanium (Ge), galliumantimonide (GaSb) or Gallium arsenide (GaAs).

Fig: Tunnel Diode

A very small tin dot is soldered to a heavily doped pellet of N-type Ge, GaSb or GaAs. The pellet issoldered on Kovar anode contact, The cathode contact is also Kovar, which is connected to the tin dotvia a mesh screen to eliminate inductance. The diode has a ceramic body & is hermetically sealed.



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Tunnel Diode Equivalent Circuit

WhereRs= Series resistance. Is a combination of ohmic contacts & spreading resistance in wafer.Ls= Series inductance being a function of conduction of diode & surrounding circuit geometry.Cj= Junction capacitance corresponding to space charge depletion width & varies with applied bias.Rj= Non- linear differential resistance of tunnel diode.

Application of Tunnel DiodeAs a high speed switchIn negative resistance & high frequency (microwave) oscillator.In pulse & digital circuits. Symbol In switching network.In timing & computer logic circuitry.Design of pulse generators & amplifiers.

The liquid crystals are one of the most fascinating material systems in nature, having propertiesof liquids as well as of a solid crystal. The term liquid crystal refers to the fact that there

compounds have a crystalline arrangement of molecules, yet they flow like a liquid. Liquid crystal display do not emit or generate light, but rather alter extremely generated

illumination. Their ability to modulate light when electrical signal is applied has made them very

useful is flat panel display technology. The crystal is made up of organic molecules which are rod-like molecule defines the “director”

of the liquid crystal. The different arrangements of these rod like molecules leads to three main categories of liquid

crystals

1- Smectic 2- Nematic 3- Cholesteric

Smetic- In this structure the rod-like molecules are arranged in layers & within each layer there isorientational order over a long range.Nematic- In this structure, the potential order between layer of molecules is lost, but the orientationorder is maintained.Cholesteric - In these crystals the rod-like molecules in each layer are oriented at a different anglewithin each layer.

4- Liquid crstal display(LCD)



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Types of LEDs

1- Dynamic scattering 2- Field effect1-Dynamic Scattering - It consists of two glass plates with a liquid crystal fluid in between. The back

plate is coated with thin transparent layer of conductive material, when as front plate has a photoetched conductive coating with seven segment pattern.

Fig: Dynamic scattering Type LCD

In the absence of the electrical signal , orientation order is maintained in the crystal allowing light to

transmit. This makes LCD display clear.

2-Field Effect Display-In these nematic liquid crystals are used.It consists of two glass plates, a liquid crystal fluid, polarizers & transparent conductors.The liquid crystal fluids sandwiched between two glass plates.Each glass plate is associated with light polarizer; the light polarizers are placed at right angle toeach other.

Advantages of LCDs Disadvantages of LCDs Less power consumption. Poor reliability.Low cost. Limited temperature range.Uniform brightness with good contrast. Poor visibility in low ambient temperature.Low operating voltage & current. Slow speed.

Requires an A.C. drive.



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Field Effect Transistor (FET) is a voltage controlled device . i.e. the output characteristics ofthis device are controlled by the input voltage and not by the input current.

FET is unipolar device. i.e. the operation of FET depends upon the flow ofmajority carriers only. For the FET, an electric field is established by the charges present thatwill control the conduction path of the output circuit without the need for direct contactbetween the controlling and controlled quantities.

JFET (Junction Field Effect Transistor)- JFET is a three terminal device with one terminal capable of controllingthe current between the other two.There are two types of JFET

1- N-channel JFET 2- P-Channel JFETN-channel JFET- It consists of an N-type semiconductor substrate with P-typeheavily doped regions diffused on opposite sides of its middle part. The spacebetween the junctions is called a channel. The top of the N-type channel isconnected through an ohmic contact to a terminal referred to as the Drain(D),while lower end of the same material is connected through an ohmic contactto a terminal referred to as the source(S). The Two P-type materials areconnected together and to the gate (G) terminal. In the absence of any appliedpotentials the JFET has two P-N junctions under No-Bias Conditions.

JFET Volt-Ampere (V-I) Characteristics

There are two types of JFET Characteristics1- Drain Characteristics 2- Transfer Characteristics

Drain Characteristics-Case-1 - When VGS= 0 V and VDS= 0 V.In this condition the drain current is zero. Fig: N-Channel Case-2 - When VGS =0 V and VDS > 0 V. i.e. some positive value.

The gate and source are connected directly i.e. V GS = 0 V. In this situation I D= IS (i.e. VDS= VDD).Under this condition the flow of charge is relatively uninhabited and limited only by resistance of the N-Channelbetween drain and source.Depletion region is wider near the drain side as compared to source side because when current lows in siliconbar, there occurs a voltage drop across the channel along whole length due to this bias voltage.

Field Effect Transistor: Construction and Characteristic of JFETs. Transfer Characteristic. CS,CD,CG amplifier andanalysis of CS amplifierMOSFET (Depletion and Enhancement) Type, Transfer Characteristic,

FET(Field Effect Transistor)



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Fig: Working of N-channel Fig: Non-Uniform Distribution of Depletion Layer

Pinch-Off- If VDS is increased to a level where it appears that the two depletion regions would “touch” , thiscondition referred to as “ pinch-off “ will result. The drain to source voltage (V DS) at which the channel pinchoff occurs, is known as pinch-off voltage (V P). IDSS- IDSS is the maximum drain current for a JFET and is defined by the conditions VGS=0 V and V DS> |V P|.

