electric circuits and electron devices 2 marks anna university

7/28/2019 Electric circuits and electron devices 2 marks anna university

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DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

TWO MARKS QUESTIONS & ANSWERS

EC2151 -- ELECTRIC CIRCUITS AND ELECTRON DEVICES


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UNIT ICIRCUIT ANALYSIS

1. What is charge?The charge is an electrical property of the atomic particles of which matter The unit of charge is the coulomb.

2. Define current?The flow of free electrons in a metal is called electric current. The unit of curreampere. Current (I) = Q/t, Where Q is total charge transferred & T is time reqfor transfer of charge.

3. What is voltage?The potential difference between two points in an electric circuit called voltageof voltage is volt. It is represented by V OR v.

Voltage = W/Q = workdone/Charge

4. Define power.The rate of doing work of electrical energy or energy supplied per unit time the power. The power denoted by either P of p. It is measured in Watts. (W).

Power = work done in electric circuit/TimeP = dw/dt = dw/dq.dq/dtP = VI

5. What is network?Interconnection of two or more simple circuit elements is called an electric netw

6. Distinguish between a branch and a node of a circuit.A part of the network which connects the various points of the network with onanother is called a branch. A point at which two or more elements are jointed tocalled node.

7. Distinguish between a mesh and a Loop of a circuit.A mesh is a loop that does not contain other loops. All meshed are loops. But aare not meshes. A loop is any closed path of branches.

8. Define active and passive elements.The sources of energy are called active element. Example: voltage sourcesource. The element which stores or dissipates energy is called passive Example: Resistor, Inductor, Capacitor.

9. Define unilateral and bilateral elements.In unilateral element, voltage current relation is not same for both the directioExample: Diode, Transistors. In bilateral element, voltage current relation is same for both the direction.Example: Resistor


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10. Define linear and non-linear elements.If the element obeys superposition principle, then it is said to be linearelements.Example : Resistor. If the given network is not obeying superposition principle then it is said to be nelements.Example: Transistor, Diode.

11. Define Lumped and distributed elements.Physically separable elements are called Lumpedelement. Example : Resistor, Capacitor, Inductor.A distributed element is one which is not separable for electrical purpose. Exam: Transmission line has distributor resistance, capacitance and inductance.

12. How are the electrical energy sources classified?The electrical energy sources are classified into:1. Ideal voltage source 2. Ideal current source.

13. Define an ideal voltage source.The voltage generated by the source does not vary with any circuit quantity. It function of time. Such a source is called an ideal voltage source.

14. Define an ideal current source.The current generated by the source does not vary with any circuit quantity.It is only a function of time. Such a source is called as an ideal current source.

15. What are independent source?Independent sources are those in which, voltage and current are independentnot affected by other part of the circuit.

16. What are dependent sources?Dependent sources are those in which source voltage or current is not fixed, budependent on the voltage or current existing at some other location in the circui

17. What are the different types of dependent or controlled sources?1. Voltage Controlled Voltage Sources (VCVS)2. Current Controlled Voltage Sources (CCVS)3. Voltage Controlled Current Sources (VCCS)4. Current Controlled Current Sources (CCCS)

18. What is resistance?It is the property of a substance which opposes the flow of current through it. Tresistance of element is denoted by the symbol R. It is measured in Ohms.

R = PL / A


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19. Write down the V-I relationship of circuit elements.

Circuit Voltage Current Power

Element

Resistance V = iR i = V / R P = vi

Inductance V = L di /dt i = 1/L vdt P = Li di / dt

Capacitance V = 1/c idt i = C dv / dt P = CV dv / dt

20. What is average value?

It is defined as area under one complete cycle to period.The average value of the sine wave is the total area under the half-cycle curve dthe distance of the curve.

Area under one complete cycleAverage value = _____________________________

Period

21. Define R.M.S. value.The r.m.s value may be determined by taking the mean of the squares of theinstantaneous value of current over one complete cycle.

(Area under hatched line) RMS = ______________________

Period

22. Define form factor.The ratio of RMS value to the average value is called the Form factor.

RMS valueForm factor (Kf) = ______________

Average Value

23. Define peak factor.Peak factor is defined as the ratio of the maximum value to the rms value.

Maximum valuePeak Factor (Kp) = ________________

RMS value

24. Define Ohms law.


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The current flowing through the electric circuit is directly proportional to the pdifference across the circuit and inversely proportional to the resistance of the provided the temperature remains constant.

