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Mumbai University Computer (Semester -III)
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Solution for Elec Circuits and
Communication Fundamentals
May 2016
INDEX
Q1 – a ………………………………………………………………………………………………………… 2-3
b ………………………………………………………………………………………………………… 4
c …………………………………………………………………………………………………………. 4-6
d ………………………………………………………………………………………………………… 7
Q2- a ………………………………………………………………………………………………………… 8
b ………………………………………………………………………………………………………… 9 -10
Q3 – a ………………………………………………………………………………………………………… 11
b ………………………………………………………………………………………………………… 12
c) ………………………………………………………………………………………………………… N.A
d ………………………………………………………………………………………………………… N.A
Q4- a ………………………………………………………………………………………………………… 13-15
B ………………………………………………………………………………………………………… 16
Q5 – a ………………………………………………………………………………………………………… 17-20
b ………………………………………………………………………………………………………… 20-21
Q6- a ………………………………………………………………………………………………………… N.A
b ………………………………………………………………………………………………………… 22-23
c ………………………………………………………………………………………………………. ……24 -25
d………………………………………………………………………………………………………………25-27
Mumbai University Computer (Semester -III)
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Q1)
(a) Justify that JFET can be used as a voltage Resistor.
Ans:
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Mumbai University Computer (Semester -III)
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(b) With neat diagram explain any one application of Zero-crossing
Detector.
Ans:
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(c) With neat block diagram explain how PLL can be used to generate
large number of frequencies from a single reference frequency.
Ans::
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PLL as Frequency Synthesizer
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(d) Explain with suitable example what do you understand by signal
multiplexing?
Ans: In telecommunications and computer networks, multiplexing (sometimes
contracted to muxing) is a method by which multiple analog or digital signals
are combined into one signal over a shared medium. The aim is to share an
expensive resource. For example, in telecommunications, several telephone
calls may be carried using one wire. Multiplexing originated in telegraphy in the
1870s, and is now widely applied in communications. In telephony, George
Owen Squier is credited with the development of telephone carrier multiplexing
in 1910.
The multiplexed signal is transmitted over a communication channel, such as a
cable. The multiplexing divides the capacity of the communication channel into
several logical channels, one for each message signal or data stream to be
transferred. A reverse process, known as demultiplexing, extracts the original
channels on the receiver end.
A device that performs the multiplexing is called a multiplexer (MUX), and a
device that performs the reverse process is called a demultiplexer (DEMUX or
DMX).
Inverse multiplexing (IMUX) has the opposite aim as multiplexing, namely to
break one data stream into several streams, transfer them simultaneously over
several communication channels, and recreate the original data stream.
Types
Multiple variable bit rate digital bit streams may be transferred efficiently over a
single fixed bandwidth channel by means of statistical multiplexing. This is
an asynchronous mode time-domain multiplexing which is a form of time-
division multiplexing.
Digital bit streams can be transferred over an analog channel by means of code-
division multiplexing techniques such as frequency-hopping spread
spectrum (FHSS) and direct-sequence spread spectrum (DSSS).
In wireless communications, multiplexing can also be accomplished through
alternating polarization (horizontal/vertical or clockwise/counterclockwise) on
each channel and satellite, or through phased multi-antenna array combined with
a multiple-input multiple-output communications (MIMO) scheme.
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Q2)
(a) Derive an expression for trans conductance for JFET.
Ans:
Mumbai University Computer (Semester -III)
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(b) List down various parameters of Opamp along with their typical
values for IC741.Also explain what the significance of CMRR and
slew rate is?
Ans:
Common mode ratio (CMMR)
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Slew rate:
Ideally the slew rate should be infinite and practically it should be as large
as possible .
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Q3)
(a) Explain how operational amplifier can be used for taking average of
three signals.
Ans:
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(b) Explain fly wheel effect in class C amplifier.
Ans:
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Q4)
(a) Explain generation of SSB using phase shift method. (10)
Ans:
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Mathematical proof of sideband suppression:
Suppression of the upper sideband:
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Mumbai University Computer (Semester -III)
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(b) Explain the use of PLL as FM detector.
Ans:
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Q5)
(a) Explain the super heterodyne receiver in detail along with the
waveform at each stage.
Ans:
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Mumbai University Computer (Semester -III)
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Mumbai University Computer (Semester -III)
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(b) Explain the concept of amplitude modulation.
Ans:
Amplitude modulation (AM) is the process of changing the amplitude of a
frequency carrier signal in proportion with the instantaneous value of
modulating signal .This is also called as double sideband full carrier (DSB-FC)
modulation
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The AM envelope:
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Q6)
(b)Write short note on generation of FM by Armstrong method.
Ans:
Armstrong method for FM generation
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Mumbai University Computer (Semester -III)
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(c)Explain the necessity and significance of modulation in
communication.
Ans:
1. Reduction in height of antenna
2. Avoid mixing of channel
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(d)Compare n-channel and p-channel JFET with respect to their device
features and voltage current characteristics.
Ans:
Following section compares both FET types with respect to their features,
symbol and construction.
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n-channel FET
As shown in figure-1, n-channel FET is constructed using a bar of N-type
material into which a pair of P-type regions are diffused. Fig-1 also mentions
circuit symbol of n-channel FET.
Following are the features of N-channel Junction FET(JFET):
• Current carriers are electrons in n-channel FET.
• Mobility of electrons is large.
• Input noise is low.
• Trans-conductance is large.
p-channel FET
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Similar to n-channel FET, p-channel FET is constucted using a bar of P-type
material into which a pair of N-type regions are diffused. Fig-2 mentions circuit
symbol of p-channel FET.
Following are the features of P-channel Junction FET(JFET):
• Current carriers are holes in p-channel FET.
• Mobility of holes is poor.
• Input noise is large.
• Trans-conductance is small.
Q3)
(c) Explain Nyquist Criteria.
Ans: N.A
(d)Determine the magnitude of gm for a JFET with IDSS = 8 mA and Vp =
-4 V at dc bias points VOs = -0.5 V and also at VGS = -2.5V.
Ans: N.A
Q6)
(a) Mention important specification of ADC and DAC required for
communication
Ans: N.A
