Edevices_circuits1.pdf

Exercise :: Electronic Devices and Circuits - Section 1 1. At room temperature the current in an intrinsic semiconductor is due to A.

holes

B. electrons C. ions D. holes and electrons Answer: Option D Explanation: Intrinsic material has equal number of holes and electrons. View Answer Discuss in Forum Workspace Report

2. Work function is the maximum energy required by the fastest electron at 0 K to escape from the metal surface. A.

True

B. False Answer: Option B Explanation: Work function is the minimum energy required by the fastest electron at 0 K to escape from the metal surface. View Answer Discuss in Forum Workspace Report

3. The most commonly used semiconductor material is A.

silicon

B. germanium C. mixture of silicon and germanium D. none of the above Answer: Option A Explanation: Germanium is rarely used. View Answer Discuss in Forum Workspace Report

4. In which of these is reverse recovery time nearly zero? A.

Zener diode

B. Tunnel diode

C. Schottky diode D. PIN diode Answer: Option C Explanation: In schottky diode there is no charge storage and hence almost zero reverse recovery time. View Answer Discuss in Forum Workspace Report

5. A transistor has a current gain of 0.99 in the CB mode. Its current gain in the CC mode is A.

100

B. 99 C. 1.01 D. 0.99 Answer: Option A Explanation:

Current gain = 1 + β = 100. 6. In p-n-p transistor the current IE has two components viz. IEP due to injection of holes from pregion to n-region and IE due to injection of electrons from n-region to p-region. Then A.

IEp and IEn are almost equal

B. IEp >> IEn C. IEn >> IEp D. either (a) or (c) Answer: Option B Explanation: Emitter is p-type in p-n-p transistor. Therefore holes are majority carriers. View Answer Discuss in Forum Workspace Report

7. In an n channel JFET, the gate is A.

n type

B. p type C. either n or p D. partially n & partially p Answer: Option B

Explanation: Since channel is n type gate must be p type. View Answer Discuss in Forum Workspace Report

8. The amount of photoelectric emission current depends on A.

frequency of incident radiation

B. intensity of incident radiation C. both frequency and intensity of incident radiation D. none of the above Answer: Option B Explanation: Only the intensity of incident radiation governs the amount of photoelectric emission. View Answer Discuss in Forum Workspace Report

9. Assertion (A): A p-n junction has high resistance in reverse direction. Reason (R): When a reverse bias is applied to p-n junction, the width of depletion layer increases. A.

Both A and R are true and R is correct explanation of A

B. Both A and R are true but R is not a correct explanation of A C. A is true but R is false D. A is false but R is true Answer: Option A Explanation: The increase in reverse resistance is due to widening of depletion layer. View Answer Discuss in Forum Workspace Report

10. In the circuit of figure the function of resistor R and diode D are

A.

to limit the current and to protect LED against over voltage

B. to limit the voltage and to protect LED against over current

C. to limit the current and protect LED against reverse breakdown voltage. D. none of the above. Answer: Option C Explanation: Resistance limits current and diode is reverse connected and therefore protects LED against reverse breakdown. 11. At very high temperatures the extrinsic semi conductors become intrinsic because A.

drive in diffusion of dopants and carriers

B. band to band transition dominants over impurity ionization C. impurity ionization dominants over band to band transition D. band to band transition is balanced by impurity ionization Answer: Option B Explanation: Covalent bonds are broken. View Answer Discuss in Forum Workspace Report

12. When a voltage is applied to a semiconductor crystal then the free electrons will flow. A.

towards positive terminal

B. towards negative terminal C. either towards positive terminal or negative terminal D. towards positive terminal for 1 μs and towards negative terminal for next 1 μs Answer: Option A Explanation: Since electrons are negatively charged they will flow towards positive terminal. View Answer Discuss in Forum Workspace Report

13. Ferrite have A.

low copper loss

B. low eddy current loss C. low resistivity D. higher specific gravity compared to iron Answer: Option C Explanation: Ferrite is a low density material of composition with Fe2O3 x O, where x is a bivalent metal, such

as Cobart, Ni, Mn. These magnetic materials having very low loss of current and used in high frequency circuit. View Answer Discuss in Forum Workspace Report

14. In a p type material the Fermi level is 0.3 eV above valence band. The concentration of acceptor atoms is increased. The new position of Fermi level is likely to be A.

