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GOVERNMENT OF KARNATAKA
KARNATAKA STATE PRE-UNIVERSITY EDUCATION EXAMINATION BOARD II YEAR PUC EXAMINATION – MARCH-2013
SCHEME OF VALUATION Subject Code: 40 Subject: ELECTRONICS Qn.
No. PART - A Marks
Allotted
01. What does the arrow mark indicate in the symbolic representation of a transistor? 1
Ans Conventional current flow. 1
02. Name the transistor amplifier which has very high input impedance. 1
Ans CC amplifier. 1
03. What happens to the distortion of the amplifier when negative feedback is given
to it?
1
Ans Decreases or reduces. 1
04. What is the value of output voltage if both the input voltages are equal in the case
of ideal differential amplifier?
1
Ans Zero OR non-zero OR two times input 1
05. Between LC and RC oscillators, which is preferred for generating low frequency? 1
Ans RC Oscillator 1
06. Define skip zone 1
Ans Skip zone: The region where there is no reception of any signal, i.e., silent zone. 1
07. Write the expression for modulation index in terms of Vmax and Vmin. 1
Ans
1
08. Write the output of XOR gate, when the inputs are A = 0 and B = 1 1
Ans 1 or high 1
09. Write the excess-3 code for (201)10. 1
Ans 0101 0011 0100 1
10. What is voice mail? 1
Ans Voice mail is an electronic telecommunication system that records, stores and
delivers voice messages.
OR
Voice mail is a device (answering machine) that records and retrieves audio
messages and play announcements to callers.
1
PART - B
11. A transistor has α = 0.98. If Ic = 5 mA, find IB. 2
Ans = IE/IC, hence IE = 5 mA/0.98 = 5.1 mA
IB = IE – IC = (5.102 – 5)m = 0.1 mA
1
1
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No. ELECTRONICS Marks
Allotted
Ans OR
β = α/(1 - α) = 0.98 / (1 - 0.98) = 49
β = IC/IB , hence IB = 5 mA / 49 = 0.102 mA = 102 µA
1
1
12. Draw the frequency response curve of a CE amplifier. Mention its bandwidth. 2
Ans
Band Width = fH - fL
1
1
13. Draw the circuit diagram of CC amplifier. Draw the input and output waveforms. 2
Ans
Input Output
1
1
14. Distinguish between positive feedback and negative feedback. 2
Ans
Any two, 1 mark each.
Positive feedback Negative feedback
1. Feedback voltage in phase with
the input
Feedback voltage out of phase with
the input
2. Af = A / (1 - Aβ) Af = A / (1 + Aβ)
3. Noise and distortion increases Noise and distortion decreases
4. Gain increases Gain decreases
2
15. Mention any two applications of a comparator 2
Ans 1. Zero crossing detector
2. Schmitt trigger
3. Phase detector
4. Voltage level detector
5. Oscillator
Any two applications, 1 mark each.
2
16. Draw the circuit symbol and electrical equivalent circuit of a crystal. 2
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Qn.
No. ELECTRONICS Marks
Allotted
Ans Circuit symbol of crystal
1
Electrical equivalent circuit of crystal
1
17. What are sky waves and space waves? 2
Ans EM waves sent towards the sky which get reflected by the ionosphere and return
back to the earth surface is called sky wave.
Line of sight waves are called space waves.
OR
The radio waves that are transmitted in the troposphere are called space waves.
OR
The radio waves which penetrate the ionosphere, get reflected by satellite and
reach the earth surface is called as space waves.
1
1
18. Draw the block diagram of AM transmitter 2
Ans
2
19. Distinguish between latch and flip-flop 2
Ans
Latch Flip-Flop
1. Store data temporarily Store data permanently
2. Clock is not required Clock is required
1
1
20. Draw the logic diagram of full adder using 3-input XOR gate and basic gates 2
Ans
1
1
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Qn.
No. ELECTRONICS Marks
Allotted
21. The truth table of a gate is given below. Identify the gate and write its logic
symbol.