Fig: Drain Characteristics when V GS = 0V

Case -3 – When VGS< 0 V and VDS>0.

Under this condition P-N Junction being reverse biased andincrease the width of the depletion layer. Now, If V GS is increased morenegatively then the situation of pinch-off occurs. At this instant, I Dmax alsodecreases because channel becomes narrower due to reverse bias.



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When VGS= -VP then I D= 0 mA. Device is turned-off.The Level of V GS that results in I D= 0mA is defined by V GS=VP withVP being a negative voltage for N-channel devices and a positivevoltage for P-Channel JFETs.

P-Channel Drain & Transfer Charcteristics

Symbols

Note:IG= 0 is the important characteristics of JFET .The drain current I D depends upon following factors-

No. of majority carriers.Length of the channel.Cross- sectional area of the channel.



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Transfer Characteristics- Following equation governs the transfer characteristics of JFETID= Drain currentIDSS= saturation (max value) of drain current at V GS= 0 VVGS= Gate to Source VoltageVP= Pinch-off voltage

Fig: Transfer Characteristics of N-channel JFET

Fig: Transfer Characteristics of P-channel JFET

Important points regarding drain characteristics of N-channel JFET.Drain current is maximum when V GS = 0 V i.e. ID = IDSS and V DS ≥ |V P|.In FET, generally avalanche breakdown occurs.VP is a particular value of V GS with either positive or negative sign dependaing upon the type of the FET.i.e. whether it is P-Channel or N-Channel.

For P-Channel V P is positive.For N-Channel V P is negative.



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For gate to source voltages V GS less than (more negative than) the pinch-off level, the drain current isOA (ID = 0A).

VGS ID 0 V IDSS

0.3 V P IDSS/2

0.5 V P IDSS/4VP 0 mA

1- Depletion Type MOSFET

N-Channel- A slab of P-Type material is formed from a silicon baseand is referred to as the substrate. The source (S) and drain (D)terminals are connected through metallic contacts to N-doped regionslinked by an N-channel. The Gate is also connected to a metal contact

surface but remains insulated from the N-Channel by a thin silicondioxide (SiO 2) layer.

This SiO 2 layer acts as a dielectric that sets upopposing electric fields within the dielectric when exposed to anexternally applied field.

“There is no direct electrical connection between the gate terminal& the channel of a MOSFET” “ It is the insulating layer of SiO 2 in the MOSFET concentration thataccounts for very desirable high input impedance of the device”.

Working

Case-1- VGS = 0 V and VDS > 0 the result is an attractionfor the positive potential at the drain by the free electrons of theN-channel and a current similar to that established through thechannel of the JFET.

Case-2- VGS = -ve and VDS > 0 In this case, electronsmove towards P-type substrate & attracts holes from P-typesubstrate.Depending on the magnitude of the negative bias established byVGS, a level of recombination between electrons and holes willoccur that will reduce the number of free electrons in the N-channel available for conduction. The resulting level of draincurrent is therefore reduced with increasing negative bias for V GS.

Case-3- VGS = +ve and VDS > 0 For +ve value of V GS , thepositive gate will draw additional electrons (free carriers ) from thep-type substrate due to the reverse leakage current andestablished new carriers through the collisions resulting betweenaccelerating particles.

MOSFET (Metal Oxide Semiconductor Field Effect Transistor)



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Fig: Drain & Transfer Characteristics of N-Channel Depletion type MOSFT

P-Channel Depletion Type MOSFET

Fig: Drain & Transfer Characteristics of P-Channel Depletion type MOSFT

Fig: P-Channel D- MOSFET

Symbol: Depletion (D) Type MOSFET

By: Anukaran KhannaBY: Anukaran Khanna UNITED GROUP OF INSTITUTIONS, ALLAHABAD


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2- Enhancement type MOSFET

N-Channel- the N-channel enhancement type MOSFET consists of alightly doped P-type substrate into which two highly doped N-regionsare diffused. The absence of a channel between the two N-dopedregion.

Case-1- V GS= 0 V If VGS is set at 0 V and a voltage applied betweenthe drain and source of the device, the absence of an N-channel willresult in a current (I D) effectively 0 A.Case-2- When both V GS > 0 V and V DS> 0 V when both V GS and V DShave been set at positive voltage greater than 0V., establishing thedrain and gate at a positive potential with respect to the source.

As the positive potential is applied between gate & source dueto the presence of SiO 2 layer which acts as a dielectric, is attracts thecharge carriers (electrons) from substrate.As VGS increases in magnitude the concentration of electrons near

the SiO 2 surface increases until eventually the induced N-type regioncan support a measurable flow between drain & source, resulting theformation of inversion layer, this inversion layer. This inversion layeris formed when a certain gate to source voltage (V GS) is applied. Thus,the minimum value of gate voltage at which inversion ofsemiconductor surface takes place is known as threshold voltage(VT).Since the channel is no-existent with V GS= 0V and “enhanced” bythe application of a positive gate to source voltage. This type ofMOSFET is called Enhancement type MOSFET.