25. Define Kirchoff s current law.Kirchhoffs current law states that in a node, sum of entering current is equal sleaving current.

I at junction point = 0

26. Define Kirchoff s voltage law.Kirchhoffs Voltage Law (KVL) states that the algebraic sum of the voltages aclosed path is zero.Around a closed path V = 0.

27. Two resistances with equal value of R are connected in series and parallel. Whatis the equivalent resistance?

Resistance in series R eq = R1+R2

R1R2Resistance in parallel R eq = __________

R1+R228. Two inductors with equal value of L are connected in series and parallel what is

the equivalent inductance?

Inductance in series L eq = L1 + L2

L1L2Inductance in parallel L eq = _______

L1+L2

29. Two capacitors with equal value of C are connected in series and parallel. Whatis the equivalent capacitance?

C1C2Capacitance in series C eq = ____________

C1+C2

Capacitance in parallel C eq = C1 + C2

30. Write down the formula for a star connected network is converted into a delta network anddelta network to star network ?


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31. Write down the formula for a delta connected network is converted into a starnetwork?

32. Write few advantages of sinusoid waveform?1. The response of the sinusoidal input for second order system is sinusoidal.2. The wave form can be written in terms of sinusoidal function according

theorem.3. The derivatives and integral also sinusoidal.4. Easy for analyses.5. Easy to generate.6. More useful in power industry

33. Distinguish between a cycle, time periods and frequency.One complete set of positive and negative instantaneous values of the voltage current is called cycle. The time taken by an alternating quantity to complete oncycle is called time period (T).

34. What is instantaneous value?The value of an alternating current, at any particular moment is called its

instantaneous value.

35. What are peak value and peak to peak value?The peak value of the sine wave during positive or negative half only.

The sum of positive and negative value is called a peak to peak value. The peak to peak value of a sinusoidal alternating voltage is equal to two times the peak valu

36. What is average value?The average value of the sine wave is the total area under the half-cycle curve

divided by the distance of the curve.


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37. Define effective value or RMS value of a sinusoidal voltage.The R.M.S value may be determined by taking the mean of the squares of theInstantaneous value of current over one complete cycle. This is often known

as the effective value.V = ____________ rms 1/T T 2

[V (t) dt0

38. Define phasor.Graphical representation of phasors V = Vm


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1 2

Admittance in parallel Y = Y YEq 1 2

43. Obtain the equivalent conductance and susceptance for series and parallelconnections.

Conductance in series

Conductance in parallel

44. Define phasor diagram.The phasor diagram is a name given to a sketch in the complex plane showing the

relationships of the phasor voltages and phasor currents throughout a specific circuit.

45. What is instantaneous power?The power at any instant of time is known as instantaneous power.

P (t) = v(t) . I (t)

46. What is average power?The average of the instantaneous power over one period is called average power.

power is also defined as the product of voltage and current.

47. What is apparent power?The product of V rms and I rms is known as the apparent power (s).

Apparent Power (S) =

48. Define power factor.The ratio of the average power to the apparent power is called the power factor.

Average power


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Power Factor = _________________ Apparent Power

49. What is power triangle ?A commonly employed graphical representation of complex power is knownthe power triangle.

50. Define complex power

The product of the rms voltage phasor and the complex conjugate of the rms cur phasor is known as complex power. It is denoted as S and it is measured in volt-amperes

51. What is reactive power?It is defined as product of the applied voltage and the reactive component of thcurrent. It is also called as imaginary component of the apparent power. It is represented b

it ismeasured in unit voltampere reactive (VAR).

Q = V Isin VAR

Eff eff

52 What is the equation for determining the number of independent loops inmesh current method?

L = b-n +1Where L = number of loops

B = number of branchesB = number of nodes.