Inputs Output
A B Y
0 0 1
1 0 0
0 1 0
1 1 1
2
21.
Ans
X-NOR gate
1
1
22. Name the two techniques that increases channel capacity in mobile
communication
2
22
Ans
Frequency reuse
Cell splitting
1
1
PART – C
23. Using the following data, calculate the experimental and theoretical values of
voltage gain for an Op-Amp non-inverting amplifier.
Input Vi = 0.5 V
Sl. No. Ri in kΩ Rf in kΩ V0 in volts Voltage gain
Theoretical Practical
1 2.2 8.2 2.36
2 1.2 5.6 2.83
4
23.
Ans AVT =
AVP = Vo / Vi
AVT1 = 1 + (8.2 k / 2.2 k) = 4.72 , AVP1 = 2.36 / 0.5 = 4.72
AVT2 = 1 + (5.6 k / 1.2 k) = 5.66 , AVP2 = 2.83 / 0.5 = 5.66
1
1
1
1
OR
23. The output waveform of Wien-bridge oscillator is shown below. Calculate the
value of resistance when C = 0.01µF (Consider R1 = R2 = R and C1 = C2 = C)
4
23
Ans
=
= 1 kHz
OR
1
1
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No. ELECTRONICS Marks
Allotted
R = 15.923 kΩ
1
1
24. Find the input base resistance and voltage gain of the CE transistor amplifier for
the data given below:
R1 = 47 kΩ, R2 = 12 kΩ, RC = 3.3 kΩ, RE = 1 kΩ, RL = 10 kΩ,
Vcc = 18 V, β = 100, VBE = 0.7 V and re’ = (52mV / IE)
4
24.
Ans V2 =
=
( ) = 3.66 V
VE = 3.66 – 0.7 = 2.96 V
IE = VE / RE = 2.96 / 1k = 2.96 mA
re’ = 52 mA / 2.96 mA
= 17.56 Ω
Zin base = β re’ = 100 x 17.56 = 1756Ω
Av = - (Rc || RL) / re’
= - (3.3k || 10k) / 17.56
= -141.29
1
1
1
1
25. Draw the circuit diagram of RC coupled amplifier. Mention its two advantages 4
25.
Ans
Any two advantages, 1 mark each.
1. It is less costly
2. It has wide frequency response
3. It provides less frequency distortion
4. It has no coils or transformers
5. Overall gain is high
2
2
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No. ELECTRONICS Marks
Allotted
26. An amplifier has an open gain of 150 and bandwidth of 200 kHz. Calculate its
gain and bandwidth after 4% negative feedback.
4
Ans Af = A / (1 + Aβ)
= 150 / (1 + (150 x 0.04)) = 21.42
1
1
BWf = (1 + Aβ) BW
= (1 + (150 x 0.04)) 200k = 1.4 MHz
OR
BWf = A BW / Af
= (150 x 100k) / 21.42 = 1.4 MHz
1
1
1
1
27. With a circuit diagram, derive an expression for the output voltage of an Op-Amp
subtractor
4
Ans
Let Vo1 be the output, with input V1 acting alone, assuming V2 = 0
------------------- (1)
Let Vo2 be the output, with input V2 acting alone, assuming V1 = 0
(
)
-------------------- (2)
Vo = Vo1 + V02
If R2 = R1 and R3 = Rf, then,
=
+
If Rf = R1 = R, then Vo = V2 – V1
1
1
1
1
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No. ELECTRONICS Marks
Allotted
28. A Hartley oscillator oscillates at 15 kHz. If the capacitor in tank circuit has a
value of 0.01 µF and one of the inductors is 1 mH, calculate the value of the other
inductor.
4
√( )
√( )
( )
= 0.01126 H
Hence L2 = 0.01126 H – 0.001 H = 0.01026 mH = 10 mH
1
1
1
1
29. Draw the amplitude modulated waveform. Derive an expression for the
instantaneous voltage of an AM wave.
4
29.
