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HKAL Extra Exercise : Part 4 Electricity and Electromagnetism Chapter 18 Electronics 18.1 Logic Gates 1. IN 1 What is the logic gate ? A. NOT gate B. OR gate C. NOR gate D. AND gate output IN 2 A certain logic gate with two inputs IN 1, IN2 and an output is shown above. The variation of the input and output voltages are as follows : 2. A B IN 1/V output 6 0 Which of the following pairs of inputs would give a HIGH output from the above combination of NAND gates ? Input A Input B (1) LOW LOW (2) HIGH LOW (3) HIGH HIGH time IN 2/V 6 0 time A. B. C. D. output/V 6 0 (1) only (3) only (1) and (2) only (2) and (3) only time 18.2 Diodes Thermionic Diodes 3. A thermionic diode is operated at 40 V and the current through it is 60 mA. How many electrons reach the anode in one second ? (Electronic charge = -1.6 × 10-19 C.) A. 2.40 × 1017 B. 3.75 × 1017 HKAL Exercise Chapter 18 Electronics C. 2.40 × 1020 D. 3.75 × 1020 1/12 Semi-conductor diodes 4. the diode shown on the screen of the CRO 2V should be A. B. C. D. 4V a.c. For the circuit shown above, which of the graphs shown below best represents the variation of current I with time t? A. B. I 7. 1 k I A B 1 k 0 t 0 C. t Which of the following graphs best represents the variation with time t of the current I through the segment AB in the above circuit ? A. B. D. I I 0 t t 0 I 5. I Which of the following circuits can give a half-wave rectification waveform on a C.R.O. ? A. t B. To C.R.O. t C. D. I I t t To C.R.O. 8. C. D. I To C.R.O. 6. a.c. source V To C.R.O. to CRO 100 k Which of the following combinations of a cell, an ideal diode and a resistor will give the above I-V relationship, where I and V are the applied current and voltage respectively ? A. B. + - + - In the above circuit, the waveform across HKAL Exercise Chapter 18 Electronics 2/12 C. D. - + 11. - + I / mA E = Asint 9. D1 D1 R D R In the above circuit, D is an ideal diode, R is a resistor of resistance 20 kΩ and the alternating e.m.f. is represented by the equation E = 5 sin (8πt), where E is in volts and t is in seconds. The power dissipated in R is A. 1.25 mW. B. 0.31 mW. C. 0.16 mW. D. zero. 0 1.2 V/V In the circuit shown above, D1, D2 are two diodes used to rectify a sinusoidal a.c. supply. Each of the diodes has the I-V characteristic as shown. For a current to flow through R, the minimum value of A is A. 0.6 V. B. 1.2 V. C. 2.4 V. D. 0.6 2 V. 12. A ‘black box’ containing two unknown components is connected to a cell, a resistor and an ammeter as shown. black box A 10. 5 In the above circuit, if the sinusoidal a.c. source has a peak-to peak voltage of 25 V, the r.m.s. current through the 5 Ω resistor is A. 0.88 A. C. 1.77 A. B. D. HKAL Exercise Chapter 18 Electronics 1.25 A. 2.50 A. A current flows steadily no matter which way the box’s terminals are connected to the cell and different ammeter readings are obtained. The two components in the ‘black box’ could be A. two diodes in parallel. B. two capacitors in parallel. C. a diode and a resistor in parallel. D. a diode and a capacitor in parallel. 3/12 Full-wave rectification 13. S is a source of alternating voltage. In which of the following circuits will the current flowing through component X be rectified, i.e. flowing in one direction only ? (1) (2) S X S X 14. R The figure shows a bridge rectifier circuit in which all the diodes are assumed ideal. The source is a sinusoidal a.c. supply. Which of the following traces (I, II or III) would be displayed on a CRO connected across the load resistor R if (1) the diode D were reversed in the circuit, (2) the diode D were removed leaving a break in the circuit ? I II III (3) S X A. B. C. D. (1) only (3) only (1) and (2) only (2) and (3) only A. B. (1) I I (2) II III C. D. II III III I Smoothing Circuit 15. input C load resistance R The above figure shows a half-wave rectifier with a smoothing circuit. The time constant of C and R should be A. large compared with the time of one cycle. B. equal to the time of one cycle. C. equal to half the time of one cycle. D. small compared with the time of one cycle. HKAL Exercise Chapter 18 Electronics 4/12 16. C. D. to CRO C to CRO R 18. The graph shows the transfer characteristic of an electronic device. In the above a.c. smoothing circuit, the ripple on the current passing through the load R can be