52. On which law is the mesh analysis based?Mesh analysis is based on Kirchoffs voltage law.

53. On which law is the nodal analysis based? Nodal analysis is based on Kirchoffs current law and Ohms law.

54. What is mesh analysis?Mesh analysis is one of the basic techniques used for finding current flowing th

loop in a network.Mesh analysis is applicable if the given network contains voltage sources. If thecurrent sources in a circuit, then it should be converted into equivalent voltage sources.

55. What is nodal analysis? Nodal analysis is one of the basic techniques used to finding solution for voltage

across the nodes in a given circuit. Nodal analysis is applicable if the given network contains current sources. If therevoltage sources in the given circuit, then it can to be converted into equivalent current sou

56. When do we go for supermesh analysis.If the branches in the network has a current source, then it is slightly difficult to a


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analysis.

One way to over come this difficulty is by applying the super mesh technique.In this case we have to choose super mesh.A super mesh is constituted by two adjacent loops that have common current sourc

57. When do we, go for supernode analysis.

If the branches in the network has a voltage source, then it is slightly difficult to apnodal analysis. One way to overcome this difficulty is by applying the supernode techniqcase, we have to choose super node. A supernode is constituted by two adjacent node thacommon voltage source.

58. State superposition theorem.Any electric circuit (linear, lumped, bilateral), is energeied by two or more

the response in any element in the network is equal to the algebraic sum of the responses individual sources acting separately.

59. State Thevenins Theorem.A complex network having linear, bilateral, lumped elements with open circ

output terminals can be reduced by a simple circuit consisting of a single voltage souseries with a impedance.

60. State Nortons theorem.Any electrical network (linear, lumped, bilateral) with short circuited termin

be reduced by a simple circuit consisting of a single current source in parallel with a Thevequivalent resistance.

61. State Maximum power transfer theorem.

Power transferred from source to load will be maximum, when source resistequal to load resistance looking back from its load terminals.

62. Define duality.Two electrical network which are governed by the same type of equations ar

duality.


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UNIT II

TRANSIENT RESONANCE IN RLC CIRCUITS

1. What is transient state?If a network contains energy storage elements, with change in excitation, th

and voltages change from one state to other state. The behaviour of the voltage or currenchanged from one state to another state is called transient state.

2. What is transient time?The time taken for the circuit to change from one steady state to another stea

is called transient time.

3. What is natural response?If we consider a circuit containing storage elements which are independentof sources, the response depends upon the nature of the circuit, it is called natural respons

4. What is transient response?The storage elements deliver their energy to the resistances, hence theresponse changes with time, gets saturated after sometime, and is referred to the transientresponse.

5. Define Laplace transform function.The laplace transform of any time dependent functionf (t) is given by F (s).Where S A complex frequency given by S = + jw

-stF (S) = L [F (T) ] = F (t) e dt

06. What is inverse Laplace transform?

Inverse Laplace Transform permits going back in the reverse direction i.e. fs domain to time domain.

-1 1 1 + j stL [F(s)] = f (t) = _________ F (s) e ds

2j 1 - j

7. Define time constant or RL Circuit.The time taken to reach 63.2% of final value in a RL Circuit is called the

time constant of RL circuit.Time constant (t) = L / R

8. Define time constant of RC Circuit.The time taken to reach 36.8% of initial current in an RC circuit is called the

constant of RC circuit.Time constant (t) = RC.


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9. What is meant by natural frequency?If the damping is made zero then the response oscillates with natural frequency witopposition, such a frequency is called natural frequency of oscillations, denoted as

n.10. Define damping ratio.

It is the ratio of actual resistance (R) in the circuit to the critical resistance (R cr). Idenoted by greek letter Zeta ().

R R = ____ = _____ C / L

R cr 2

11. Define initial value theorem.The initial value theorem states that if f (t) and f (t) both are laplace transfo

ThenLim f (t) = lim s F (s)T 0 s

12. Define final value theorem.The final value theorem states that, if f (t) and f (t) both are laplace transfo

thenLim f (t) = lim s F (s)T s 0

13. What is driving point impedence ?The ratio of the Laplace transform of the voltage at the point to the laplace transfor

current at the same port is called driving point impedance.The driving point impedance of the network is define as

V(s)Z (S) = _____________

B(s)14. What is transfer point impedance?

