HKAL Exercise : Part 4 Electricity and Electromagnetism
Chapter 18 Electronics
Revision
18.1 Diodes 18.7 Logic Gates
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HKAL Exercise Chapter 18 Electronics
1.
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 :
What is the logic gate ?
A. NOT gate
B. NOR gate
C. NAND gate
D. none of the above
2. A thermionic diode is operated at 150 V and the current through it is 40 mA. How many electrons reach the anode in one second ?
(Electronic charge = -1.6 × 10-19 C.)
A. 2.50 × 1017 B. 6.00 × 1017
C. 2.50 × 1020 D. 6.00 × 1020
3.
Which of the following pairs of inputs would give a LOW output from the above combination of NAND gates ?
Input A Input B
(1) LOW HIGH
(2) LOW LOW
(3) HIGH HIGH
A. (2) only
B. (3) only
C. (1) and (2) only
D. (2) and (3) only
4.
For the circuit shown above, which of the graphs shown below best represents the variation of current I with time t?
A. B.
C. D.
5. Which of the following circuits can give a half-wave rectification waveform on a C.R.O. ?
A. B.
C. D.
6.
In the above circuit, the waveform across the diode shown on the screen of the CRO should be
A. B.
C. D.
7.
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.
C. D.
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HKAL Exercise Chapter 18 Electronics
(Structural question)
8. Figure 8.1 shows a circuit in which a light dependent resistor (LDR) is connected in series with a 10 kΩ resistor and a 8 V battery. The resistance of the LDR is 400 kΩ in the dark.
Figure 8.1
(a) When the LDR is in the dark, the voltage across the LDR is approximately 8 V, which takes the same value as the e.m.f. of the battery. Briefly explain the result and suggest a suitable instrument for the measurement of this voltage. (2 marks)
(b) The voltage across the LDR is 6 V when the illumination is high. Find the resistance of the LDR at this level of illumination. (1 mark)
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HKAL Exercise Chapter 18 Electronics
9.
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.
C. D.
10.
In the above circuit, D is an ideal diode, R is a resistor of resistance 10 kΩ and the alternating e.m.f. is represented by the equation
E = 15 cos (2πt),
where E is in volts and t is in seconds. The power dissipated in R is
A. 22.5 mW. B. 11.3 mW.
C. 5.6 mW. D. zero.
11.
In the above circuit, if the sinusoidal a.c. source has a peak-to peak voltage of 15 V, the r.m.s. current through the 40 Ω resistor is
A. 0.05 A.
B. 0.07 A.
C. 0.09 A.
D. 0.19 A.
12.
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. 0.5 V.
B. 1.0 V.
C. 2.0 V.
D. 2.0V.
13. A ‘black box’ containing two unknown components is connected to a cell, a resistor and an ammeter as shown.
A current flows steadily no matter which way the box’s terminals are connected to the cell and the same ammeter reading is 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 series.
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HKAL Exercise Chapter 18 Electronics
Full-wave rectification
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HKAL Exercise Chapter 18 Electronics
14. 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)
(3)
A. (1) only
B. (1) and (3) only
C. (2) and (3) only
D. (1), (2) and (3)
15.
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
(1) (2)
A. I II
B. II I
C. II III
D. III III
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HKAL Exercise Chapter 18 Electronics
Smoothing Circuit
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HKAL Exercise Chapter 18 Electronics
16.
The above figure shows a half-wave rectifier with a smoothing circuit. The time constant of C and R should be
A. small compared with the time of one cycle.
B. large compared with the time of one cycle.
C. equal to half the time of one cycle.
D. equal to twice the time of one cycle.
17.
In the above a.c. smoothing circuit, the ripple on the current passing through the load R can be reduced by
(1) decreasing the capacitance C.
(2) increasing the load resistance R.
(3) increasing the a.c. supply frequency.
A. (1) only
B. (3) only
C. (1) and (2) only
D. (2) and (3) only
18.
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.
C. D.
19. The graph shows the transfer characteristic of an electronic device.
The input is a sinusoidal voltage with a peak value of 1.0 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.
C. D.
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HKAL Exercise Chapter 18 Electronics
18.2 Transistors
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HKAL Exercise Chapter 18 Electronics
20. 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 ib respectively. What are the possible values of ie/ib and ic/ie as obtained from the measurement of the various currents?
ie/ib ic/ie
A. 0.99 100
B. 100 0.99
C. 1 1
D. 100 1.01
21.
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 emitter. The current gain for this transistor is
A. 80.
B. 200.
C. 400.
D. 500.
22.
The circuit above shows an NPN transistor and two resistors 3 kΩ and 20 kΩ connected to a 4 V d.c. supply. The current gain of the transistor is 50. What is the value of the collector current ?
A. 8.3 μA
B. 10 μA
C. 0.2 mA
D. 1.3 mA
23.
In the above transistor circuit, the voltage across the base and the emitter is 1 V when the transistor conducts. The current amplification factor of the transistor is 50. If the output voltage is 4 V, what is the value of R ?
