ISRO Electronics & Communication (ECE) 2008 Question Paper & Solutions

Question 1

Topic: Analog Circuits

The circuit shown in figure is a

• A. Low-pass filter
• B. High-pass filter
• C. Band-pass filter
• D. Band-reject filter

Answer: A. Low-pass filter

Question 2

Topic: Analog Circuits

In the op-amp circuit given in figure, the load current $i_L$ is

• A. $-\frac{V_s}{R_2}$
• B. $\frac{V_s}{R_2}$
• C. $-\frac{V_s}{R_L}$
• D. $-\frac{V_s}{R_1}$

Answer: A. $-\frac{V_s}{R_2}$

Explanation: $\frac{V_s - V}{R_1} = \frac{V - V_0}{R_1} \Rightarrow V_s - 2V = -V_0$ ...(ii)\n$\frac{V}{R_2} + \frac{V}{R_L} = \frac{V_0 - V}{R_2} \Rightarrow 2\frac{V}{R_2} + \frac{V}{R_L} = \frac{V_0}{R_2}$ ...(i)\nPutting $V_0$ from equation (i) in equation (ii) we get\n$\frac{-V_s + 2V}{R_2} = -(\frac{2V}{R_2} + \frac{V}{R_L})$\n$I_L = \frac{V}{R_L} = -\frac{V_s}{R_2}$

Question 3

Topic: Analog Circuits

An op-amp has an offset voltage of 1 mV and is ideal in all other respects. If this op-amp is used in the circuit shown in figure, the output voltage will be

• A. 1 mV
• B. 1V
• C. $\pm 1~V$
• D. OV

Answer: C. $\pm 1~V$

Explanation: $V_0 = (1 + \frac{R_f}{R_1})V_{i0}$\nWhere $V_{i0} =$ input offset voltage\n$V_0 = (1 + \frac{1 \times 10^6}{1 \times 10^3}) \times 1 \times 10^{-3} = 1001 \times 1 \times 10^{-3} = 1.001~V \simeq 1~V$\nNature can not be predicted\nSo $V_0 = \pm 1~V$

Question 4

Topic: Analog Circuits

For the circuit shown below if $g_m = 3 \times 10^{-3}$ and $R_s = 3000~\Omega$, then the approximate value of $R_o$ is

• A. 3000 $\Omega$
• B. 1000/3 $\Omega$
• C. 300 $\Omega$
• D. 100 $\Omega$

Answer: C. 300 $\Omega$

Explanation: $r_d = \frac{1}{g_m} = \frac{1000}{3} \Omega$\n$R_o = r_d || R_s = \frac{\frac{1000}{3} \times 3000}{\frac{1000}{3} + 3000} = \frac{1000 \times 3000}{3} \times \frac{3}{10000} = 300~\Omega$

Question 5

Topic: Analog Circuits

Where does the operating point of a class-B power amplifier lie?

• A. At the middle of a.c. load line
• B. Approximately at collector cut-off on both the d.c. and a.c. load lines
• C. Inside the collector cut-off region on a.c. load line
• D. At the middle point of d.c. load line

Answer: B. Approximately at collector cut-off on both the d.c. and a.c. load lines

Question 6

Topic: Signals and Systems

The Laplace transform of $e^{-2t}$ is

• A. $\frac{1}{2s}$
• B. $\frac{2}{s}$
• C. $\frac{2}{s+1}$
• D. $\frac{1}{s+2}$

Answer: D. $\frac{1}{s+2}$

Question 7

Topic: Control Systems

The system with characteristic equation $s^4 + 3s^3 + 6s^2 + 9s + 12 = 0$

• A. Stable
• B. Unstable
• C. Marginally stable
• D. Marginally unstable

Answer: B. Unstable

Explanation: According to R-H array\nTwo sign change. So it has two poles in RHS.\nSo, system is unstable.