reduced by (1) increasing the a.c. supply frequency. (2) decreasing the load resistance R. (3) decreasing the capacitance C. A. C. D. (1) only B. (1) and (2) only (2) and (3) only output voltage/V +4 +3 +2 +1 (3) only -4 -3 -2 -1 0 1 2 input voltage/V 3 4 The input is a sinusoidal voltage with a peak value of 2.5 V and a mean value of zero. Which one of the following waveforms best represents the variation of the output voltage with time ? A. B. 17. When a CRO is connected to a circuit, the trace obtained is as shown above. To which of the following circuits has the CRO been connected ? A. B. to CRO C. D. to CRO 18.3 Transistors 19. A transistor is used for current amplification in the common emitter configuration. The measured currents through the emitter, collector and base of the transistor are ie, ic and ie respectively. What are the possible values of ie/ic and ib/ie as obtained from the measurement of the various currents? ie/ic ib/ie A. 0.5 0.5 B. 20 1.05 C. 20 0.95 D. 0.95 20 HKAL Exercise Chapter 18 Electronics 5/12 20. In the above transistor circuit, the voltage across the base and the emitter is 2 V IC / mA 20 15 10 5 0 when the transistor conducts. The current amplification factor of the transistor is 40. If the output voltage is 1 V, what is the value of R ? A. 1.3 kΩ B. 5.0 kΩ C. 7.5 kΩ D. 10.0 kΩ IB = 45 A IB = 30 A IB = 15 A VCE / V The graph show the characteristics for a transistor operating in the common emitter mode. IC is the collector current, IB is the base current and VCE is the potential difference between the collector and 23. L2 emitter. The current gain for this transistor is A. 75. B. 167. C. 300. D. 333. E1 +7 V 3 k 30 k C. 0 D. The circuit above shows an NPN transistor and two resistors 3 k Ω and 30 k Ω C. 0.2 mA D. 2.3 mA E2 The diagram shows a transistor circuit with two similar light bulbs L1 and L2. The bulb L2 lights up brightly, but L1 does not glow at all. Which of the following could be a possible reason for this? A. The filament of L1 is burnt out. B. The current gain is very high. 21. connected to a 7 V d.c. supply. The current gain of the transistor is 150. What is the value of the collector current ? A. 31.5 μA B. 35 μA L1 The cell E1 should be connected the other way round. The cell E2 should be connected the other way round. 24. X R 2V S b c 5V e 22. 4V R 200 k Vout 0V HKAL Exercise Chapter 18 Electronics Y In the above circuit, the sliding contact S is moved between X and Y to give different voltages across be. Which of the following graphs best represents the variation of the base current Ib with the 6/12 voltage Vbe across the base and the emitter ? A. 26. +6 V B. Ib R Ib 0 0 Vbe C. 0 Vbe Input 2 R V 0V In the above circuit, the reading of the voltmeter is 6 V. What should be the voltages applied at Input 1 and Input 2 Ib 0 R Vbe D. Ib Input 1 Vbe 25. respectively ? Input 1 A. 0 V B. 0 V C. 3 V D. 6 V Input 2 0V 6V 3V 6V A + 6V R d.c. - 50 25 k P C B E In the transistor circuit shown above , the voltages VAE, VPE, VCE and VBE were measured and tabulated as follows: VAE VPE VCE VBE 3.5 V 2.5 V 2.3 V 0.7 V What are the values for the base current IB and the current gain? IB / mA current gain A. 0.07 240 B. 0.10 240 C. 0.07 343 D. 0.10 343 HKAL Exercise Chapter 18 Electronics 7/12 18.4 Applications of Transistors : Linear voltage amplification 27. A NPN transistor is operated as a linear 29. voltage amplifier and the output voltage is displayed on a CRO screen as shown : +6 V 2.3 k RL Rb Vout 1.5 V Vin 13 k 6V Vout Vin 0V An n-p-n transistor is used in the above circuit as a pulse shaper or a squarer. When a Vout Vpp 3V Vdc time What changes will occur when Rb is slightly increased ? Vdc Vpp A. decreases decreases B. increases decreases C. decreases increases D. increases increases sinusoidal voltage whose magnitude varies between +2 V and –2 V is applied to the input, what will be the output voltage ? A. B. Vout Vout time 0 time 0 -6 V C. D. Vout Vout 4V 0 28. 