Ans
Instantaneous voltage of modulating signal, vm = Vm Sin mt
Instantaneous voltage of carrier signal, vc = Vc Sin ct
Vm and Vc are maximum amplitudes of modulating and carrier signals
respectively.
Instantaneous voltage of amplitude modulated wave
vAM = (Vc + vm) Sin ct
= (Vc + Vm Sin mt) Sin ct
= Vc (1 + ma Sin mt) Sin ct because, (Vm / Vc) = ma
Where ma is the modulation index.
( )
( )
1
1
1
1
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No. ELECTRONICS Marks
Allotted
30. With a circuit diagram, explain the working of linear diode detector. 4
Circuit Diagram:
1
3
Working:
Diode D rectifies AM wave.
Capacitor C2 provides low reactance to carrier and high reactance to signal.
Resistor R provides discharging path to C2.
Capacitor C adds zero level to detected signal.
31. Simplify the following Boolean expression by using K-map,
Y = f(A, B, C, D) = ∑m(0, 1, 2, 4, 5, 8, 9, 10, 12, 13). Draw the logic circuit for
the simplified expression using only NAND gates.
4
Ans
Y = f(A, B, C, D) = ∑m(0, 1, 2, 4, 5, 8, 9, 10, 12, 13)
PART – D
32. Draw the PIN diagram of IC-741. Describe an experiment to study Op-Amp
adder.
6
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No. ELECTRONICS Marks
Allotted
32.
Ans
PIN Diagram:
Aim: To construct an op-amp adder and to compare theoretical and practical
values of the output voltages (for DC inputs).
Equipments and Components: Dual power supply ±12 V, variable RPS, resistors,
IC 741, voltmeter
Circuit Diagram:
Procedure:
1. Rig up the circuit as shown in figure
2. Set the different values of V1 and V2 and record the output voltages
Tabular column or observation:
Input voltage
(in volts)
Theoretical o/p voltage
(in volts)
Practical o/p voltage
(in volts)
V1 V2 Vo = - ( V1 + V2)
Result: The theoretical and practical values are agreeing with each other.
1
1
2
1
1
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No. ELECTRONICS Marks
Allotted
OR
32. Draw the pin configuration of IC 7400. Describe an experiment to realize half-
adder using NAND gates.
6
32.
Ans
PIN diagram of IC 7400:
Aim: To construct half adder using IC7400 and to verify its truth table.
Equipments and components: Digital trainer kit, IC7400, LED, RPS 5V
Logic Diagram:
Procedure:
1. Rig up the circuit as shown in figure.
2. For various combinations of inputs A and B, as given in the truth table,
the logic levels at the output are verified, by on and off of LEDs.
Inputs Outputs LED response
A B Sum Carry Sum Carry
0 0 0 0 Off Off
0 1 1 0 On Off
1 0 1 0 On Off
1 1 0 1 Off On
Result: Truth table of the half adder is verified.
1
1
2
1
1
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Qn.
No. ELECTRONICS Marks
Allotted
33. With the circuit diagram, describe the procedure to draw input and output
characteristic curves of a transistor in CB configuration.
6
Ans
CB Amplifier
Input characteristics:
1. Set VCB constant.
2. Set different values of VEB and find the corresponding values of IE.
3. Plot the graph of IE versus VEB
Output Characteristics:
1. Set IE constant
2. Set different values of VCB and find the corresponding values of IC.
3. Plot the graph of IC versus VCB.
2
1
1
1
1
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No. ELECTRONICS Marks
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34a. What is dark current? Draw the circuit diagram of opto-coupler. Mention its one
application.
4
34a.
Ans
When there is no incident light a small, thermally generated collector to emitter
leakage current, ICEO flows. This is called dark current.
Circuit diagram:
Applications (any one).
1. For telephone line coupling
2. In audio applications
3. In peripheral equipment isolation
1
2
1
34b. A single stage CE amplifier has a gain of 60 dB. If the input signal is 40 mV,
calculate the output voltage.
2
34b.