Question 8

Topic: Electronic Devices

Compared to field effect photo transistors, bipolar photo transistors are

• A. More sensitive and faster
• B. Less sensitive and slower
• C. More sensitive and slower
• D. Less sensitive and faster

Answer: A. More sensitive and faster

Question 9

Topic: Electronic Devices

The output V-I characteristics of an enhancement type MOSFET has

• A. Only an ohmic region
• B. Only a saturation region
• C. An ohmic region at low voltage value followed by a saturation region at higher voltages
• D. An ohmic region at large voltage values preceded by a saturation region at lower voltages

Answer: C. An ohmic region at low voltage value followed by a saturation region at higher voltages

Question 10

Topic: Reliability Engineering

Which of the following relation is valid? Where MTBF = Mean Time Between Failure, MTTF = Mean Time To Failure, MTTR = Mean Time To Repair

• A. $MTBF=MTTF+MTTR$
• B. $MTR+MTTF+MTBF=1$
• C. $\frac{1}{MTTR}+\frac{1}{MTTF}=\frac{1}{MTBF}$
• D. MTBF. MTTF. $MTTR=1$

Answer: A. $MTBF=MTTF+MTTR$

Question 11

Topic: Electronic Devices

Two transistors have the same value of $\alpha$ but different gain bandwidth products. One of them is a germanium transistor and the other is a silicon transistor. Both the transistors have similar geometries and base width. The transistor with lower GB product

• A. is the germanium
• B. is the silicon
• C. both are same
• D. Cannot be identified unless more information is available

Answer: B. is the silicon

Question 12

Topic: Analog Circuits

The following transistor configuration has the highest input impedance

• A. CC
• B. CE
• C. CB
• D. All of the above

Answer: A. CC

Question 13

Topic: Computer Organization

If $t_c$, $h$ and $t_m$ specify the cache access time, hit ratio and main memory access time then the average access time is (Given $t_c = 160$ ns, $t_m = 960$ ns, $h = 0.99$)

• A. 160 ns
• B. 960 ns
• C. 256 ns
• D. $950 \times 0.9$ ns

Answer: A. 160 ns

Explanation: Average access time $t_{avg} = ht_c + (1 - h)(t_m + t_c)$\nWhere, $h =$ hit ratio\n$t_c =$ cache access time\n$t_m =$ main memory access time\n$t_{avg} = 0.99 \times 160 + 0.01(160 + 960) = 169.6$ ns

Question 14

Topic: Computer Organization

The advantage of write (copy) back data cache organization over write through organization is

• A. Main memory consistency
• B. Write allocate on write miss
• C. Less memory bandwidth requirement
• D. Higher capacity

Answer: C. Less memory bandwidth requirement

Question 15

Topic: Digital Electronics

E2PROM storage element is

• A. Cross-coupled latch
• B. Isolated gate transistor
• C. Capacitor
• D. Flip flop

Answer: B. Isolated gate transistor

Question 16

Topic: Engineering Mathematics

The modulus of $1+\cos \alpha+i \sin \alpha$ is

• A. $2 \sin\frac{\alpha}{2}$
• B. $2 \cos\frac{\alpha}{2}$
• C. $\sin^{2}\frac{\alpha}{2}-1$
• D. $\cos^{2}\frac{\alpha}{2}-1$

Answer: B. $2 \cos\frac{\alpha}{2}$

Explanation: $z=1+\cos \alpha+i \sin \alpha$\n$|z|=\sqrt{(1+\cos \alpha)^{2}+\sin^{2}\alpha}$\n$=\sqrt{1+2 \cos \alpha+\cos^{2}\alpha+\sin^{2}\alpha}$\n$=\sqrt{2(1+\cos \alpha)}$\n$=\sqrt{2\times2 \cos^{2}(\frac{\alpha}{2})}=2 \cos(\frac{\alpha}{2})$

Question 17

Topic: Digital Electronics

The 8 bit DAC produces 1.0 V for a digital input of 00110010. What is the largest output it can produce?

