Unit 9b tủ tài liệu training

VERY SHORT AND SHORT-ANSWERS QUESTIONS 51 52 53 54 55 56 57 58 59 60 61 62 Define a.c current gain βac for common-emitter configuration What happens when both the emitter and the collector of a transistor are forward biased? What is an electrical oscillator? Draw a circuit diagram of a transistor as an oscillator What is a digital circuit? What is a logic gate? What is a truth table? Draw the logic symbol of an OR gate (A.I.S.S.C.E 1991) Write down the truth table of an OR gate (A.I.S.S.C.E 1995, A.I.S.S.C.E Delhi 1991) Draw the logic symbol of an AND gate (A.I.S.S.C.E 1990,1994) How is an npn transistor represented symbolically? (A.I.S.S.C.E 1994) In the circuits shown, the switch which is open represents the logic state and the switch which in closed represents the logic state The lamp L is lit when output is logic state What type of gates are represented by the circuits in (i) and (ii)? (A.I.S.S.C.E 1995) A A B B L Fig 9.2 (i) Fig 9.2 (ii) S Chand & Company Limited 63 64 65 66 67 Write down the truth table of an AND gate (A.I.S.S.C.E 1991) How does the mobility of carriers in a semiconductor vary with temperature? Draw the logic symbol of a NOR gate Write down the truth table of a NOR gate 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 69 70 71 72 73 74 75 76 77 78 79 80 Draw the logic symbol of a NAND gate (A.I.S.S.C.E 1993) Write down the truth table of a NAND gate (A.I.S.S.C.E Delhi) What is the relation between α and β? What is dynamic resistance of a junction diode? Drawing a labelled diagram, explain how a npn transistor can be used as an amplifier in common-base configuration (A.I.S.S.C.E Delhi 1997) Explain, by drawing a labelled diagram, how pnp transistor can be used as an amplifier in common-base configuration (A.I.S.S.C.E Delhi 1997) Briefly describe a solar cell Why semiconductors behave as insulators at absolute zero? Why does the conductivity of a semiconductor increase with rise of temperature? (C.B.S.E Sample Paper 1995) Give the ratio of number of holes and number of conduction electrons in an intrinsic semiconductor (A.I.S.S.C.E 1997) In an intrinsic semiconductor where does the Fermi level lie? 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) S Chand & Company Limited 81 82 83 84 85 86 87 88 89 90 91 92 93 94 Give symbolic representation of Zener diode Write one application of Zener diode What is tank circuit in an oscillator? What is the expression for the frequency of oscillations produced by a tank circuit What you mean by feedback? What are various types of feedback? What type of feedback is used in an oscillator? What are integrated circuits? Briefly describe a photodiode Show how NAND gate can be used as NOT gate (A.I.S.S.C.E Delhi 1999) When both the inputs are high what is the output of NAND gate? Give logic circuit of an AND gate using NAND gates only Give logic circuit of an OR gate using NAND gates only Write the function of base region of a transistor Why is this region made thin and slightly doped? (A.I.S.S.C.E Delhi 1998) 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 (ii) reversed biased (A.I.S.S.C.E 1998) 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) S Chand & Company Limited 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., δI C δI B 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 frequency It converts d.c power into a.c power 54 Transistor as an oscillator : βac = L′ VCE L K C VEB Fig 9.10 S Chand & Company Limited 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 A B Y A 0 Y 1 B 1 Fig 9.11 1 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 C E A Y B Fig 9.12 B Fig 9.13 62 Circuit (i) represents an AND gate and circuit (ii) represents an OR gate S Chand & Company Limited 63 Truth table for AND gate is A B Y 0 0 1 0 1 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 Y′ A B Y 0 1 1 1 1 67 Logic symbol of NOT gate is shown in Fig 9.15 Truth table : A Y 1 A Y B Fig 9.14 A 68 A = 1, B = 1, so A B = or A B = i.e λ = 69 Logic symbol for NAND gate: See Fig 9.16 S Chand & Company Limited Y Fig 9.15 70 Truth table for NAND gate : A 0 1 71 β= B 1 α , 1− α Y′ 0 Y 1 α= A Y B Fig 9.16 β 1+β 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 Rd = δV δI 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 AV = Vo (δI C ) RL = Vi (δI E ) RC S Chand & Company Limited IE VO RL Vi IB VCE IC VBE or AV = α ac Fig 9.17 RL , Ri 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 S Chand & Company Limited Li gh t Emitter + P n – I P RL n Base Fig 9.18 76 At absolute zero electrons not have energies to move to conduction band and hence the semiconductors not show any conductivity 77 See Q 28 78 : 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 S Chand & Company Limited 83 The parallel combination of an inductor and a capacitor used in an oscillator is called tank circuit 84 ν= 2π LC 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 S Chand & Company Limited Li gh t R Fig 9.20 90 A NAND gate works as NOT gate when the two inputs are joined together X Y Fig 9.21 91 Zero 92 A Y B Fig 9.22 S Chand & Company Limited 93 A Y 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 S Chand & Company Limited 96 IC IB VO RL Vi VO IE Input Output VCE VBE Fig 9.24 When no input a.c signal is applied, the potential difference Vo across the collector and the emitter is given by .(1) VO = VCE – IC RL 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 (Long-Answer Type) S Chand & Company Limited