Download Electronics

AL-LQ-Electronics / p.1
1.
(88-IIB-11)
Figure (a)
Figure (b)
Figure (a) shows a simple amplifier circuit which consists of a NPN silicon transistor. The
input/output voltage characteristic of the transistor is shown in Figure (b).
(a)
The circuit is designed in such a way that, without any input signal, the d.c. potential at the
collector of the transistor (Vout) is 3 V.
(i)
Explain the advantage of setting the collector voltage at this value.
(2 marks)
(ii)
Assuming that the base current is negligible, what is the ratio of resistances R1
to R2? (2
marks)
(b)
An input signal from a signal generator is fed into the amplifier via a 10F capacitor. Explain the
use of such a large value capacitor.
(c)
(2 marks)
Suppose the signal generator superposes on Vin a voltage varying between
 0.1 V as shown in
Figure (c). Making use of Figure (b), sketch on Figure (c) the expected variation in voltage of the
(d)
output signal.
(2 marks)
Figure (c)
Estimate the current gain (  ) of the transistor within the linear region.
(2 marks)
(e)
Figure (d)
(i)
When the input signal voltage from the signal generator is gradually increased, the output
signal voltage appears in the form shown in Figure (d). Explain why this should happen.
(3 marks)
(ii)
Estimate the p.d. between the collector and the emitter ( VCE) and the collector current (IC)
during time interval t1 and t2 when the voltage curve appears ‘flat’. Give your answers in
the table below.
(2 marks)
Time interval
VCE
IC
t1
____________
_____________
t2
____________
_____________
AL-LQ-Electronics / p.2
2.
(89-IIB-11)
Consider the transistor circuit in Figure (a) which has an input-output characteristic curve as shown in Figure (b).
Figure (a)
Figure (b)
(a)
Account for the shape of the characteristic curve.
(b)
Suppose the transistor circuit shown in Figure (a) is symbolically represented by Figure (c).
Figure (c)
(3 marks)
Figure (d)
The transistor circuit is now connected to an RC circuit, as shown, with the capacitor initially
uncharged. Now at time = t1, S is switched from the OFF position to the ON position. S stays at
the ON position from time = t1 to time = t3. At time = t3, S is switched back to the OFF position
and remains there. It is found that at time = t2, the potential at point I has dropped below 1 V.
(i) Sketch on Figure (e) the variation of the potential at point I with respect to time, from
time = 0 to a time later than t3.
(ii)
(4 marks)
With the help of the input-output characteristic curve, sketch on Figure (f) the
corresponding variation of the output potential at O against time.
(3 marks)
AL-LQ-Electronics / p.3
3.
(90-IIB-11)
(a)
Draw a diagram of a circuit you could use to find the
input voltage Vi / output voltage Vo characteristic of an NPN transistor in the
common-emitter configuration.
(3 marks)
(b)
The results obtained are shown above. Suppose the circuit in this experiment is to be used as an alternating
voltage amplifier. The input voltage must first be fixed at a suitable value. Use the above graph to
(i)
choose the most suitable value for this fixed input voltage, giving an explanation , and
(2 marks)
(ii)
(c)
determine the voltage amplification.
(2 marks)
A sinusoidal voltage of amplitude 0.4 V is now superimposed on the fixed input voltage. The
input and output voltages are then observed on a CRO set in AC mode. The input voltage is
shown in Figure (a). On the same diagram, sketch the corresponding variations in output voltage
as observed on the CRO.
(d)
(2 marks)
If the amplitude of the superimposed sinusoidal voltage is increased to 1.5 V (Figure (b)), sketch
on the same diagram the corresponding variations in output voltage, as observed on the CRO.
(3 marks)
Figure (a)
Figure (b)
AL-LQ-Electronics / p.4
4.
(92-IIB-10)
The temperature of an air-conditioned room can be controlled by means of a thermostat circuit which
consists of a thermistor bridge circuit and an ideal operational amplifier OA. T is the thermistor. An
indicator LED light is to be switched on if the room temperature is higher than the preset value and air
conditioner is then switched on automatically.
(a)
(i)
With the variable resistor R set at its midpoint position, the LED lights up when the room
temperature is 25oC. If now R is decreased slightly the LED turn off automatically. Explain
why this happens. (You may assume that the LED lights when V0 > 0).