• A. 5 V
• B. -5 V
• C. 5.5 V
• D. 5.10 V

Answer: D. 5.10 V

Explanation: Output = Resolution $\times$ D\n$D =$ Decimal value\n$1.0 = \text{Resolution} \times 50$\n$\therefore \text{Resolution} = \frac{1}{50}$\n$V_{FS} = \text{Resolution} \times 255$\n$= \frac{1}{50} \times 255 = 5.10~V$

Question 18

Topic: Digital Electronics

The fastest ADC among the following is

• A. Successive approximation type
• B. Dual slope type
• C. Sigma-Delta ADC
• D. Flash Converter

Answer: D. Flash Converter

Question 19

Topic: Digital Electronics

The mod number of a Johnson counter will be always equal to the number of flip flops used

• A. same
• B. twice
• C. $2^{N}$ where N is the number of flip flops
• D. None of these

Answer: B. twice

Question 20

Topic: Digital Electronics

Odd parity generator uses .... logic

• A. XNOR
• B. XOR
• C. Sequential
• D. OR

Answer: B. XOR

Question 21

Topic: Digital Electronics

Which type of memory has fast erase and write times

• A. EPROM
• B. EEPROM
• C. Flash memory
• D. None of these

Answer: C. Flash memory

Question 22

Topic: Computer Organization

The performance gain that can be obtained by improving some portion of a computer can be calculated using

• A. Moore's law
• B. Djikstra's algorithm
• C. Amdahl's law
• D. Murphy's law

Answer: A. Moore's law

Question 23

Topic: Computer Organization

Microprogramming refers to

• A. Emulation
• B. Programming at micro level
• C. The use of storage to implement the control unit
• D. Array processing

Answer: A. Emulation

Question 24

Topic: Computer Organization

An array processor is a ... machine

• A. SIMD
• B. MIMD
• C. SISD
• D. MISD

Answer: A. SIMD

Explanation: An array processor is also called vector processor which is a single instruction and multiple data machine.

Question 25

Topic: Computer Organization

...... machines tend to make use of internal resources of the processor, a rich set of registers and a pipelined organization

• A. CISC
• B. Parallel processor
• C. RISC
• D. Array processor

Answer: C. RISC

Question 26

Topic: Computer Networks

Difference between a switch and Hub

• A. Both are same
• B. Switch avoids flooding
• C. Hub avoids broadcast
• D. None of the above

Answer: B. Switch avoids flooding

Question 27

Topic: Computer Networks

100 Base T refers to

• A. Fibre connectivity
• B. BNC connectors
• C. SONET
• D. Twisted pair

Answer: D. Twisted pair

Question 28

Topic: Digital Electronics

A S-R flip flop with a clock input can be converted to a 'D' flip flop using

• A. Two inverters
• B. the flip flop output (Q & Q) connected to its inputs (S & R)
• C. One inverter
• D. Not possible

Answer: C. One inverter

Explanation: Logic diagram (conversion of S-R flip-flops to D flip-flop) involves using one inverter between the D input and the R input, with the D input directly connected to S.

Question 29

Topic: Digital Electronics

An 8 bit ADC has a full scale input of 2.55 V. If other cumulative errors are 2.55 mV, determine the maximum error

• A. 10 mV
• B. 12.55 V
• C. 7.45 mV
• D. 2.55 mV

Answer: B. 12.55 V

Explanation: Resolution = $\frac{V_{FS}}{D}$ ($\because D=255(11111111)_b$)\n$= \frac{2.55}{255} = 10~mV$\nmaximum error = cumulative error + resolution\n$= (2.55 + 10)mV = 12.55~mV$

Question 30

Topic: Digital Electronics

A counter is designed with six stages of flip flops. Determine the output frequency at the last (sixth) stage, when input frequency is 1 MHz

• A. 1 MHz
• B. 166 KHz
• C. 15.625 KHz
• D. zero

Answer: C. 15.625 KHz

Explanation: The output of flip-flop will give half of input frequency. So, the output frequency at the last (sixth) stage = $\frac{1 \times 10^6}{2^6} = 15.625~kHz$

Question 31

Topic: Computer Networks

The ethernet protocol uses

• A. CSMA/CA
• B. SCPC
• C. CSMA/CD
• D. Slotted ALOHA

Answer: C. CSMA/CD

Question 32

Topic: Engineering Mathematics

The value of c which makes the angle $45^{\circ}$ between $\vec{a}=i+cj$ and $\vec{b}=i+j$ is