0.5 eV above valence band

B. 0.28 eV above valence band C. 0.1 eV above valence band D. below the valence band Answer: Option B Explanation: Addition of acceptor atom brings Fermi level closer to valence band. View Answer Discuss in Forum Workspace Report

15. In an n-p-n transistor, the majority carriers in the base are A.

electrons

B. holes C. both holes and electrons D. either holes or electrons Answer: Option A Explanation: Emitter is n type and emits electrons which diffuse through the base. 16. An LED has a rating of 2 V and 10 mA. It is used along with 6V battery. The range of series resistance is A.

0 to 200 Ω

B. 200 - 400 Ω C. 200 Ω and above D. 400 Ω and above Answer: Option D Explanation: R=

= 400Ω.

R must be at least 400Ω so that current in LED does not exceed 10 mA. View Answer Discuss in Forum Workspace Report

17. The number of doped regions in PIN diode is A.

1

B. 2 C. 3 D. 1 or 2 Answer: Option B Explanation: A PIN diode has p and n doped regions separated by intrinsic layer. View Answer Discuss in Forum Workspace Report

18. A transistor has two p-n junctions. The batteries should be connected such that A.

both junctions are forward biased

B. both junctions are reverse biased C. one junction is forward biased and the other is reverse biased D. none of the above Answer: Option C Explanation: Emitter-base junction is forward biased and base collector junction is reverse biased. View Answer Discuss in Forum Workspace Report

19. A silicon (PN) junction at a temperature of 20°C has a reverse saturation current of 10 pico Ampere. The reverse saturation current at 40°C for the same bias is approximately. A.

30 pA

B. 40 pA C. 50 pA D. 60 pA Answer: Option B Explanation: By increasing of temperature by 10°C, Io become double so by increasing temperature 20°C, Io become 4 time than initial value... and it is 40 PA. View Answer Discuss in Forum Workspace Report

20. In a bipolar transistor the barrier potential A.

0

B. a total of 0.7 V

C. 0.7 V across each depletion layer D. 0.35 V Answer: Option C Explanation: Since there are two p-n junctions, there are two depletion layers and 0.7 V across each layer. 21. Recombination produces new electron-hole pairs A.

True

B. False Answer: Option B Explanation: Due to recombination the number of electron-hole pairs is reduced. View Answer Discuss in Forum Workspace Report

22. An amplifier without feedback has a voltage gain of 50, input resistance of 1 kΩ and output resistance of 2.5 kΩ. The input resistance of the current shunt -ve feedback amplifier using the above amplifier with a feedback factor of 0.2 is A.

1/11 kΩ

B. 1/5 kΩ C. 5 kW D. 11 kW Answer: Option A Explanation:

Input Resistance with feedback for current shunt,

.

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23. As compared to an ordinary semiconductor diode, a Schottky diode A.

has lower cut in voltage

B. has higher cut in voltage C. lower reverse saturation current D. both (b) and (c) Answer: Option A Explanation: Cut in voltage in Schottky diode is about 0.3 V as compared to 0.7 V in ordinary semiconductor diode.

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24. Assertion (A): When a high reverse voltage is applied to a p-n junction the diode breaks down. Reason (R): High reverse voltage causes Avalanche effect. A.