A. 600 kΩ
B. 188 kΩ
C. 63 kΩ
D. 21 kΩ
24.
In the transistor circuit shown above , the voltages VAE, VPE, VCE and VBE were measured and tabulated as follows:
VAE / VPE / VCE / VBE4.1 V / 2.9 V / 2.7 V / 0.8 V
What are the values for the base current IB and the collector current IC?
IB / mA IC / mA IB / mA IC / mA
A. 0.15 68.3 B. 0.15 23.3
C. 0.11 68.3 D. 0.11 23.3
25.
In the above circuit, the sliding contact S is moved between X and Y to give different voltages across ce. Which of the following graphs best represents the variation of the collector current Ic with the voltage Vce across the emitter and the collector ?
A. B.
C. D.
26.
In the above circuit, the reading of the voltmeter is zero. What should be the voltages applied at Input 1 and Input 2 respectively ?
Input 1 Input 2 Input 1 Input 2
A. 0 V 0 V B. 3 V 3 V
C. 3 V 6 V D. 6 V 0 V
27.
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 cell E1 should be connected the other way round.
C. The collector current is very much less than the emitter current.
D. The base current is very much less than the emitter current.
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HKAL Exercise Chapter 18 Electronics
18.3 Applications of Transistors
Linear voltage amplification
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HKAL Exercise Chapter 18 Electronics
28. A NPN transistor is operated as a linear voltage amplifier and the output voltage is displayed on a CRO screen as shown :
What changes will occur when RL is slightly decreased ?
Vdc Vpp
A. increases increases
B. decreases increases
C. increases decreases
D. decreases decreases
29.
In the above circuit, if the current amplification factor β = 100, what is the output voltage Vout ?
A. 0 V
B. 0.5 V
C. 7.5 V
D. 8.0 V
30.
An n-p-n transistor is used in the above circuit as a pulse shaper or a squarer. When a sinusoidal voltage whose magnitude varies between +4 V and –4 V is applied to the input, what will be the output voltage ?
A. B.
C. D.
31.
The figure shows a typical voltage-amplifier circuit built from a transistor with current gain 100. It operates normally on a quiescent collector current of 2 mA and with the base-emitter junction voltage VBE = +0.8 V. What should be the value of the base-bias resistor RB ?
A. 250 kΩ B. 210 kΩ
C. 40 kΩ D. 21 kΩ
32.
The above diagrams show an NPN transistor circuit and its input/output voltage characteristic. What is the current amplification factor of the transistor ?
A. 20 B. 30
C. 60 D. 120
33.
In the above transistor circuit, the voltage across the base and the emitter is 1.0 V when the transistor works. The current amplification factor of the transistor is 90. What input voltage (Vin) would give an output voltage (Vout) of 4.5 V ?
A. 0.83 V B. 1.17 V
C. 1.50 V D. 1.67 V
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HKAL Exercise Chapter 18 Electronics
18.4 Operational Amplifiers
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HKAL Exercise Chapter 18 Electronics
34. Which of the following statements about an operational amplifier is/are correct ?
(1) For d.c., the open loop voltage gain is of the order 105.
(2) For a.c., the open loop voltage gain increases with decreasing frequency.
(3) It amplifies the voltage at the non-inverting input.
A. (1) only
B. (1) and (2) only
C. (2) and (3) only
D. (1), (2) and (3)
35.
In the operational amplifier circuit shown the feedback is provided by a potentiometer PQ. The gain of the amplifier circuit is
(1) dependent on the resistance value of the potentiometer PQ.
(2) infinite when the sliding contact is at P.
(3) zero when the sliding contact is at Q.
A. (1) only
B. (2) only
C. (1) and (2) only
D. (2) and (3) only
36.
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 ?
A. B.
C. D.
Light Sensitive Switch
37.
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) Decreasing the light intensity on the LDR.
(2) Increasing the value of R3.
(3) Decreasing the value of R1.
A. (2) only
B. (3) only
C. (1) and (2) only
D. (2) and (3) only
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HKAL Exercise Chapter 18 Electronics
18.5 Negative Feedback
Negative feedback Inverting Amplifier
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HKAL Exercise Chapter 18 Electronics
38. Which of the following is/are the advantage(s) of using negative feedback in an operational amplifier for voltage amplification ?
(1) greater stability
(2) no distortion of the output
(3) predictable gain
A. (1) only
B. (3) only
C. (1) and (3) only
D. (2) and (3) only
39.
In the circuit above, what is the voltage amplification ?
A. 50 000 with inversion
B. 2.5 with inversion
C. 50 000 without inversion
D. 2.5 without inversion
40.
The above figure shows an operational amplifier circuit which uses a 15 V supply (not shown). If the input potential is +0.5 V, what is the potential at point X ?
A. -0.5 V B. -4 V
C. +0.5 V D. +4 V
43.
In the above operational amplifier circuit, the output voltage Vout is
A. –2 V. B. –1 V.
C. + 1 V. D. + 2 V.
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HKAL Exercise Chapter 18 Electronics
18.6 Non-Inverting Amplifiers