It is defined as the ratio of voltage transform at one port to the current transformthe other port. It is defined by


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15. Define network function.A network function N (S) is defined as the ratio of the complex amplitude oan excponential output P (S) to the complex amplitude of an exponential input Q (S).

16. Define pole and zero.The network function N (S) will become infinite. Hence the roots of denomi polynomial P1, P2, P3,Pm are called poles of network function.The network function N(S) will become zero. Hence the roots of numerator polynomial Z1, Z2, Z3..Zn. are called zeros of network function.

17. Define resonant circuit.The circuit that treat a narrow range of frequencies very differently than all

frequencies. These are referred to as resonant circuit. The gain of a highly resonant circuisharp maximum or minimum as its resonant frequency.

18. When the circuit is said to be in resonance?1. A network is in resonance when the voltage and current at the network inpu

are in phase.2. If inductive reactance of a network equals capacitive reactance then the networ

be resonance

19. What is resonant frequency ?The frequency at which resonance occurs is called resonance

frequency. 1f = ________ B 2LC

20. Define bandwidth.

The bandwidth (BW) is defined as the frequency difference between upper frequency (f2) and lower cut-off frequency (f1)

Bandwidth = f2-f1

Where f2 upper cut-off frequency

f1 lower cut-off frequency

21. Define selectivity.

Selectivity is defined as the ratio of bandwidth to the resonant frequency of circuit. BandwidthSelectivity = _______________________

Resonant frequency

22. Define quality factor.The quality factor is defined as the ratio of maximum energy stored to the en

dissipated per cycle.Maximum energy stored per cycleQuality factor (Q) = 2 * ______________________________

Energy dissipated per cycle


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23. Define half power frequencies ?The frequencies at which the power is half the maximum power are called h

frequencies.R Lower half power frequency, f1 = f r - ____ 4 L

R Upper half power frequency, f2 = f r + ____ 4 L

24. Write down the formula for inductive reactance and capacitive reactance?

Inductive reactance is given by X = 2 flL

1Capacitive reactance is given by X = _________

C 2 fcWhereF supply frequencyL Inductance of the coilC Capacitance of the capacitor.

25. Give the expression for quality factor of series RLC Circuit.Quality factor is Q = 1 / R L /C

26. Give the expression for quality factor of parallel RLC Circuit.Quality factor is Q = R C / L


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UNIT III

SEMICONDUCTOR DIODES

1. Give the value of Charge, Mass of an electron.

Charge of an electron 1.6 x 10 -19 coloumbs & Mass of an electron - 9.11 x 10 -31 Kgs2. Define Potential.A potential of V volts at point B with respect to point A, is defined as the work done in taking unit positive charge from A to B , against the electric field.

3. Define Current density.It is defined as the current per unit area of the conducting medium. J = I / A

4. Define Electron volts.If an electron falls through a potential of one volt then its energy is 1 electron volt.1 eV = 1.6 x 10 -19 joules

5. What is Electrostatic deflection sensitivity?Electrostatic deflection sensitivity of a pair of deflecting plates of a cathode rayoscilloscope ( CRO) is defined as the amount of deflection of electron spot producedwhen a voltage of 1 Volt DC is applied between the corresponding plates.

6. What is the relation for the maximum number of electrons in each shell?Ans: 2n2 7. What are valence electrons?Electron in the outermost shell of an atom is called valence electron.

8. What is forbidden energy gap?The space between the valence and conduction band is said to be forbidden energy gap.

9. What are conductors? Give examples?Conductors are materials in which the valence and conduction band overlap each other soswift movement of electrons which leads to conduction. Ex. Copper, silver.

10. What are insulators? Give examples?Insulators are materials in which the valence and conduction band are far awayfrom each other. So no movement of free electrons and thus no conduction. Exglass, plastic.

11. Give the energy band structure of Insulator.In Insulators there is a wide forbidden energy gap. So movement of valenceelectron from valence to conduction band is not possible.

12. Give the energy band structure of Semi conductor.In Semiconductors there is a small forbidden energy gap. So movement of


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valence electron from valence to conduction band is possible if the valenceelectrons are supplied with some energy.

13. Give the energy band structure of conductor.In conductors there is no forbidden energy gap, valence band and conductionand overlap each other. so there is a heavy movement of valence electrons.