• A. 1
• B. $\frac{1}{\sqrt{2}}$
• C. $\frac{-1}{\sqrt{2}}$
• D. 0

Answer: D. 0

Explanation: $\vec{a}\cdot\vec{b}=|\vec{a}|.|\vec{b}|\cos~\theta$\n$\cos~\theta=\frac{\vec{a}\cdot\vec{b}}{|\vec{a}||\vec{b}|}$\n$\cos~45^{\circ}=\frac{1+c}{\sqrt{2}\cdot\sqrt{1+c^{2}}}$\n$\frac{1}{\sqrt{2}}=\frac{1+c}{\sqrt{2}\cdot\sqrt{1+c^{2}}}$\n$(1+c)^{2}=1+c^{2}$\n$\Rightarrow c^{2}+2c+1=c^{2}+1$\n$c=0$

Question 33

Topic: Computer Organization

In a daisy chained connection to the CPU, the peripheral whose interrupt request has the highest priority is the one

• A. with the largest vector address
• B. with highest speed of operation
• C. electrically nearest to the CPU
• D. electrically farthest from the CPU

Answer: C. electrically nearest to the CPU

Explanation: Daisy chaining method: The daisy chaining method of establishing priority consist of a serial connection of all devices that request an interrupt. The device with the highest priority is placed in the first position, followed by lower priority devices microprocessor to the device with the lowest priority, which is placed last in the chain. This method of connection between these devices and the CPU is known as daisy chain priority interrupt.

Question 34

Topic: Computer Networks

RS-232 interface

• A. Uses only positive voltage
• B. Cannot transmit signals over long distance
• C. Uses only negative voltage
• D. A logic high uses positive voltage

Answer: D. A logic high uses positive voltage

Question 35

Topic: Computer Organization

Assuming ideal conditions, the speed up obtained from a balanced Nstage pipeline is

• A. 2N
• B. $N^{2}$
• C. N
• D. N!

Answer: C. N

Explanation: Speed-up = number of stages

Question 36

Topic: Digital Electronics

The number of comparators required in an 8-bit flash type A/D converter is

• A. 256
• B. 255
• C. 9
• D. 8

Answer: B. 255

Explanation: Number of comparator = $2^{n}-1=2^{8}-1=255$

Question 37

Topic: Digital Electronics

Minimum number of 2-input NAND gates that will be required to implement the function: $Y=AB+CD+EF$ is

• A. 4
• B. 5
• C. 6
• D. 7

Answer: A. 4

Explanation: $Y=\overline{AB}\cdot\overline{CD}\cdot\overline{EF} = AB+CD+EF$. According to the logic diagram, it requires 4 NAND gates.

Question 38

Topic: Digital Electronics

In a ripple counter how many changes in state happen when count changes from 7 to 8?

• A. 1
• B. 2
• C. 3
• D. 4

Answer: D. 4

Explanation: $7 \rightarrow 0111$, $8 \rightarrow 1000$. Totally 4 changes.

Question 39

Topic: Digital Electronics

A programmable device (PROM) is

• A. programmable OR and fixed AND array
• B. programmable AND and fixed OR array
• C. programmable AND and programmable OR array
• D. none

Answer: A. programmable OR and fixed AND array

Question 40

Topic: Digital Electronics

In asychronous circuits, which of the following best explains a dynamic hazard

• A. Output changes several times for a single change in an input
• B. Output changes to a different state for a single change in an input
• C. Output changes momentarily when it is supposed to remain constant for a single change in an input
• D. none of these

Answer: A. Output changes several times for a single change in an input

Question 41

Topic: Computer Organization

Digital signal processors use a computer architecture derived from

• A. Harvard Architecture
• B. Von-Neumann Architecture
• C. Cray Architecture
• D. None of the above

Answer: A. Harvard Architecture

Explanation: Harvard Architecture: Separate program and data memory for faster operation.

Question 42

Topic: Digital Electronics

Which of the following represents the Moore model for sequential circuits?

• A. Logic -> X, Z
• B. Clock -> D Flip Flop -> X, Z
• C. Logic -> D Flip-flop -> Logic -> X, Z
• D. Logic -> D Flip-flop -> Logic -> Z

Answer: C. Logic -> D Flip-flop -> Logic -> X, Z

Explanation: The output of sequential circuit depends only on the present state of the flip-flop is called Moore model or Moore machine.

