Unit 9b tủ tài liệu training

Draw a circuit diagram of a transistor as an oscillator.. Draw the logic symbol of a NOT gate and write its truth table.. Write down the truth table of a NAND gate.. Give logic circuit o

VERY SHORT AND SHORT-ANSWERS QUESTIONS

51 Define a.c current gain βac for common-emitter configuration

52. What happens when both the emitter and the collector of a transistor are forward biased?

53. What is an electrical oscillator?

54. Draw a circuit diagram of a transistor as an oscillator

55. What is a digital circuit?

56. What is a logic gate?

57. What is a truth table?

59. Write down the truth table of an OR gate (A.I.S.S.C.E 1995, A.I.S.S.C.E Delhi 1991)

60. Draw the logic symbol of an AND gate (A.I.S.S.C.E 1990,1994)

61. How is an npn transistor represented symbolically? (A.I.S.S.C.E 1994)

62. In the circuits shown, the switch which is open represents the logic state 0 and the switch which

in closed represents the logic state 1 The lamp L is lit when output is logic state 1 What type of gates are represented by the circuits in (i) and (ii)? (A.I.S.S.C.E 1995)A

Fig 9.2 (ii)

B A

Fig 9.2 (i)

B

L

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63 Write down the truth table of an AND gate (A.I.S.S.C.E 1991)

64 How does the mobility of carriers in a semiconductor vary with temperature?

65 Draw the logic symbol of a NOR gate.

66 Write down the truth table of a NOR gate.

67 Draw the logic symbol of a NOT gate and write its truth table.

68 Given the Booean expression λ =A B. If A = B = 1, what is the value of λ

70 Write down the truth table of a NAND gate (A.I.S.S.C.E Delhi)

71 What is the relation between α and β?

72 What is dynamic resistance of a junction diode?

73 Drawing a labelled diagram, explain how a npn transistor can be used as an amplifier in

74 Explain, by drawing a labelled diagram, how pnp transistor can be used as an amplifier in

75 Briefly describe a solar cell.

76 Why semiconductors behave as insulators at absolute zero?

77 Why does the conductivity of a semiconductor increase with rise of temperature?

(C.B.S.E Sample Paper 1995)

78. Give the ratio of number of holes and number of conduction electrons in an intrinsic

79 In an intrinsic semiconductor where does the Fermi level lie?

80 Is the ratio of number of holes and number of conduction electrons in a p-type extrinsic

semiconductor more than, less than, or equal to 1? (A.I.S.S.C.E 1997)

81 Give symbolic representation of Zener diode.

82 Write one application of Zener diode.

83 What is tank circuit in an oscillator?

84 What is the expression for the frequency of oscillations produced by a tank circuit.

85 What do you mean by feedback?

86 What are various types of feedback?

87 What type of feedback is used in an oscillator?

88 What are integrated circuits?

89 Briefly describe a photodiode.

90 Show how NAND gate can be used as NOT gate (A.I.S.S.C.E Delhi 1999)

91 When both the inputs are high what is the output of NAND gate?

92 Give logic circuit of an AND gate using NAND gates only.

93 Give logic circuit of an OR gate using NAND gates only.

94 Write the function of base region of a transistor Why is this region made thin and slightly

95. Define the terms potential barrier and depletion region for a pn junction diode State how the thickness of the depletion region will change when the pn junction diode is

(i) forward biased

96 Explain with the help of a circuit diagram why the output voltage is out of phase with the input

voltage in a common-emitter transistor amplifier (A.I.S.S.C.E Delhi 1999)

97 With the help of a circuit diagram explain the working principle of a full wave rectifier.

(A.I.S.S.C.E 1999)

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ANSWERS

51 In common emitter configuration, the a.c current gain is defined as the ratio of small change in

collector current to the corresponding change in base current, i.e.,

C ac B

I I

δ

β = δ

52 The transistor will work as two p-n junctions with common base terminals.

53 Oscillator is an electronic device that generates electromagnetic oscillations of required

fre-quency It converts d.c power into a.c power

54 Transistor as an oscillator :

L′

L

C

VCE

K

VEB

Fig 9.10

55 A digital circuit is an electronic circuit in which the current and voltage signals have only two

levels, either on or off.

56. A digital circuit that follows a certain logical relationship between input and output voltages is

called a logic gate.