14. what are Semiconductors? Give examples?The materials whose electrical property lies between those of conductors andinsulators are known as Semiconductors. Ex germanium, silicon.

15. What are the types of Semiconductor?1. Intrinsic semiconductor 2. Extrinsic semiconductor.

16. What is Intrinsic Semiconductor?Pure form of semiconductors are said to be intrinsic semiconductor.Ex: germanium, silicon.

17. Define Mass action law.Under thermal equilibrium the product of free electron concentration (n) and holeconcentration (p) is constant regardless of the individual magnitude.n.p = ni2

18. What is Extrinsic Semiconductor?If certain amount of impurity atom is added to intrinsic semiconductor theresulting semiconductor is Extrinsic or impure Semiconductor.

19. What are the types of Extrinsic Semiconductor?1. P-type Semiconductor 2. N- Type Semiconductor.

20. What is P-type Semiconductor?The Semiconductor which are obtained by introducing pentavalent impurity atom(phosphorous, antimony) are known as P-type Semiconductor.

21. What is N-type Semiconductor?The Semiconductor which is obtained by introducing trivalent impurity atom (gallium, inknown as N-type Semiconductor.

22. What is doping?Process of adding impurity to a intrinsic semiconductor atom is doping. The impurity is cdopant.

23. Which charge carriers is majority and minority carrier in N-typeSemiconductor?majority carrier: electron and minority carrier: holes.

24.which charge carriers is majority and minority carrier in P-type


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Semiconductor?Majority carrier: holes and minority carrier: electron

25. Why n - type or penta valent impurities are called as Donor impurities?n- type impurities will donate the excess negative charge carriers ( Electrons) and thereforeffered to as donor impurities.

26. Why P type or trivalent impurities are called as acceptor impurity? p- type impurities make available positive carriers because they create holes which can aelectron, so these impurities are said to be as acceptor impurity.

27.Give the relation for concentration of holes in the n- type material? pn = ni2 /NDWhere pn - concentration of holes in the n type semiconductor ND - concentration of donor atoms in the n type semiconductor

28. Give the relation for concentration of electrons in the p - type material?np=ni2 /NA Wherenp - concentration of electrons in p- type semiconductor ND - concentration of acceptor atoms in the p type semiconductor

29. Define drift current?When an electric field is applied across the semiconductor, the holes move towards the nterminal of the battery and electron move towards the positive terminal of the battery. Thmovement of charge carriers will result in a current termed as drift current.

30. Give the expression for drift current density due to electron.Jn = q nnE Where,

Jn - drift current density due to electron q- Charge of electron

n - Mobility of electron E - applied electric field31. Give the expression for drift current density due to holes.Jp = q p p EWhere, Jn - drift current density due to holes q - Charge of holes p - Mobility of holes E - applied electric field

32. Define the term diffusion current?A concentration gradient exists, if the number of either electrons or holes is greater in onea semiconductor as compared to the rest of the region. The holes and electron tend to movregion of higher concentration to the region of lower concentration. This process in calledand the current produced due this movement is diffusion current.


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33. Define mean life time of a hole or and electron.The electron hole pair created due to thermal agitation woll disappear as a rerecombination. Thus an average time for which a hole or an electron exist before recombcan be said as the mean life time of a hole or electron.

34. What is the other name of continuity equation? What does it indicate?

The other name of continuity equation is equation of conservation of charge.This equation indicates that the rate at which holes are generated thermally just equals tat which holes are lost because of recombination under equilibrium conditions.

35. Define Hall effect?If a metal or semiconductor carrying current I is placed in a transverse magnetic fieelectric field E is induced in the direction perpendicular to both I and B This phenoknown as Hall effect.

36. Give some application of Hall Effect.i). Hall Effect can be used to measure the strength of a magnetic field in terms of electricavoltage. ii).It is used to determine whether the semiconductor is p type or n- type materiii).It is used to determine the carrier concentration iv).It is used to determine themobility.37. Define the term transition capacitance?When a PN junction is reverse biased, the depletion layer acts like a dielectric material w N type regions on either side which has low resistance act as the plates. In this way a rev biased PN junction may be regarded as parallel plate capacitor and thus the capacitance aset up is called as the transition capacitance.CT = A / WWhereCT - transition capacitanceA - Cross section area of the junction W Width of the depletionregion

38. What is a varactor diode?A diode which is based on the voltage variable capacitance of the reverse biased p-n juncsaid to be varactor diode. It has other names such as varicaps, voltacaps.