(ii)
Briefly explain how the circuit controls the switching on and the switching off of the
air-conditioner.
(3 marks)
(iii) What is the use of the variable resistor R in the bridge circuit?
(b)
(3 marks)
For a room temperature of
25oC,
(1 mark)
and with R set at its mid-position (R = 25 k), calculate the
potential at point X with reference to earth.
(3 marks)
AL-LQ-Electronics / p.5
5.
(93-IIB-11)
(a)
Figure (a) shows an operational amplifier circuit (power supply not shown.)
Figure (a)
(b)
(i)
What is the relation between the output voltage V out and the input voltage Vin?
(1 mark)
(ii)
Mention two different types of application of the above circuit.
(2 marks)
A voltmeter V, a 10 F capacitor C and a switch S are connected to the circuit as shown in
Figure (b).
Figure (b)
Suppose the input resistance of the operational amplifier, R = 2.0 M  , and the open-loop voltage
gain, A0 = 1.0  10 .
(i) When the switch is closed, the voltmeter reads 5.0 V. Find the initial quantity of charge on
5
the capacitor.
(ii)
(1 mark)
The circuit shown in Figure (b) can be represented by that in Figure (c).
Figure (c)
Show that the rate of change of charge on the capacitor is given by
dQ
Q

dt
(1  A0 ) RC
(4 marks)
(iii) Find the quantity of charge on the capacitor after the switch has been closed for 10 hours.
(3 marks)
AL-LQ-Electronics / p.6
6.
(94-I-10)
An ideal operational amplifier (op amp) is used in the circuit shown below. The two input potentials
are
VA  3 V and VB  4 V respectively. The power supply for the amplifier is provided by two
batteries, each of e.m.f. 15 V and zero internal resistance.
(a)
State TWO basic assumptions for an ideal op amp.
(2 marks)
(b)
Briefly explain why point X is said to be ‘virtually earthed’.
(2 marks)
(c)
(i)
Find the current flowing through the 10 k  resistor.
(ii)
Find the current flowing through the 50 k  resistor and state its direction of flow.
(1 mark)
(2 marks)
(d)
What is the output potential V0?
(e)
VA is now replaced by a square-wave voltage which varies between 0 V and +3 V (as shown
below). In the spaces provided, sketch the variation of the output potential, V0, with time, t, for
(i) VB unchanged (i.e. remains at –4 V);
(2 marks)
(ii) VB changed to +4 V.
(2 marks)
Label the V0 axes with appropriate values.
(2 marks)
AL-LQ-Electronics / p.7
7.
(95-I-5)
The figure shows a typical silicon transistor circuit together with its input/output voltage
characteristics.
(a)
If the input voltage Vi to the circuit is varied in the way shown on the graphs below, sketch the
variation of the output voltage V0
with time for each case. Label the axes with appropriate
scales.
(b)
(4 marks)
It is given that the current amplification factor of the silicon transistor is 80 and Rc=1.5k.
(i) When Vi is 1 V , what are the collector current and base current?
(ii)
Hence determine the value of RB. (The voltage between the base and the emitter is constantly
0.5 V when conducting.)
(c)
(3 marks)
If an input signal varying between
(2 marks)
 0.24 V and  0.24 V is to be accurately amplified
without distortion, two resistors and two capacitors are added to the original circuit. The
resulting circuit is shown below.
(i)
What should be the value of R1/R2 for providing an optimal bias voltage for the transistor?
(2 marks)
(ii)
What is the peak voltage of the output signal?
(2 marks)
AL-LQ-Electronics / p.8
8.
(98-I-9)
The figure shows a common-emitter transistor circuit and also its input/output voltage characteristics.
(a)
(i)
Find the voltage
Vc .
(ii)
What is the voltage between the base and the emitter when Vi = 1.5 V?
(1 mark)
(1 mark)
(iii) Find the voltage gain and the current amplification factor of this transistor circuit.. (4
marks)
(b)
In order to amplify the rectified a.c. signal shown below, two resistors and two capacitors are
added to the circuit as shown in the figure.
(i)
State the function of the capacitors in the circuit.
(ii)
What is the maximum value of V p such that the signal can be amplified without chopping
off the peaks?
(iii) Draw the corresponding output signal in the space provided if V p = 0.2 V
(1 mark)
(2 marks)
(2 marks)
AL-LQ-Electronics / p.9
9.