Drain and Transfer characteristics of N-Channel Enhancementtype MOSFET.

r levels of V GS > VT the drain current is related to the appliedgate to source voltage by the following non-relationship.

ID = K (VGS- VT)2

Where V GS =applied gate to source voltageVT = Threshold VoltageID = Drain CurrentK = Constant

The values of K can be determined from the followingequation. I D(on) & VGS (on) are the values for each at aparticular part on the characteristics of the device.

K =



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Fig: Drain and Transfer characteristics of N-Channel Enhancement type MOSFET

Symbols: Enhancement Type MOSFET

Important Points for FET Biasing

IG ≈ 0 mA.

ID = IS

For JFETs and Depletion Type MOSFET

ID = K (VGS- VT)2 For Enhancement Type MOSFET



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OP-Amp (Operational Amplifier)

Introduction

1. Op-Amp is a important linear Integrated circuit. 2. It is a multi-terminal device, which internally is quite complex.3. An Op-Amp is a direct-coupled high-gain differential amplifier usually consisting of one or more

differential amplifier and usually followed by a level translator and an output stage .4. The output stage is generally a push-pull or push-pull complementary- symmetry pair .5. The Op-Amp is a versatile device that can be used to amplify DC as well as AC input signal and was

originally designed for computing mathematical function such as addition, subtraction, multiplication,integration and differentiation etc.

6. Op-Amp can be used for a variety of applications such as ac and dc amplification, active filter,comparators, regulations etc.

Basic Circuit Symbol

Single Ended Input

Inverting Input Vo = - A x Vin

Non- InvertingInput Vo = A x Vin

Unit III Operational AmplifiersIntroduction, Differential Amplifier Circuits, Op-Amp Basic, Practical Op-Amp Circuits (Inverting Amplifier,Non inverting Amplifier, Unit Follower, Summing Amplifier, Integrator, Differentiator).Differential andCommon-Mode Operation



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Double- Ended (Differential) Input

Differential Amplifier - A differential amplifier is a type of electronic amplifier that multiplies the differencebetween two inputs be same constant factor.

Block-Diagram of Op-Amp

1. Input Stage:- The input stage is the dual input balance output (DIBO) differential amplifier. This stagegenerally provides most of the voltage gain of the amplifier and also established the input resistanceto the Op-Amp.

2. Intermediate Stage: this stage is usually another differential amplifier, which is driven by the output ofthe first stage. In most of amplifier this stage is dual input unbalanced output.

3. Level Shifting Stage (LSS): The level shifting circuit is used after the intermediate stage to shift the dclevel at the output of the intermediate state downward to zero with respect to ground.

4. Output stage: this is final stage which is usually consist of a push-pull complementary amplifier. Theoutput stage increase the output voltage swing and raise the current supplying capability of the Op-Amp..

Ideal Op-Amp Characteristics

1- Infinite input resistance (Rin = ∞ Ώ)2- Infinite Open loop voltage gain (A = ∞ )3- Infinite Bandwidth4- Infinite Slew Rate5- Infinite CMRR (common mode rejection ratio)

6- Zero output resistance (Ro = O Ώ )7- Zero offset voltage8- Zero power supply rejection ratio (PSRR) 741 – Op-Amp IC PIN Diagram

BY: Anukaran Khanna UNITED GROUP OF INSTITUTIONS ALLAHABAD


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Where Vid = V 1 – V2

so, A= 21 V V Vo

→ V1 – V2 = AVo

=Vo

= 0 → V1 – V2 = 0→ V1 = V2 Given V 1 is ground so V 2 = 0 , means Inverting terminal is virtual ground.

Close Loop Voltage Gain for Inverting Amplifier

From circuit Iin = If

1

2

RV V

Iin IN

and f

O

RV V

If 2

So,

1

2

RV V IN

= f

O

RV V 2

By concept of virtual ground V 2 = 0

1 RV

IN =

f

O

RV

OV =1 R

V Rf O

Closed loop voltage gain A f =in

o

V V

=1 R

R f

Close Loop Voltage Gain Non Inverting Amplifier

Voltage at point V 2

2V = f

O

R RV R

1

1 (By voltage divider rule)

2V V O

=1

1

R

R R f …………… 1

But Vo = A (Vin- V2)If A=∞

V2 = Vin



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Putting this value in equation 1

in

O

V V

=1

1

R

R R f So, Closed Loop voltage gain A f = 1+1 R

R f

Feedback Factor β = f A

1

So, for inverting amplifier β = - f R

R1

and

For non inverting amplifier

β =

f R R

R

1

1

Unity gain amplifier/Voltage Follower

We know that closed loop voltage gain for non inverting amplifier is A f = 1+1 R

R f

Now let R f = 0 (shorting)

R1 = ∞ (opening)

So, Af = 1+0 =1

= 1

Vo =Vin

So, output voltage is equal to input voltage and both are in phase, therefore this configuration is called Op-Amp voltage follower amplifier.