39. Define the term diffusion capacitance.The diffusion capacitance of a forward biased diode is defined as the rate of change of injcharge with voltage.CD = I / VTWhere, Cd time constant

I current across the diodevT threshold voltage

40. what is recovery time? Give its types.When a diode has its state changed from one type of bias to other a transient accompanieresponse, i.e., the diode reaches steady state only after an interval of time tr called as rtime. The recovery time can be divided in to two types such as(i) forward recovery time(ii) reverse recovery time

41. What is meant by forward recovery time?The forward recovery time may be defined as the time interval from the instant of 10% di


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voltage to the instant this voltage reaches 90% of the final value. It is represented as t f r.

42. What is meant by reverse recovery time?The reverse recovery time can be defined as the time required for injected or the excessminority carrier density reduced to zero , when external voltage is suddenly reversed.

43. Define storage time.

The interval time for the stored minority charge to become zero is called storage time. Itis represented as t s.

44. Define transition time.The time when the diode has normally recovered and the diode reverse current reachesreverse saturaton current I0 is called as transition time. It is represented as t t

45. What are break down diodes?Diodes which are designed with adequate power dissipation capabilities to operate in the down region are called as break down or zener diodes.

46. What is break down? What are its types?When the reverse voltage across the pn junction is increased rapidly at a voltage the breaks down leading to a current flow across the device. This phenomenon is called as band the voltage is break down voltage. The types of break down arei) zener break downii)Avalanche breakdown

47. What is zener breakdown?Zener break down takes place when both sides of the junction are very heavily doped andConsequently the depletion layer is thin and consequently the depletion layer is tin. Whenvalue of reverse bias voltage is applied , a very strong electric field is set up across the thdepletion layer. This electric field is enough to break the covalent bonds. Now extremely number of free charge carriers are produced which constitute the zener current. This procknown as zener break down.

48. What is avalanche break down?When bias is applied , thermally generated carriers which are already present in the diodesufficient energy from the applied potential to produce new carriers by removing valencefrom their bonds. These newly generated additional carriers acquire more energy from thand they strike the lattice and create more number of free electrons and holes. This proceas long as bias is increased and the number of free carriers get multiplied. This process isavalanche multiplication. Thus the break down which occur in the junction resulting in heof current is termed as avalanche break down.


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49. How does the avalanche breakdown voltage vary with temperature?In lightly doped diode an increase in temperature increases the probability of collision of and thus increases the depletion width. Thus the electrons and holes needs a high voltage junction. Thus the avalanche voltage is increased with increased temperature.

50. How does the zener breakdown voltage vary with temperature?In heavily doped diodes, an increase in temperature increases the energies of valence elechence makes it easier for these electrons to escape from covalent bonds. Thus less voltagesufficient to knock or pull these electrons from their position in the crystal and convert thto conduction electrons. Thus zener break down voltage decreases with temperature.

UNIT IV

TRANSISTORS51 . What is a transistor (BJT)?Transistor is a three terminal device whose output current, voltage and /or

power is controlled by input current.52 . What are the terminals present in a transistor?Three terminals: emitter, base, collector.

53 . What is FET?FET is abbreviated for field effect transistor. It is a three terminal device with itsoutput characteristics controlled by input voltage.

54 . Why FET is called voltage controlled device?


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The output characteristics of FET is controlled by its input voltage thus it is voltage contr

55 . What are the two main types of FET?1. JFET 2. MOSFET.

56. What are the terminals available in FET?1). Drain, 2).Source and 3). Gate

57 . What is JFET?JFET- Junction Field Effect Transistor.

58 . What are the types of JFET? N- Channel JFET and P- Channel JFET

59. What are the two important characteristics of JFET?1. Drain characteristics 2. Transfer characteristics.

60 . What is transconductance in JFET?

It is the ratio of small change in drain current to the corresponding change in drain to sou61 . What is amplification factor in JFET?It is the ratio of small change in drain to source voltage to the corresponding change in Gto source voltage.