(99-I-4)
(a)
Figure (a) shows an operational amplifier circuit.
Figure (a)
A graph of output voltage Vout plotted against input voltage Vin is shown below.
(i)
What is the resistance of the resistor Rf ?
(2 marks)
(ii)
With reference to the graph, explain the function of the above circuit.
(3 marks)
(b)
Figure (b)
Figure (b) shows the circuit of a comparator. The LED lights up when the input voltage Vin is less
than 4.5 V. Find the resistance of R.
(c)
(2 marks)
A stable power supply is essential for the operation of computer parts, which usually work at
5 V d.c. A fluctuation in the supply voltage Vin of more than 10% is certainly not tolerable, so it is
therefore important to keep a regular check on it. Figure (c) shows a warning device designed for
such a purpose.
Figure (c)
(i)
State the potentials at P and at Q. Briefly explain the operation of the circuit.
(ii)
Calculate the resistance of the resistors R1 and R2.
(3 marks)
(3 marks)
AL-LQ-Electronics / p.10
10.
(00-I-10)
Figure (a) shows an operational amplifier circuit.
Figure (a)
(a)
(i)
(ii)
What is meant by negative feedback in an operational amplifier circuit? State TWO effects of using
negative feedback.
(3 marks)
What will be the effect on the above circuit if Rf increases?
(1 mark)
(iii) Find Vout when Vin equals
(I)
10 mV
(II)
2V
(2 marks)
(iv) The graph below represents the variation of the input voltage Vin with time. Sketch the corresponding
variation of the output voltage Vout.
(b)
(i)
(2 marks)
The circuit in Figure (a) can be used to amplify the signals of the order of 10 mV from an electronic
stethoscope monitoring heart beats. An earphone requiring a peak voltage of at least 0.5 V to operate
is connected to the output of the circuit. What should be the minimum resistance value of the
feedback resistor Rf?
(ii)
(2 marks)
Although a step-up transformer can give a larger output voltage from a small input voltage, it cannot
serve the same purpose as the amplifier circuit in Figure (a). Explain briefly.
(2 marks)
AL-LQ-Electronics / p.11
11.
(01-IB-9)
(a)
Figure 9.1 shows a circuit in which a light dependent resistor (LDR) is connected in series with a 8kΩ
resistor and a 6V battery. The resistance of the LDR is 500kΩin the dark.
Figure 9.1
(i) When the LDR is in the dark, the voltage across the LDR is approximately 6V, 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.
(ii) The voltage across the LDR is 2V when the illumination is high. Find the resistance of the LDR at this
level of illumination.
(b) (i) Describe the difference in function between an operational amplifier used as an amplifier and one used as
a comparator.
(ii) With an operational amplifier used as a comparator, draw a switching circuit for a street light powered by
220V a.c. mains which allows both manual and automatic operation. Briefly describe how the circuit
works.
AL-LQ-Electronics / p.12
12.
(03-IIB-5)
(a)
Explain why a typical operational amplifier (op amp) does not alter the value of the value of the voltage
applied to its input(s) while its output voltage can be transferred with little loss to a load greater than a few
kilohms.
(2 marks)
(b)
+15 V
V1
+
V2
Vo
-15 V
0V
(i)
What is the function of the op amp circuit in Figure 5.1?
(1 mark)
(ii)
If the open-loop voltage gain of the op amp is 10 and its output voltage saturates at  13 V ,
calculate the minimum voltage difference between V1 and V2 for saturating the op amp. (2 marks)
5
(iii) A sinusoidal a.c. voltage of peak value 200 V is applied at V1. Sketch the output voltage Vo
when
(I)
V2 is connected to the ground;
(II)
V2  100 V .
(4 marks)
V1/V
200
0
T time
(I)
Vo/V
0
T time
(II)
Vo/V
0
(c)
T time
Figure 5.2 shows an op amp circuit and the table indicates how the output varies with the inputs.
AL-LQ-Electronics / p.13
Figure 5.2
Rf
10 k
V1
+15 V
R1
+
V2
Vo
-15 V
0V
V1/V
V2/V
Vo/V
+1
-2
-1
-1
+2
+3
(i)
What is the major function of the resistor Rf?
(ii)
Write down an equation to show how V1, V2 and Vo are related. Find the resistance values of resistors
R1 and Rf.
(1 mark)
(4 marks)