Download 香港考試局

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 = Asint
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