Both A and R are true and R is correct explanation of A

B. Both A and R are true but R is not a correct explanation of A C. A is true but R is false D. A is false but R is true Answer: Option A Explanation: Avalanche breakdown occurs at high reverse voltage. View Answer Discuss in Forum Workspace Report

25. As compared to an ordinary semiconductor diode, a Schottky diode A.

has higher reverse saturation current

B. has higher reverse saturation current and higher cut in voltage C. has higher reverse saturation current and lower cut in voltage D. has lower reverse saturation current and lower cut in voltage Answer: Option C Explanation: This is due to high electron concentration in metals. 26. Crossover distortion behaviour is characteristic of A.

class A O/P stage

B. class B O/P stage C. class AB output stage D. common pulse O/P state Answer: Option B Explanation: It is a characteristics of class B output stage as the amplifier is biased in cut-off region. In class B amplifier, two transistor are operated in such a way that one is amplify the half cycle and second is amplify -ve half cycle.

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27. If ac for transistor is 0.98 then βac is equal to A.

51

B. 49 C. 47 D. 45 Answer: Option B Explanation:

. View Answer Discuss in Forum Workspace Report

28. Assertion (A): The conductivity of p type semiconductor is higher than that of intrinsic semiconductor. Reason (R): The addition of donor impurity creates additional energy levels below conduction band. A.

Both A and R are true and R is correct explanation of A

B. Both A and R are true but R is not a correct explanation of A C. A is true but R is false D. A is false but R is true Answer: Option B Explanation: A refers to type semiconductor while R refers to n type semiconductor. Both A and R are correct but independent. View Answer Discuss in Forum Workspace Report

29. In an n-p-n transistor biased for operation in forward active region

A.

emitter is positive with respect to base

B. collector is positive with respect to base C. base is positive with respect to emitter and collector is positive with respect to base D. none of the above Answer: Option C Explanation: In forward active mode emitter base junction is forward biased and base collector junction is reverse biased. View Answer Discuss in Forum Workspace Report

30. An increase in temperature increases the width of depletion layer. A.

True

B. False Answer: Option B Explanation: With increase in temperature width of depletion layer decreases. 31. A zener diode is used in A.

voltage regulator circuit

B. amplifier circuits C. both voltage regulator and amplifier circuit D. none of the above Answer: Option A Explanation: Zener diode is used only in voltage regulator circuits. View Answer Discuss in Forum Workspace Report

32. A particular green LED emits light of wavelength 5490, Å, the energy bandgap of the semiconductor material used there is .. h = 6.6 x 10-34 J sec. A.

2.26 eV

B. 1.98 eV C. 1.17 eV D. 0.74 eV Answer: Option A Explanation:

From Plank equation

joule

to convert it into electron volt it will be divided by 1.6 x 10-19. View Answer Discuss in Forum Workspace Report

33. In a zener diode A.

the forward current is very high

B. sharp breakdown occurs at a certain reverse voltage C. the ratio v-i can be negative D. there are two p-n junctions Answer: Option B Explanation: When reverse voltage equals breakdown value it starts conducting and voltage does not increase further. View Answer Discuss in Forum Workspace Report

34. In a bipolar transistor which current is largest A.

collector current

B. base current C. emitter current D. base current or emitter current Answer: Option C Explanation: Emitter current is larger, collector current is slightly less than emitter current and base current is very small. View Answer Discuss in Forum Workspace Report

35. The v-i characteristics of a FET is shown in figure. In which region is the device biased for small signal amplification

A.

AB

B. BC C. CD D. BD Answer: Option B Explanation: Small signal amplifier operation is in constant current region of characteristics. 36. Secondary emission is always decremental. A.

True

B. False Answer: Option B Explanation: Sometimes it can be useful also. View Answer Discuss in Forum Workspace Report

37. In a degenerate n type semiconductor material, the Fermi level, A.

is in valence band

B. is in conduction band C. is at the centre in between valence and conduction bands D. is very near valence band Answer: Option B Explanation: This is due to high level of doping. View Answer Discuss in Forum Workspace Report

38. The types of carriers in a semiconductor are

A.