57. A table which shows all the input and output possibilities of a logic gate is called a truth table.

58 Logic symbol of OR gate is shown Fig 9.11.

59 The truth table of an OR gate is

60 Logic symbol of an AND gate is shown in Fig 9.12.

61 An n-p-n transistor is represented as shown in Fig 9.13.

B

Y A

B

Fig 9.12

Fig 9.13

62 Circuit (i) represents an AND gate and circuit (ii) represents an OR gate.

Y A

B

Fig 9.11

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63 Truth table for AND gate is

64 Mobility of carriers decreases slightly with rise in temperature.

65 Logic symbol of a NOR gate is shown in Fig 9.14.

66 The truth table of NOR gate is

67 Logic symbol of NOT gate is shown in Fig 9.15.

Truth table :

68 A = 1, B = 1, so A B = 1.

or A B = 0 i.e.. λ =0

69 Logic symbol for NAND gate: See Fig 9.16.

Y A

B

Fig 9.14

Y A

Fig 9.15

70 Truth table for NAND gate :

72 The dynamic resistance of a junction diode is defined as the ratio of the small change in voltage

to the corresponding small change in current Let δV be the small change in voltage which

produces a change δI in current Then the dynamic resistance Rd is

d

V R I

δ

= δ

73 n-p-n transistor as common-base amplifier

The signal to be amptified, Vi is applied in the emitter base circuit The output, Vo, is taken across the load in the collector, The variation in the input signal voltage causes variation in the emitter current, which in turn, produces variation in collector current and hence output voltage

( ) Voltage Gain

( )

V

V I R A

V I R

δ

= = δ

Y A

B

Fig 9.16

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Vi

VO

RL

IE

IB

IC

VCE

VBE Fig 9.17

L ac i

R

= α

where Ri is the input resistance and RL is the load resistance αac is the a.c current gain

74. Same as in Q.73 Circuit should be redrawn for a pnp transistor.

75 A solar cell is a pn junction diode in which one of the regions ( p or n) is made very thin The thin

region is called emitter and the other base Light is allowed to fall on the emitter Since the

emitter is very thin, light energy is not greatly absorbed before reaching the junction and a

current flows through the resistor RL Thus light energy is converted into electrical energy The magnitude of the current depends on the intensity of light

Soler cells can be used to charge storage batteries in daytime, which can be used for power during nights

I

Emitter

Base

P

n

P n +

Light

Fig 9.18

–

76 At absolute zero electrons do not have energies to move to conduction band and hence the

semiconductors do not show any conductivity

77 See Q 28.

78 1 : 1

79 In the middle of the forbidden gap.

80 It is more than one.

81.

Fig 9.19

82 Zener diode in used as a voltage regulator.

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83 The parallel combination of an inductor and a capacitor used in an oscillator is called tank circuit.

84.

1

2 LC

ν =

π

85. When a fraction of output is fed to the input of an electronic circuit, the process in called

feedback.

86. The feedback is of two types

:-(a) Positive feedback - The signal in feedback is in phase with the input signal.

(b) Negative feedback - The signal in feed back is 180o out of phase with the input signal

87. Positive feedback

88 Integrated circuits are complete electronic circuits fabricated on a single small semiconductor

wafer (chip)

89 Photodiode : It is a pn junction diode in which light is allowed to fall at the junction The diode

is reverse biased with voltage less than the breakdown voltage If the photon energy (hν) is more than the energy required to take an electron from the valence to the conduction band, the current

in the resistor R increases If the intensity of light is increased the current increases.

Light

Fig 9.20

90 A NAND gate works as NOT gate when the two inputs are joined together.

Y X

Fig 9.21

91 Zero.

92.

A

B

Y

Fig 9.22

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93.

Y A

B

Fig 9.23

94 The base controls the flow of electros or holes from the emitter to the collector Base is made

very thin and is doped lightly so that most of the carriers are attracted straight into the collector and very few combine in the base

95 Potential Barrier : See Q 21.

Depletion Region : See Q 31.

The thickness of the depletion region decreases when the junction diode is forward baised and increases when the diode is reverse biased

V O

V i

RL

IE

IC

IB

V CE

V O

V BE

Fig 9.24

When no input a.c signal is applied, the potential difference Vo across the collector and the emitter is given by

When the input signal Vi is applied, the forward bias increases during the positive half cycle

This results in an increase in IC and hence a decrease in Vo, according to Eq (1) Thus during the positive half cycle of the input, the collector becomes less positive

During the negative half cycle, the forward bias is decreased resulting in a decrease in IC and

hence an increase in VO Thus the collector becomes more positive

This shows that in a common-emitter amplifier, the output voltage is out of phase with the input voltage

97 See Q 6 (Long-Answer Type).