62. Why do we choose q point at the center of the loadline?The operating point of a transistor is kept fixed usually at the center of the active region ithe input signal is well amplified. If the point is fixed in the saturation region or the cuthe positive and negative half cycle gets clipped off respectively.

63 . List out the different types of biasing. ._ Voltage divider bias, Base bias, Emitter feed back bias, Collector feedback bias, Emitter b

64 . What do you meant by thermal runway?Due to the self heating at the collector junction, the collector current rises. This causes dathe device. This phenomenon is called thermal runway.

65. Why is the transistor called a current controlled device?The output characteristics of the transistor depend on the input current. So thtransistor is a current controlled device.

66. Define current amplification factor?It is defined as the ratio of change in output current to the change in input current at const

67. What are the requirements for biasing circuits? The q point must be taken at the Centre of the active region of the output characteristics Stabilize the collector current against the temperature variations.


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Make the q point independent of the transistor parameters. When the transistor is replaced, it must be of same type.

68 . When does a transistor act as a switch?The transistor acts as a switch when it is operated at either cutoff region or saturation regi

69 . What is biasing?To use the transistor in any application it is necessary to provide sufficient voltage and cuoperate the transistor. This is called biasing.

70. What is stability factor?Stability factor is defined as the rate of change of collector current with respect to the rateof reverse saturation current.

71 . Explain about the various regions in a transistor?The three regions are active region saturation region cutoff region.

72 . Explain about the characteristics of a transistor?Input characteristics: it is drawn between input voltage & input current while keeping outas constant. Output characteristics: It is drawn between the output voltage &output currenkeeping input current as constant.

UNIT V


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SPECIAL SEMICONDUCTOR DEVICES

78. What is a TRIAC?TRIAC is a three terminal bidirectional semiconductor switching device. It can conduct idirections for any desired period. In operation it is equivalent to two SCRs connected inantiparallel.79. Give the application of TRIAC.1. Heater control2. Motor speed control3. Phase control4. Static switches80. What are the different operating modes of TRIAC?1. Keeping MT2 and G positive2. Keeping MT2 and G negative.3. Keeping MT2 positive and G negative.4. Keeping MT2 negative and G positive.81. What are the regions in the VI characteristics of UJT?

1. Cut-off region2. Negative resistance region.3. Saturation region82. What is meant by negative resistance region of UJT?In a UJT when the emitter voltage reaches the peak point voltage, emitter current starts flAfter the peak point any effort to increase in emitter voltage further leads to sudden increemitter current with corresponding decrease in emitter voltage, exhibiting negative resistatakes place until the valley point is reached. This region between the peak point and vallecalled negative resistance region.83. Mention the applications of UJT.1. It is used in timing circuits2. It is used in switching circuits3. It is used in phase control circuits


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4. It can be used as trigger device for SCR and triac.5. It is used in saw tooth generator.6. It is used for pulse generation.84. What is a DIAC?DIAC is a two terminal bidirectional semiconductor switching device. . It can conduct in direction depending upon the polarity of the voltage applied across its main terminals. In DIAC is equivalent to two 4 layer diodes connected in antiparallel.85. Give some applications of DIAC.1. To trigger TRIAC2. Motor speed control3. Heat control4. Light dimmer circuits87. What is a SCR?A silicon controller rectifier (SCR) is a three terminal, three junction semiconductor devi

as a true electronic switch. It is a unidirectional device. It converts alternating current intocurrent and controls the amount of power fed to the load.88. Define break over voltage of SCR.Break over voltage is defined as the minimum forward voltage with gate open at which thstarts conducting heavily.89.Why SCR cannot be used as a bidirectional switch.SCR can do conduction only when anode is positive with respect to cathode with proper gcurrent. Therefore, SCR operates only in one direction and cannot be used as bidirectiona90. How turning on of SCR is done?1. By increasing the voltage across SCR above forward break over voltage.2. By applying a small positive voltage at gate.3. By rapidly increasing the anode to cathode voltage.4. By irradiating SCR with light.91. How turning off of SCR is done?1. By reversing the polarity of anode to cathode voltage.2. By reducing the current through the SCR below holding current.3.By interrupting anode current by means of momentarily series or parallel switching92. Define holding current in a SCR.Holding current is defined as the minimum value of anode current to keep the SCR ON.93. List the advantages of SCR.1. SCR can handle and control large currents.2. Its switching speed is very high3. It has no moving parts, therefore it gives noiseless operation.4. Its operating efficiency is high.94. List the application of SCR.1. It can be used as a speed controller in DC and AC motors.2. It can be used as an inverter.3. It can be used as a converter 4. It is used in battery chargers.5. It is used for phase control and heater control.6. It is used in light dimming control circuits.95. What is meant by latching.