Sign Changer or InverterTthis configuration is an inverting amplifier but gain of the circuit is made unity by selecting precisely equalresistors R F = R1

If RF= R1

Than, A f = -1 R

R f

Af = -1

in

O

V V



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Applications Of Operation Amplifier

1- Adder/ Summer

Due to virtual ground V1 =V2 = 0V

I1 + I2 + I3 = If

If R1 =R2 =R3 =R , then

If R=Rf , then the output voltage V o is equal to the negative sum of input voltages. i.e.

2- Subtractor

The Subtractor circuit can be implemented by usingOp-Amp differential configuration. i.e. by applying thevoltages, to be subtracted. at the both (inverting and non

inverting ) terminals simultaneously.

Vo= Va –Vb

3- Integrator-

An integrator is a circuit in which the output voltagewaveform is the integral of the input voltage waveform.

Due to virtual ground V 1 = V2 =0 & IR = IC

1

0

RVa

1

0

RVb

1

0

RVc

f

o

RV

c f

b f

a f V

R

RV

R

RV

R

RVo

321

cba f V V V

R

RVo

cba V V V Vo

C in I R

V V 2

C in I R

V 0

RV

I inC



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But I C = current through the capacitorCurrent through a capacitor may be expressed as

Thereforedt

dV C C =

RV in

But VC = V2- VO

dt V V d

C O2 = RV in

By concept of virtual ground V 2= 0

dt V d

C O = RV in

Integrating both sides of above equation with respect to t ,we get

–

VO= -

4- Differentiator

A differentiator is a circuit in which the output voltage waveform is the

differential of the input voltage waveform.

Due to vertical ground V 1= V2= 0

IC= IR

dt dV

C C =

dt dV

C C = By concept of virtual ground V 2=0

dt V dV

C in 2 =

dt dV

C in 0

=

Vo=-R Cdt

dV in

Parameters of Practical Op=Amp

dt dV

C I C C



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1- Input offset voltage - Input offset voltage is defined as the voltage that must be applied between thetwo input terminals of an OPAMP to null or zero the output.

2- Input offset Current: The input offset current I io is the differencebetween the currents into inverting and non-inverting terminals

of a balanced amplifier.

Iio = | I B1 – IB2 |

3- Input Bias Current: The input bias current I B is the average of the current entering the input terminalsof a balanced amplifier i.e.

IB = (I B1 + IB2 ) / 2

4- Differential Input Resistance: (Ri) Ri is the equivalent resistance that can be measured at either theinverting or non-inverting input terminal with the other terminal grounded.

5-

CMRR- previously described 6- Slew Rate: previously described

Transostorised Differential Amplifier

The transistorized differential amplifier basically uses the emitter biased circuits which are identical incharacteristics.The two transistors Q 1 and Q 2 have exactly matchedcharacteristics. R C1 = RC2 and R E1 = RE2 .+VCC and –VEE are also same. The differential amplifier canbe obtained by using such two emitter baised circuits. Thisis achieved by connecting emitter E 1 of Q 1 to the emitter E 2 of Q 2.Due to this, R E1 appears in parallel with R E2 and thecombination can be replaced by a single resistancedenoted as R E. The base B 1 of Q 1 connected to input 1which is V S1 while the base B 2 of Q 2 is connected to theinput 2 which is V S2. The balanced output is taken betweenthe collector C 1 of Q 1 and the collector C 2 of Q 2 . Such anamplifier is called emitter coupled differential amplifier.The two collector resistances are same hence can bedenoted by R C.

Dual Input balanced outputDifferential amplifier



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Two modes: 1-Differential mode amplifier 2- Common mode operation

Differential mode amplifier- In the differential mode,the two input signals are different from each other.Consider the two input signals which are same in

magnitude but 180 degree. Out of phase. These signals,with opposite phase can be obtained from center taptransformer.

Common mode operation - In this mode, thesignals applied to the base Q 1 and Q 2 are derivedfrom the same source. So the two signals are equal

in magnitude as well as in phase.



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ByB y : N a v n e e t P a l E m a i l : e r . n a v n e e t p a l @ g m a i l . c o m Page 1

Cathode Ray Oscilloscope is probably the most versatile tool for development of electronic circuits and systems.

CRO allows the amplitude of electrical signals, whether they are voltage, current or power to be displayedas a function of time . CRO depends on the movement of an electron beam, which is bombarded on the screen,coated with a fluorescent material, to produce a visible spot. If the electron beam is deflected on both theconventional axis. I.e. X-axis & Y-axis, atwo dimensional display is produced.“The oscilloscope is a kind of voltmeter which uses beam instead of a pointer and a kind of recorder which uses anelectron beam. ”

The heart of CRO is cathode ray tube (CRT) which makes the applied signal visible by the deflectionof a thin beam of electrons.

Fig: Cathode Ray Tube

Constructional features of CRT

1- Glass Envelope - It is conical highly evacuated glass covering & maintains vacuum inside & support thevarious electrodes. The inner walls of CRT between neck & screen are coated with conducting materialscalled aquadag . This coating is electrically connected to the accelerating anode so that electrons whichaccidentally strikes.

2- Electron Gun Assembly- Electron gun in CRT is combined name for indirectly heated cathode , a control grid ,a focusing anode & an accelerating anode . The control grid is held at negative potential w.r.t. cathodewhereas the two anodes are maintained at high positive potential w.r.t. cathode.

