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 1 www.brainheaters.in
Q1) Ans: (a) Justify that JFET can be used as a voltage Resistor. 2 www.brainheaters.in
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Ans: (b) With neat diagram explain any one application of Zero-crossing Detector. 4 www.brainheaters.in
Ans:: (c) With neat block diagram explain how PLL can be used to generate large number of frequencies from a single reference frequency. 5 www.brainheaters.in
PLL as Frequency Synthesizer 6 www.brainheaters.in
(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 timedivision multiplexing. Digital bit streams can be transferred over an analog channel by means of codedivision 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. 7 www.brainheaters.in
Q2) Ans: (a) Derive an expression for trans conductance for JFET. 8 www.brainheaters.in
Ans: (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? Common mode ratio (CMMR) 9 www.brainheaters.in
Slew rate: Ideally the slew rate should be infinite and practically it should be as large as possible. 10 www.brainheaters.in
Q3) Ans: (a) Explain how operational amplifier can be used for taking average of three signals. 11 www.brainheaters.in
(b) Explain fly wheel effect in class C amplifier. Ans: 12 www.brainheaters.in
Q4) Ans: (a) Explain generation of SSB using phase shift method. (10) 13 www.brainheaters.in
Mathematical proof of sideband suppression: Suppression of the upper sideband: 14 www.brainheaters.in
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Ans: (b) Explain the use of PLL as FM detector. 16 www.brainheaters.in
Q5) Ans: (a) Explain the super heterodyne receiver in detail along with the waveform at each stage. 17 www.brainheaters.in
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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 20 www.brainheaters.in
The AM envelope: 21 www.brainheaters.in
Q6) Ans: (b)write short note on generation of FM by Armstrong method. Armstrong method for FM generation 22 www.brainheaters.in
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Ans: (c)explain the necessity and significance of modulation in communication. 1. Reduction in height of antenna 2. Avoid mixing of channel 24 www.brainheaters.in
(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. 25 www.brainheaters.in
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 26 www.brainheaters.in
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) Ans: N.A (c) Explain Nyquist Criteria. (d)determine the magnitude of g m for a JFET with I DSS = 8 ma and V p = -4 V at dc bias points V Os = -0.5 V and also at V GS = -2.5V. Ans: N.A Q6) Ans: N.A (a) Mention important specification of ADC and DAC required for communication 27 www.brainheaters.in