Question 43

Topic: Signals and Systems

An anti-aliasing filter is

• A. An analog filter
• B. A digital filter
• C. Can be analog or digital
• D. None of the above

Answer: A. An analog filter

Question 44

Topic: Analog Circuits

Which of the following filter has the faster roll-off?

• A. R-C
• B. Bessel
• C. Butterworth
• D. Chebyshev

Answer: D. Chebyshev

Question 45

Topic: Signals and Systems

Find the rms value of signal $x(t)=5~cos(50t+\frac{\pi}{3})$

• A. $\sqrt{5}$
• B. $\sqrt{5\times50t}$
• C. $\sqrt{50+\frac{\pi}{3}}$
• D. $\sqrt{12.5}$

Answer: D. $\sqrt{12.5}$

Explanation: $x(t)=5~cos(50t+\frac{\pi}{3}) = 5\{cos(50t)\cdot cos(\frac{\pi}{3})-sin(50t).sin(\frac{\pi}{3})\}$\n$=5\{cos(50t)\cdot\frac{1}{2}-sin(50t)\frac{\sqrt{3}}{2}\}$\n$x(t)=\frac{5}{2}cos(50t)-\frac{5\sqrt{3}}{2}sin(50t)$\n$x(t)_{rms}=\sqrt{(\frac{5}{2\sqrt{2}})^{2}+(\frac{5\sqrt{3}}{2\sqrt{2}})^{2}}$\n$=\sqrt{(\frac{5}{2\sqrt{2}})^{2}(1+\sqrt{3})} = \sqrt{\frac{25}{8}\times4} = \sqrt{12.5}$

Question 46

Topic: Measurements and Instrumentation

Determine the value indicated by a DC voltmeter for the waveform given below

• A. OV
• B. 2.5 V
• C. 10 V
• D. 5 V

Answer: C. 10 V

Explanation: $V_{DC}=\frac{1}{50}\int_{0}^{50}10~dt = \frac{1}{50}\times 10\times (50-0)=10~V$

Question 47

Topic: Signals and Systems

Evaluate the function $\int_{0}^{\infty}t^{2}\delta(t-3)dt$

• A. $e^{-t^{2}}$
• B. $\frac{t^{3}}{3}$
• C. $t^{2}(t-3)$
• D. 9

Answer: D. 9

Explanation: $I=\int_{0}^{\infty}t^{2}\delta(t-3)dt=3^{2}=9;$ at $t=3$ $=0$; otherwise

Question 48

Topic: Engineering Mathematics

A husband and wife appear in an interview for two vacancies for same post. The probability of husband getting selected is $\frac{1}{5}$ while the probability of wife getting selected is $\frac{1}{7}$ Then the probability that anyone of them getting selected is

• A. $\frac{11}{35}$
• B. $\frac{12}{35}$
• C. $\frac{10}{35}$
• D. $\frac{34}{35}$

Answer: D. $\frac{34}{35}$

Explanation: The probability of getting anyone selected $=1-\frac{1}{5}\times\frac{1}{7}=\frac{34}{35}$

Question 49

Topic: Measurements and Instrumentation

The rms value of Complex waveforms are measured using

• A. Transistor Voltmeter
• B. Differential Voltmeter
• C. High bandwidth Voltmeter
• D. Voltmeter containing heat sensing element such as thermocouples

Answer: D. Voltmeter containing heat sensing element such as thermocouples

Question 50

Topic: Signals and Systems

Find the fundamental period T of the following continuous signal $x(t)=20~cos(10\pi t+\frac{\pi}{6})$

• A. $\frac{\pi}{6}sec$
• B. $\frac{20\times10\pi}{T}sec$
• C. 5 sec
• D. $\frac{1}{5}sec$

Answer: D. $\frac{1}{5}sec$

Explanation: $x(t)=20~cos(10~\pi t)+\frac{\pi}{6}$\n$\omega=\frac{2\pi}{T}$\n$T=\frac{2\pi}{\omega}=\frac{2\pi}{10\pi}=\frac{1}{5}sec.$