3- Control Grid- Control grid in the CRT is cylindrical with a small aperture in line with the cathode. Theelectrons emitted from the cathode emerge from this. The negative bias voltage applied to the grid, controlsthe beam current. The intensity (or brightness) of spot depends on the beam current. There are two anodes Focusing Anode (Focuses the beam) and Accelerating Anode (accelerates/incresesthe speed of beam)

4- Focusing Anode- The beam of electron is converged and focused on the screen by two anodes, which form anelectronic lens. The first anode ( focusing anode ) is kept highly position with respect to the cathode, widercylinder following the first. Both the cylinders have narrow apertures in lines with the electron beam.

5- Accelerating Anode- The second anode is still operated at higher positive potential (about 10,000V) and doesmost of the acceleration of the beam, and increases in velocity.

6- Deflection Plate Assembly- Electrostatic deflection of an electron beam is used in a general purpose oscilloscope. The deflecting system consists of a pair of horizontal (H 1& H2) and vertical deflecting plates (V 1& V2).

When the plates are at zero voltage, the beam is midway between them and the spot is in thecentre of the screen.

Unit- IV Digital Voltmeter: Introduction, RAMP Techniques & Digital Multimeters Oscilloscope : Introduction, Basic Principle, CRT , Block Diagram of Oscilloscope, Simple CRO,

Measurement of voltage , current phase and frequency using CRO

Cathode Ray Oscilloscope(CRO)



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When H1 is made positive with respect to cathode andall other plates are at zero voltage. It attracts he beamand the spot moves horizontally to the right.Similarly when V1 is made positive, the spot movesvertically and where V2 is made positive. It movesvertically downwards.

7- Screen- screen is coated with fluorescent material such as Zinc Orthosilicate, Zinc Oxide etc. The bombardingelectrons striking the screen, release secondary emission electrons. These electrons are collected or trappedby an aqueous solution of graphite called “ Aquadag ” which is connected to the second anode.Collectio n ofthe secondary electrons is necessary to keep the screen in a state of electrical equilibrium.

Block Diagram Of CRO

The CRO consists of the following component:(i) CRT (ii) Vertical amplifier (iii) Delay line (iv) Horizontal amplifier (v) Time-base generator(vi)Triggering circuit (vii) Power supply

(i) CRT- This is a cathode ray tube which emits electrons that strikes the phosphor screen internally toprovide a visible display of signals.

(ii) Vertical Amplifier- Vertical amplifier determines the sensitivity and bandwidth of an oscilloscope.Sensitivity, which is expressed in terms of V/cm of vertical deflection at the mid-band frequency.

(iii) Delay Line- It is used to delay the signal for some time in the vertical sections till the time horizontalamplifier also receives signal.

(iv) Time-Base Generators- The CRO is used to display a waveform that varies as a function of time. If thewave form is to be accurately reproduced, the beam should have a constant horizontal velocity. It is used

to generate the saw tooth voltage required to deflect the beam in the horizontal section.(v) Horizontal Amplifier- This is used to amplify the saw tooth voltage before it is applied to horizontaldeflection plates.

(vi) Trigger Circuit- This is used to convert the incoming signal into trigger pulses so that the input signal andthe sweep frequency can be synchronized.

(vii) Power Supply- There are two power supplies, a – Ve high voltage (HV) supply and a +ve low voltage (LV)supply. The –Ve supply is from -1000V to -1500V. This voltage is passed through a bleeder resistor at afew mA. The intermediate voltages are obtained from the bleeder resistor for intensity, focus andpositioning controls resistor for intensity.



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Working of CRO: The electron beam generated from an electron gun will be accelerated toward a phosphorousscreen by a potential difference between the electron gun and the screen. As the beam emerges from the gun, itpasses through a set of parallel plates (the vertical deflection plates ) oriented horizontally. The voltage to bedisplayed is amplified by a vertical amplifier , and applied across these plates producing an electric field whichdeflects the path of the electron vertically. The polarity of the signal determines whether the deflections will be up ordown and the magnitude of the signal determines the amount of vertical displacement of the electrons. After the

beam has passed through the vertical deflection Plates, it passed through a second set of similar plates that areoriented horizontally. A potential difference applied to these plates produce an electric field which deflects left orright. Under normal configuration, these horizontal deflection plates are connected to a time-base circuitry. Thiscircuit can control how fast the electron beam sweeps from the left to right (use Sweep Knob). Adjusting the sweepspeed, the resulting trace on the screen can be spread out or compressed.

Types of CRO1- Dual Beam CRO 2- Dual Trace CRO

Application of CRO(a) Examination of waveforms. (b) Voltage measurement. (c) Current measurement.(e) Frequency measurement. (f) Phase measurement.

Measurement of voltage and currentVoltage Measurement- For measurement of direct voltage, firstly the spot on the screen without applying any voltagesignal in the deflection plates. Then direct voltage to be measures is applied between a pair of deflection plates anddeflection of the spot is observed on the screen. The magnitude of the deflection multiplied by the deflection factorgives the values of direct voltage applied.Current Measurement- For measurement of current, the current under measurement is passed through a knownnon-conductive resistance and the voltage drop across it is measured by CRO. The current can be determined simplyby dividing the voltage drop measured by the value of non-inductive resistance.Measurement of frequency- The pattern obtained on CRO screen is called Lissajous Pattern. They are stationary onthe screen of CRO. They are used to know unknown frequency of a signal.