Ans
60 dB = 20 log Av
Av = 1000
Av = Vo / Vi
Hence Vo = 40 x 10-3
x 1000 = 40 V
1
1
35a. What is an integrator? Draw the circuit diagram and obtain an expression for its
output voltage.
4
Ans The circuit whose output is proportional to the integral of the input is referred to
as an integrator.
Circuit Diagram:
1
1
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No. ELECTRONICS Marks
Allotted
From the figure if = ii ..…..(1)
if = dQ/dt = Cd(V0 - VG)/dt = Cd(V0)/dt …….(2)
ii = (VG-Vi)/R = -Vi/R ……(3)
Therefore, Cd(V0)/dt = -Vi/R
(Proper steps must be considered)
1
1
35b. Calculate the output voltage in the circuit shown below:
2
35b.
Ans
Note: If the quastion number (35b) is written by the student, full (2) marks must be
awarded because signs of inverting and non inverting terminals of the first op-amp
is printed wrongly in question paper.
2
36a. With a circuit diagram, explain the working of phase shift oscillator. Write the
expression for frequency of oscillations.
4
Ans Circuit diagram:
√
3 RC sections produce a phase shift of 60o each and the total phase shift is 180
o in
the RC network.
The op-amp inverting amplifier introduces a phase shift of 180o between its input
and output.
Total phase shift is 360o and loop gain Aβ = 1, which are required for the
sustained oscillations.
1
1
1
1
dtVRC
V i1
- 0
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No. ELECTRONICS Marks
Allotted
36b. Calculate the feedback factor in the tank circuit shown below.
C1 = 1 µF, C2 = 10 µF, L = 2 mH
2
36.b
Ans
β = C1 / C2
= 1µ / 10µ = 0.1
1
1
37a. Draw the block diagram of FM receiver. Mention two advantages of FM. 4
37a.
Ans
Block diagram:
Any two advantages: (1 mark each)
1. All transmitted power is useful.
2. Noise supression is better than AM
3. Signal interference from adjacent channel is very less
4. Signal to noise ratio is high
2
2
37b. A 50 MHz carrier is modulated by a 40 Hz audio signal. If the carrier voltage is
4V, and the maximum deviation is 10 kHz, calculate the bandwidth.
2
37b.
Ans BW = 2 ( + fm)
= 2 (10 k + 400) = 20.8 kHz
1
1
38a. Draw the block diagram of digital computer. Explain the function of ALU and
memory unit.
4
38a.
Ans
Block diagram:
\
2
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No. ELECTRONICS Marks
Allotted
ALU: Arithmetic Logic Unit is designed to perform basic arithmetic operations
(add, subtract), logical operations (AND, OR, etc)
Memory unit: The function of the memory unit is to store programs and data.
1
1
38b.
Express the expression into canonical SOP form.
2
Ans.
( ) ( )
1
1
39a. With a block diagram, explain the working of RS flip-flop 4
39a.
Ans
Block diagram:
The output of an RS flip-flop remains in the same state as long as the clock pulse
is zero, regardless of the R&S inputs.
1. When clock=1, R=0, S=0, the output remains in the previous state.
2. When clock=1, R=1, S=0, the output is reset
3. When clock=1, R=0, S=1, the output is set
4. When clock=1, R=1, S=1, the output is in forbidden state
Truth table:
Inputs Output Clock State
S R Q
X X NC NC 0 Remains in the previous state
0 0 NC NC 1 Remains in the previous state
1 0 1 0 1 SET
0 1 0 1 1 RESET
1 1 1? 1? 1 Forbidden or invalid condition
1
1
1
1
39b. Construct OR and AND gates using NOR gates 2
Ans OR Gate using NOR gate
1
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No. ELECTRONICS Marks
Allotted
AND Gate using NOR gate
1
40a. Draw the block diagram of monochrome TV receiver. 4
Ans
Block diagram of monochrome TV receiver
4
40b. What are uplink and downlink signals? 2
Ans The signal transmitted from the earth to satellite is called uplink signal.
The signal transmitted from the satellite to earth is called downlink signal.
1
1
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