6V time 2V 0 -2 V time +6 V 1.9 k 20 k Vin 0.7 V 30. Vout = 3.8 V +6 V RB 0V In the above circuit, if the current amplification factor β = 120, what is the input voltage Vin ? A. 0 V B. 0.51 V C. 0.89 V D. 6 V HKAL Exercise Chapter 18 Electronics C B E 0V The figure shows a typical voltage-amplifier circuit built from a transistor with current gain 120. It operates normally on a quiescent collector current of 4 mA and with the base-emitter 8/12 junction voltage VBE = +0.8 V. What should be the value of the base-bias resistor RB ? A. 108 kΩ C. 180 kΩ B. D. 32. +6 V 3 k 156 kΩ 240 kΩ 18 k Vout 0V Vin 31. Vout / V In the above transistor circuit, the voltage across the base and the emitter is 0.4 V when the transistor works. The current amplification factor of the transistor is 60. +4 V 6 4 5 k 2 20 k Vout Vin 0V 0 1 2 3 Vin / V The above diagrams show an NPN transistor circuit and its input/output voltage characteristic. What is the current amplification factor of the transistor ? A. 5 B. 20 C. 40 D. 80 What output voltage (Vout) would be with an input voltage (Vin) of 0.6 V ? A. 0 V B. 2.0 V C. 4.0 V D. 5.3 V 18.5 Operational Amplifiers 32. Which of the following statements about an operational amplifier is/are correct ? (1) For a.c., the open loop voltage gain decreases with increasing frequency. (2) For d.c., the open loop voltage gain is of the order 10. (3) It amplifies the voltage at the inverting input. A. (1) only B. (3) only C. D. (1) and (2) only (2) and (3) only 33. V1 / V 5 0 1 2 3 4 V2 / V 10 0 1 2 3 4 time/ms +15 V V1 V2 + Vout -15 V Two electrical signals V1 and V2 are fed into an operational amplifier. The variations of V1 and V2 with time are shown above. Which of the following graphs represents the variation of the output Vout with time ? time/ms HKAL Exercise Chapter 18 Electronics 9/12 A. B. Vout Vout 0 time 0 C. D. Vout time (1) only (2) only C. D. (1) and (2) only (2) and (3) only Light Sensitive Switch 35. Vout time 0 0 A. B. time V+ Thermistor R1 + 34. R3 Vin P Q R2 V+15 V Vout -15 V In the operational amplifier circuit shown the feedback is provided by a potentiometer PQ. The gain of the amplifier circuit is (1) one when the sliding contact is at Q. (2) negative infinite when the sliding contact is at P. (3) independent of the resistance value of the potentiometer PQ. Vout 0V In the above circuit, the output voltage from the operational amplifier is negative. Which of the following changes could cause the output voltage to change to positive ? (1) Increasing the temperature of the thermistor. (2) Decreasing the value of R2. (3) Increasing the value of R3. A. (1) only B. (1) and (2) only C. (2) and (3) only D. (1), (2) and (3) 18.6 Negative Feedback Negative feedback operational amplifier 36. Which of the following is/are the advantage(s) of using negative feedback in an operational amplifier for voltage amplification ? (1) larger voltage gain (2) less distortion of the output (3) greater stability A. (1) only B. (3) only C. (1) and (2) only D. (2) and (3) only HKAL Exercise Chapter 18 Electronics 10/12 37. 40. 40 k 600 k 30 k +15 V 10 k + + 10 k In the circuit above, what is the voltage amplification ? A. 20 with inversion B. 600 000 with inversion C. 20 without inversion D. 600 000 without inversion V2 20 k X 0V 40 k what is the potential at P ? A. 0 V B. 0.54 V C. 1.56 V D. 1.62 V 120 k + Vo The figure shows an operational amplifier circuit. When V1 = +2.1 V and Vo = -0.6 V, 38. +0.2 V -15 V V1 41. +24 V The above figure shows an operational amplifier circuit which uses a 15 V supply (not shown). If the input potential is +0.2 V, what is the potential at point X ? A. -1.2 V B. -0.2 V C. +1.2 V D. +0.2 V 4 k 10 k 4 k 20 k 30 k + 4 k Vout 39. -24 V 4 k +15 V Y X Vin + -15 V Vout 2 k In the above operational amplifier circuit, the output voltage Vout is A. –12 V. B. –6 V. C. D. + 6 V. + 12 V. An input voltage (Vin) of 8.0 V is applied to an ideal operational amplifier connected as shown. The output voltage Vout is A. -16 V. B. 16 V. C. -24 V. D. 24 V. HKAL Exercise Chapter 18 Electronics 11/12 18.7 Non-Inverting Amplifiers 42. An operational amplifier is connected as shown below with input voltage Vi = +4 V. 30 k +15 V 10 k Vi 5 k Vo + -15 V 0V What is the output voltage Vo ? A. -6 V B. -8 V C. +6 V D. +12 V 43. A circuit is required for doubling the amplitude of a sinusoidal alternating voltage. Which of the following circuits might be suitable ? (1) (2) + 20 k input output output input 500 turns 10 k 1000 turns (3) 5 k output input 5 k A. Key : (1) only B. (2) only C. (1) and (2) only D. (2) and (3) only 1 – 10 : DABAC, BDBBB, 11 – 20 : BCCDA, ACDCD, 21 – 30 : DCBAC, ABCBB, 31 – 40 : CACDD, DAADC, 41 – 43 : ADB. HKAL Exercise Chapter 18 Electronics 12/12