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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

Qn.

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

Qn.


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

Qn.


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

Qn.


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

Qn.


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

Qn.


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

Qn.


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

Qn.


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

Qn.


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

Qn.


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

Qn.


Allotted

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

Qn.


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

Qn.


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

Qn.


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

Qn.


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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