The unknown frequency can be accurately determined with the help of a CRO.Steps-

1. A known frequency is applied to horizontal input and unknown frequency to the vertical input.2. The various controls are adjusted.3. A pattern with loops is obtained.4. The number of loops cut by the horizontal line gives the frequency on the vertical plates(f V) and the number

of loops cut by the vertical lines gives the frequency on the horizontal plates(f H).



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Measurement of Phase Difference- The two sinusoidal voltage signals of equal frequency having some phasedifference are applied to the deflection plates of a CRO, a straight line or eclipse appears on the screen.

Let there is two sinusoidal voltage signals given by expressionsѵ h= Vh sin ωt & ѵ v = Vv sint (ωt+φ) where φ is the phase difference. Since deflection is directly propotional to amplitude of voltage.d h= Dh sin ωt & d v= Dv sin(ωt+φ) at t=0, values of d h and d v aredh o = 0 & Where dv o=Vertical deflection, at t = 0dvo= Dv sin φ Dv=Maximum vertical deflection.

(a) First Signal (b) Phase shifted (c) CRO displaySecond signal (on screen)

Digital voltmeter(DVM) is an indicating device. it measures the voltage passing between any two points. DVM is oftenused in data processing system. An ideal voltmeter has an infinitely high input resistance so that it does not draw anycurrent from the circuit.

RAMP TECHNIQUE-The block diagram of a ramp type digital voltmeter has three stages.Signal propagation - It modifies the signal amplitude according to the requirement and it also protects fromloading. Hence, resistive attenuator is used to decrease the large incoming signal and amplifier is used toamplify the small incoming signal to the measurable range.Analog to digital conversion - it converts analog information to digital or in discrete form.Display Unit- It displays the measured value of voltage via display devices.

Fig: Simplified or basic building blocks of digital voltmeter Signal Propagation: Digital voltmeter employs operational amplifier Op-Amp is

multistage integrated circuit is used in input circuit in DVM.

Gain of Op-Amp

Fig: Input circuit of DVM

Sin φ=

A= -

Digital Voltmeter

By: Anukaran KhannaBY: ANUKARAN KHANNABY: Anukaran Khanna UNITED GROUP OF INSTITUTIONS, ALLAHABAD


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Analog to Digital Converter (ADC)- in ADC unit, the analog input signal is typically converted into digital signals in theform of binary or BCD data and displayed in the display unit.

Fig Block Diagram of Analog to Digital converter V input is positive voltage but ‘Vc’ comes out to be negative ramp.

The output from the integrator is compared with zero volt reference in the comparator.The output from the comparator will be positive voltage.The AND gate is opened & during this period T, the pulses from the crystal clock oscillator is counted in thecounter.At the end of known time period T, the counters are reset by the control circuit and it activates the reflectionswitch so that the negative known reference voltage V R is applied to the input of the integrator.In this case, the output from the integrator will be a positive going ramp.

With values-

At the end of this time period t, Vc is equal to zero volts.At this time, this counter is counter is counting and hence t, can be known

The unknown voltage Vin could be known from it.

Vc= x t

T = Vin = t

Vc= - T

Fixed Time Fixed SlopeVc= - T Vc= x t

-Vc



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Multimeter is used to measure voltage, current at resistance. It has high input resistance, greater accuracy, betterresolution and easy readability.Block Diagram

Fig: Block Diagram of MultimeterMeasurement of voltage- this multimeter is used to measure unknown input voltage as same as digital voltmeter.Measurement of Current- Sensing resistance or a series of current sensing resistors are used to measure either a.c.or d.c. current. The current is to be measured is passed through one of the sensing resistor and DMM digitizes thevoltages developed across the resistors.

Applying KVL,

Where, Rf= feedback resistance (Known values)Vs= It is output voltage proportional to unknown source current Is.Is= displayed on the display unit

Measurement of resistance- DMM measures the resistance by applying a known current from an internal currentsource to the unknown resistance and then digitizing the voltage developed in the resistance.

Vi, Ri are known parameters.

The output voltage which is proportional to the unknownresistance Rf is applied to DVM section of DMM and thevalue of unknown resistance Rf is displayed.

Multimeter is also used to test whether the diode, transmitter or SCR is good or faulty and to check thecontinuity.

Display unit used in voltmeter or multimeter can be either liquid crystal display(LCD), Light EmittingDiode(LED) or Seven segment display.

Vs= -Is Rf

Vo= - Vi

Digital Multimeter



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Deflection Sensitivity of CRT- The shift of spot of light on the screen per unit change in voltage across the deflectionplates is known as deflection sensitivity of CRT.

The deflection sensitivity depends not only on the design of the tube but also on the voltage appliedto the accelerating anode. The deflection sensitivity is low at high accelerating voltages and vica-versa.

Example-1: The deflection sensitivity of a CRT is 0.01 mm/V . Find the shift produced in the spot when 400 Vis appliedto the plate (vertical).Solution: voltage is applied to vertical plates so spot moves vertically.