1

B. 2 C. 3 D. 4 Answer: Option B Explanation: Holes and electrons. View Answer Discuss in Forum Workspace Report

39. A potential of 7 V is applied to a silicon diode. A resistance of 1 K ohm is also in series with the diode. The current is A.

7 mA

B. 6.3 mA C. 0.7 mA D. 0 Answer: Option B Explanation:

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40. Assertion (A): The reverse saturation current in a semiconductor diode is 4nA at 20°C and 32 nA at 50°C. Reason (R): The reverse saturation current in a semiconductor diode doubles for every 10°C rise in temperature. A.

Both A and R are true and R is correct explanation of A

B. Both A and R are true but R is not a correct explanation of A C. A is true but R is false D. A is false but R is true Answer: Option A Explanation: At 20°C, 4 nA, at 30°C, 8 nA, at 40°C, 16 nA, at 50°C, 32 nA. 41. Calculate the stability factor and change in IC from 25°C to 100°C for, β = 50, RB/ RE = 250, ΔIC0 = 19.9 nA for emitter bias configuration. A.

42.53, 0.85 μA

B. 40.91, 0.58 μA C. 40.91, 0.58 μA D. 41.10, 0.39 μA Answer: Option A Explanation: SICO = (1 + β). 51.

= 42.53

ΔIC = (SICO).ΔICO = 42.53 x 19.9 nA = 0.85 μA. View Answer Discuss in Forum Workspace Report

42. A periodic voltage has following value for equal time intervals changing suddenly from one value to next... 0, 5, 10, 20, 50, 60, 50, 20, 10, 5, 0, -5, -10 etc. Then rms value of the waveform is A.

31 V

B. 32 V C. insufficient data D. none of these Answer: Option A Explanation:

RMS value = 965 = 31.064 Volt. View Answer Discuss in Forum Workspace Report

43. Work function of oxide coated cathode is much lower than that of tungsten cathode. A.

True

B. False Answer: Option A Explanation: Therefore emission current from oxide coated cathode is more. View Answer Discuss in Forum Workspace Report

44. The word enhancement mode is associated with A.

tunnel diode

B. MOSFET C. JFET D. photo diode Answer: Option B Explanation: MOSFET may be depletion mode or enhancement mode. View Answer Discuss in Forum Workspace Report

45. In which region of a CE bipolar transistor is collector current almost constant? A.

Saturation region

B. Active region C. Breakdown region D. Both saturation and active region Answer: Option B Explanation: It is used as amplifier when it operates in this region. 46. A p-n junction diode has A.

low forward and high reverse resistance

B. a non-linear v-i characteristics C. zero forward current till the forward voltage reaches cut in value D. all of the above Answer: Option D Explanation: A p-n Junction has all these features. View Answer Discuss in Forum Workspace Report

47. Which of the following is true as regards photo emission? A.

Velocity of emitted electrons is dependent on light intensity

B. Rate of photo emission is inversely proportional to light intensity C. Maximum velocity of electron increases with decreasing wave length

D. Both holes and electrons are produced Answer: Option C Explanation: As wavelength decreases, frequency increases and maximum velocity of electron increases. View Answer Discuss in Forum Workspace Report

48. The power dissipation in a transistor is the product of A.

emitter current and emitter to base voltage

B. emitter current and collector to emitter voltage C. collector current and collector to emitter voltage D. none of the above Answer: Option C Explanation: Maximum power dissipation occurs at collector junction. View Answer Discuss in Forum Workspace Report

49. The normal operation of JFET is A.

constant voltage region

B. constant current region C. both constant voltage and constant current regions D. either constant voltage or constant current region Answer: Option B Explanation: In major portion of drain characteristics ID is constant. View Answer Discuss in Forum Workspace Report

50. The minority carrier life time and diffusion constant in a semiconductor material are respectively 100 microsecond and 100 cm2/sec. The diffusion length is A.

0.1 cm

B. 0.01 cm C. 0.0141 cm D. 1 cm Answer: Option A Explanation:

Diffusion length =

.