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The ability of SCR to remain conducting even when the gate signal is removed is called a

96. Define forward current rating of a SCR. Forward current rating of a SCR is the maximum anode current that it can handle withoutdestruction.

97. List the important ratings of SCR.1. Forward break over voltage2. Holding current3. Gate trigger current4. Average forward current5. Reverse break down voltage.

98. Compare SCR with TRIAC.SCR TRIAC

1. unidirectional current 1. bidirectional current2. triggered by positive pulse at gate 2. triggered by pulse of positive or negative

at gate3. fast turn off time 3,. Longer turn off time4. large current ratings 4. lower current ratings

99. Differentiate BJT and UJT.BJT UJT

1. It has two PN junctions 1. It has only one PN junctions2. three terminals present 2. three terminals present are emitter,are emitter, base,collector base1,base2

3. basically a amplifying device 3. basically a switching device

100. What is Shockley diode (PNPN diode)?Shockley diode is a four layered PNPN silicon diode. It is a low- current SCR without a gdevice is switched ON when the anode to cathode voltage is increased to forward switchivoltageVS which is equivalent to SCR forward break over voltage.

101. What is a thyristor?

Thyristor is a semiconductor device having three or more junctions .Such a device acts without any bias and can be fabricated to have voltage ratings of severalhundred volts aratings from a few amperes to almost thousand amperes.

102. What are the types of thyristors?1. Unidirectional thyristors2. Bidirectional thyristors3. Low-power thyristors


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103. Give the various triggering devices for thyristors.1. SCR 2. UJT3. DIAC4. TRIAC

104. what is backward diode?The backward diode is a diode in which the doping level is moderate. The forward currencase is very small, very much similar to that of the reverse current in the conventional dio

105. what is a photo diode? The photo diode is a diode in which the current sensitivity to radiation can be made m

by the use of the reverse biased PN junction. Thus this diode conducts heavily in the revewhen there is some radiaton allowed to fall on the PN junction.

106. What is a LED?A PN junction diode which emits light when forward biased is known as Light emittingdiode (LED).

107. What is a tunnel diode?The tunnel diode is a pn junction diode in which the impurity concentration is greatly incabout 1000 times higher than a conventional PN junction diode thus yielding a very thin dlayer. This diode utilizes a phenomenon called tunneling and hence the diode is referred adiode.

108. What is tunneling phenomenon?The phenomenon of penetration of the charge carriers directly though the potential barrieclimbing over it is called as tunneling.

16 marks-Hints73. Explain the construction, operation, volt ampere characteristics, andapplication of SCR, also explain its two transistor model.Maximum mark for this question: 16marks Construction (2marks)Equivalent circuit and two transistor model(2marks) Operation (4marks)Volt ampere characteristics(4marks) Application (2marks)

74. Explain the construction, operation, equivalent circuit, voltampere characteristics, and application of UJT.Maximum mark for this question: 16marks Construction (4marks)


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Equivalent circuit(2marks) Operation(4marks)Volt ampere characteristics(4marks) Application (2marks)

75. Explain the construction, operation, equivalent circuit, voltampere characteristics, and application of DIAC.Maximum mark for this question: 16marks Construction (4marks)Equivalent circuit(2marks) Operation(4marks)Volt ampere characteristics(4marks) Application (2marks)

76. Explain the construction, operation, equivalent circuit, voltampere characteristics, and application of TRIACMaximum mark for this question: 16 marksConstruction (4marks)Equivalent circuit (2marks)Operation (4marks)Volt ampere characteristics (4marks)Application (2marks)

electric circuits and electron devices 2 marks anna university

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