Spot Shift = deflection sensitivity x Applied VoltageSpot shift= 0.01 x 400 = 4 mm

Example-2: The deflection sensitivity of a CRT is 0.03 mm/V. If an unknown voltage is applied to horizontal plates.The spot shifts 3 mm horizontally. Find the values of unknown voltage.

Solution: Sensitivity=

Applied voltage = =100 V

Example-3: The vertical gain control of a CRO is set at a deflection sensitivity of 5V/cm. the unknown a.c. voltage issupplied to Y- input. A 10cm long straight line trace is observed. Find a.c. voltage.Solution: Unknown voltage = sensitivity x length of trace = 5 x 10 = 50 V.

This 50V is peak to peak voltage value so, for only one direction either positive or negative is

Vpeak= = = 25 V

RMS value of 25V = = = 17.667 V

Example-4: Find the unknown frequency for the following pattern. When f h = 100Hz.

=4

So , fv(Unknown frequency) = 4 x f h =4 x 100 =400 Hz

Theory Questions Explain working of Ramp type DVM.

Explain briefly the working principle of a DMM with block diagram. Explain the working of CRO with help of a block diagram. Sketch a cathode ray tube used in a CRO. State the main application of a CRO. Briefly explain each of them. Explain how you will quickly measure the

frequency of waveform displayed on the CRO. How you measure current, voltage, frequency and phase from CRO? Explain Lisaajous Pattern.



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B y : - N a v n e e t P a l E m a i l : - e r . n a v n e e t p a l @ g m a i l . c o m Page 1

Unit V (Communication System)

Communication System:- A Communication is a set up used in the transmission & reception of information from oneplace to another place.In communication system, transmitter is located at one place & receiver is located at another

place & the communication channel connects the transmitter & receiver. the channel is physical mediums.

Fig: 1: Block Diagram of Communication System

Elements1- Information Source 2-Transmitter 3=Communication channel 4- Noise 5- Receiver 6-Destination

Description1. Information Source- The information source provides a message signal being non- electrical signal like voice

signal. The transducer converts the non-electrical voice signal into electrical form.2. Transmitter- The transmitter is a collection of electronic circuit designed to convert the information into a

signal suitable for transmission over a given communication medium.

3. Communication Channel- The communication channel is the medium by which the electronic signal istransmitted from one place to another.4. Noise- Noise is random, undesirable electrical energy that enters the communication system via medium &

interferes with transmitted message. Some noise is also produced in the receiver.5. Receiver- A receiver is a collection of electronic circuits designed to convert the signal back to the original

information.

1. Signal- It is a signal valued function of time that carries the information. It is in the electrical form suitable fortransmission. Signals may be analog or digital signal.

2. Transducer- Transducer is a device, which is used to convert non electrical signal to the electrical signal.3. Noise- It refers to the disturbance or distortion in the transmission & processing of message signals in

communication system.4. Transmitter- The transmitter is a collection of electronic circuits designed to convert the information into a

signal suitable for transmission over a given communication medium.5. Receiver- A receiver is a collection of electronic circuits designed to convert the signal back to the original

information.6. Spectrum- The spectrum is the entire range of wavelengths of electromagnetic radiations . The spectrum of

frequency component is the frequency domain representation of the signal.7. Amplification - In this process, The strength of the transmitted signal is increased by using some electronic

circuit.

Channel

Elements of Communication System

Terminology in Communication System

Content:- Fundamentals of Communication Engineering : Elements of a Communication System, Need of modulation,electromagnetic spectrum and typical applications, terminologies in communication systems, Basics of signal representation andanalysis, Fundamentals of amplitude and angle modulation, modulation and demodulation techniques.



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8. Attenuation- It refers to the loss of signal strength during its propagation through the communication channel.9. Baseband Signal- The information/ message signal in its original frequency range is known as baseband signal.10. Modulation - Modulation is the process of super imposing the low frequency message signal (modulating signal)

on a high frequency wave (called the carrier signal). The Resulting wave is the modulated wave which istransmitted over the channel.

11. Demodulation- A reverse process of retrieve the information from the modulated wave at receiver is known asdemodulation.

12. Bandwidth- Bandwidth refers to the range of frequencies over which communication system works. 13. Broadband Signal- This information/ Message signal in its modulated frequency range is known as broadband

signal.14. Broadband Transmission- The Transmission of information/ Message signal in the modulated frequency range is

known as broadband transmission.

