Download Revision on Op-amp

Exercise 16(Revision on Op-amp.)
95’ MC
36.
43. An operational amplifier is connected as shown below with
100 k
input voltage Vi = +2V.
10 k 

+ 0.5 V
X
+
60 k 
+15 V
10 k 
Vi
0V
_
+
Vo
20 k 
-15 V
The above figure shows an operational amplifier circuit
0V
which uses a 15 V supply (not shown).
If the input
potential is +0.5 V, what is the potential at point X?
What is the output voltage Vo?
A.
-5 V
A. -6 V
B.
-15 V
B. -4 V
C.
+0.5 V
C. +4 V
D.
+5 V
D. +6 V
E.
+15 V
E. +12 V
93’ MC
94’ MC
41.
V1 /V
35. Which of the following statements about an operational
amplifier is/are correct?
10
+15 V
0
(1) It amplifies the difference between the voltages at its
two inputs.
1 2 3 4 time/ms
V1
V2
V2 /V
(2) For d.c., the open loop voltage gain is of the order 105.
5
(3) For a.c., the open loop voltage gain decreases with
0
_
+
Vout
-15 V
1 2 3 4 time/ms
increasing frequency.
Two electrical signals V1 and V2 are fed into an operational
A.
(1) only
amplifier.
The variations of V1 and V2 with time are
B.
(3) only
shown above.
C.
(1) and (2) only
the variation of the output Vout with time?
D.
(2) and (3) only
E.
(1), (2) and (3)
Which of the following graphs represents
A.
Vout
0
time
1
Vout and the input voltage Vin?
B.
(ii)
Vout
Mention two different types of applications of
the above circuit.
0
time
(b) A voltmeter V, a 10 F capacitor C and a switch S are
connected to the circuit as shown in Figure 11.2.
C.
Vout
_
C = 10  F
0
+
time
V
S
0V
D.
Figure 11.2
Vout
Suppose the input resistance of the operational
0
time
amplifier, R = 2.0 M, and the open-loop voltage gain,
Ao = 1.0  105.
E.
(i)
When the switch is closed, the voltmeter reads
5.0 V.
Vout
Find the initial quantity of charge on the
capacitor.
0
time
(ii)
93’ IIB
The circuit shown in Figure 11.2 can be
represented by that in Figure 11.3.
11. (a) Figure 11.1 shows an operational amplifier circuit
(power supply not shown).
R
i
current
_
V in
+
V out
Vout
Q
+
_
C
0V
0V
Figure 11.3
Figure 11.1
Show that the rate of change of charge on the
(i)
What is the relation between the output voltage
capacitor is given by
2
potential, Vo, with time, t, for
dQ
Q

dt
(1  Ao )RC
(i)
VB unchanged (i.e. remains at -4 V);
(4 marks)
(ii)
(iii)
VB changed to +4 V.
Find the quantity of charge on the capacitor after
the switch has been closed for 10 hours.
Label the Vo axes
(3 marks)
with appropriate values.
94’ IIB
10. An ideal operational amplifier (op amp) is used in the
circuit shown in Figure 10.1.
The two input potentials are
VA = +3 V and VB = -4 V respectively.
VA / V
The power supply
for the amplifier is provided by two batteries, each of e.m.f.
15 V and zero internal resistance.
3
0
t
50 k 
10 k 
+15 V
X _
20 k 
(i)
+
VA
Z
-15 V
V0 / V
V0
VB
Y
0V
0
t
Figure 10.1
(a) State TWO basic assumptions for an ideal op amp.
(b) Briefly explain why point X is said to be
(2 marks)
’virtually
(ii)
earthed’. (2 marks)
V0 / V
(c) (i)
Find the current flowing through the 10 k
resistor. (1 mark)
(ii)
Find the current flowing through the 50 k
0
resistor and state its direction of flow.
(d) What is the output potential Vo?
(2 marks)
t
(2 marks)
(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
3
92’ IIB
2001’
9. (a) Figure 9.1 shows a circuit in which a light
dependent resistor (LDR) is
connected in series with a 8kΩ resistor
10.
+6 V
20 k
30 k
10 k
Y
X
50 k
and a 6V battery. The resistance of the
LDR is 500kΩ in the dark.
4k
_
OA
Vo
+
R
to air-conditioner
switching circuit
LED
T
Figure 9.1
-6 V
Figure 10.1
(i)
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 the air conditioner is then
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.
(2 marks)
switched on automatically.
(a) (i)
With the variable resistor R set at its midpoint
position, the LED lights up when the room
temperature is 25 ºC.
If now R is decreased
slightly the LED turns off automatically.
Explain why this happens.
(You may assume
that the LED lights when Vo > 0).
(ii)
Briefly explain how the circuit controls the
switching on and the switching off of the
air-conditioner.
(iii)
What is the use of the variable resistor R in the
bridge circuit? (1 mark)
(b) For a room temperature of 25 ºC, and with R set at its
(ii) The voltage across the LDR is 2V
when the illumination is high. Find the
resistance of the LDR at this level of
illumination.
(1 marks)
(b) (i) Describe the difference in function
between an operational amplifier
used (3
asmarks)
an amplifier and one used as a
comparator.
(2 marks)
(ii) With an operational amplifier used as
a comparator, draw a switching circuit
marks)
for a (3street
light powered by 220V a.c.
mains which allows both manual and
automatic operation Briefly describe
how the circuit works.
(6 marks)
mid-position (R = 25 k), calculate the potential at point
X with reference to earth.
(3 marks)
4
2000’
10. Figure 10.1 shows an operational amplifier
circuit.
Rf=100kΩ
Vin
10kΩ
V+15V
out
Vout
Figure 10.1
-15V
0V
(a) (i) What is meant by negative feedback in
an operational amplifier circuit?
State TWO effects of using negative
feedback.
(3 marks)
(ii) What will be the effect on the above
circuit if Rf increases? (1 marks)
(iii) Find Vout when Vin equals (I) 10mV;
(II) 2V
(2 marks)
(iv) The graph below represents the of he
input voltage Vin with time. Sketch the
corresponding variation of the output
voltage Vout.(2 marks)
5
(b) (i) The circuit in Figure 10.1 can be used
to amplify the signals of the
order of 10mV from an electronic
stethoscope monitoring heart beats.
An earphone requiring a peak voltage
of at least 0.5 V to operated to the
output of the circuit. What should be
the minimum resistance value of the
feedback resistor Rf?
(ii) 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 10.1. Explain briefly.
(2 marks)
6