Radio Frequency- Radio Frequency or RF refers to that portion of the electromagnetic spectrum in whichelectromagnetic waves can be generated by alternating current fed to an antenna. The radio frequency range is dividedby the ITU (International Telecommunication Unit) into a number of bands of frequencies, where all frequencies in theband exhibit similar properties. The bands are ELF, VF, VLF, LF, MF, HF, VHF, UHF, SHF & EHF.S.No. Frequency Band Frequency Applications

1 Very Low Frequency (VLF) 3 KHz - 30 KHz Long distance point to point communication2 Low Frequency (LF) 30 KHz – 300 KHz Marine & navigational purpose3 Medium frequency (MF) 300 KHz – 3 MHz Marine & broadcasting purpose4 High Frequency (HF) 3 MHz – 30 MHz Communication of all types landline telephony5 Very High frequency (VHF) 30 MHz – 300 MHz FM, TV, radar & Air navigation6 Ultra High Frequency (UHF) 300 MHz – 3000 MHz Radar & microwave communication7 Super High Frequency (SHF) 3 GHz – 30 GHz Wireless LAN, Radar, Radar relays & navigation

purpose

8 Extremely High Frequency (EHF) 30 GHz – 300 GHz Optical fiber communication

In electronic communication system, a high frequency signal called the carrier signal is used to transmit original signal tothe destination.Modulation- The process by which the baseband signal modifies the carrier signal is called Modulation & the resultingsignal is called modulated signal.Demodulation- At the receiving end, the original signal & carrier signal are separated from modulated signal by theprocess of demodulation.

Need of Modulation The original signal either in analog or digital form (baseband signals) are incompatible for direct transmission over themedium. They cannot travel longer distances in air. For ex. Voice signal. Therefore, we have to use modulationtechnique for the communication of baseband signal.

To decrease the length of transmitting & the receiving antenna.Long distance communication is possible.To obtain higher value of signal to noise ratio for modulation.To have frequency division multiplexing where large number of signals is transmitted at different frequencies.Avoids mixing of signals.Allows multiplexing of signals.Allows Adjustments in the bandwidth.Improves Quality of reception.

Electromagnetic Spectrum & Typical Applications

Modulation & Demodulation



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Height of Antenna The height of antenna required for transmission & reception of radio waves in radio transmission is afunction of wavelength of the frequency used. The minimum height of the antenna is given as λ/4 . The wavelength λ isgiven as

where c velocity of lightf frequency

ModulationIt is defined as the process by which some characteristics of signal called “CARRIER” is varied in accordance w ith a‘modulating signal’ or ‘base -band signal’ or ‘message signal’. The result of the modulation process is referred as‘modulated wave’ . At the receiving end of the system we reconstruct the original base-band signal & this isaccomplished by using a process called “DEMODULATION”.“Demodulation” is the reverse process of “Modulation”.

Continuous Wave Modulation

Amplitude Modulation (AM) Angle Modulation

Frequency Modulation (FM) Phase Modulation (PM)

In AM, the maximum amplitude of the high frequency carrier wave isvaried in accordance to the instantaneous value (amplitude) of themodulating or base-band signal.

Equation for AM : Let C(t) = Ac Cos ω ct …………….is a carrier wave havingWhere A c= carrier amplitude ω c= carrier frequency

& m(t)=modulating signal band limited to maximum frequency ‘ω m’. Hence ω c=2πf c >> ω m Now the AM wave is described as

Where, A=A c +m(t)

So,

= Ac[1+Kam(t)] Cos ω ct Where Ka= constant called amplitude sensitivity of modulation Also, we can write

Sinusoidal AM: Let C(t) = Ac Cos ω ct ……….……Carrier Wave m(t) = A m Cos ω mt …………. Modulating Wave Then after modulation, we get X AM(t)= A Cos ω ct Where A= Ac + Am Cos ω mt

So we get,

so, Where m a= = modulation index

Frequency Spectrum: The Modulated carrier has new signals at different frequencies, called side frequencies or

XAM(t)= A Cos ω ct

XAM(t)= [Ac +m(t)] Cos ω ct

XAM(t)= Ac Cos ω ct + m(t) Cos ω ct

XAM(t)= [Ac + Am Cos ω mt] Cos ω ct XAM(t)= Ac [1+ Cos ω mt] Cos ω ct

XAM(t)= Ac [1+ m a Cos ω mt] Cos ω ct

λ=

Amplitude Modulation (AM)



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sidebands, which occur in the frequency spectrum directly above & below the carrier frequency. We know that

XAM(t)= Ac [1+ m a Cos ω mt] Cos ω ct= Ac Cos ω ct + m a Ac Cos ω mt Cos ω ct

XAM(t)= Ac Cos ω ct + m a Ac Cos (ω c + ω m)t + m a Ac Cos (ω c - ω m)t

Full Carrier Upper LowerSide band Side band

Modulation Index: In Amplitude modulation, the modulation index is defined as the ratio of maximum amplitude ofmodulating signal to the maximum amplitude of the carrier signal.

% modulation = m a x 100Modulation index gives the depth to which the carrier signal is modulated.

for

Power Relation in AM wave: Where P T= Total PowerPC= Carrier Power

Transmission Efficiency (η):

η=

Current Relation in AM wave: Let R be the resistance into which the current flow.where I t = Total transmitted current

Ic= Carrier currentm = Modulating index

Effective Modulation Index : If a carrier is modulated by more than a single sine wave, the effective modulation indexis given by:

Note that the total modulation index must not exceed 1 or distortion will occur.m eff can be used in all previously developed equations using m.

Bandwidth of AM wave :

Bandwidth required for the amplitude modulation is twice the frequency of modulating signal.

Ma= =

0 ≤m a≤1 Am ≤ Ac

BW= 2f m



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Total phase angle of high frequency carrier wave is varied in accordance with the modulating or mes

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Basic Elex Notes