Question 51

Topic: Network Theory

In the circuit shown the switch closes at t = 0. The voltage across 4 $\mu$F capacitor in ideal condition changes to

• A. 0
• B. 16 V
• C. 15 V
• D. 24 V

Answer: B. 16 V

Explanation: $I(s)=\frac{\frac{V(0^+)}{s}}{\frac{1}{2s}+\frac{1}{4s}}$\n$V_c(s)=\frac{\frac{V(0^+)}{s}}{\frac{1}{2s}+\frac{1}{4s}}\cdot\frac{1}{4s} = \frac{\frac{24}{s}}{\frac{3}{4s}}\times\frac{1}{4s}=\frac{8}{s}$.\nAt steady state, final value theorem is applicable, so\n$V=\lim_{s\rightarrow0}s\cdot v_c(s)=s\cdot\frac{8}{s}=8~V$.\nChange in voltage $=(24-8)V=16~V$.

Question 52

Topic: Network Theory

The 2-port network of figure A has open circuit impedance parameters given by matrix

• A. $\begin{bmatrix}R&R\\ R&R\end{bmatrix}$
• B. $\begin{bmatrix}R&0\\ 0&R\end{bmatrix}$
• C. $\begin{bmatrix}\infty&R\\ R&\infty\end{bmatrix}$
• D. $\begin{bmatrix}R&\infty\\ \infty&R\end{bmatrix}$

Answer: A. $\begin{bmatrix}R&R\\ R&R\end{bmatrix}$

Explanation: $Z_{11}=Z_a+Z_c$\n$Z_{22}=Z_b+Z_c$\n$Z_{12}=Z_{21}=Z_c$\nCompare figure (a) and figure (b)\n$Z_{11}=0+R=R$\n$Z_{22}=0+R=R$\n$Z_{12}=Z_{21}=R$\nOpen circuit impedance $\begin{bmatrix}Z_{11}&Z_{12}\\ Z_{21}&Z_{22}\end{bmatrix}=\begin{bmatrix}R&R\\ R&R\end{bmatrix}$

Question 53

Topic: Network Theory

The network shown in the figure draw a current I. If the ends a and b are shorted, the current drawn would be

• A. 6I
• B. 4I
• C. 2I
• D. I

Answer: D. I

Explanation: No current flow through this section due to shorting at the end.

Question 54

Topic: Analog Circuits

As compared to a full-wave rectifier using two diodes, the four diode bridge rectifier has the dominant advantage of

• A. higher current carrying capacity
• B. lower peak inverse voltage
• C. lower ripple factor
• D. higher efficiency

Answer: B. lower peak inverse voltage

Question 55

Topic: Network Theory

When a unit impulse voltage is applied to an inductor of 1 H, the energy supplied by the source is

• A. $\infty$
• B. 1 J
• C. $1/2~J$
• D. 0

Answer: C. $1/2~J$

Explanation: $E=\frac{1}{2}LI^{2}=\frac{1}{2}\times1\times1^{2}=\frac{1}{2}J.$

Question 56

Topic: Network Theory

The value of R for $i=2~A$ is

• A. 5
• B. 10
• C. 40
• D. 60

Answer: D. 60

Explanation: Voltage at point $b=120~V$\n$R=\frac{V}{I}=\frac{120}{2}=60~\Omega.$

Question 57

Topic: Network Theory

The time constant for the circuit shown below is

• A. 0.2 microsecond
• B. 0.8 microsecond
• C. 0.4 microsecond
• D. 0.2 millisecond

Answer: D. 0.2 millisecond

Explanation: $R_{eq}=1k+(2k||2k)$\n$=1k+\frac{2k\times2k}{4k}=2~k\Omega$\nTime constant $=R_{eq}C_{eq}$\n$=2\times10^{3}\times0.1\times10^{-6}$\n$= 0.2$ milli second

Question 58

Topic: Analog Circuits

For a sinusoidal input, the circuit shown in the figure will act as a

• A. Pulse generator
• B. Ramp generator
• C. Full-wave rectifier
• D. Voltage doubler

Answer: C. Full-wave rectifier

Question 59

Topic: Analog Circuits

A second-order band-pass active filter can be obtained by cascading a low pass second order section having cut off frequency $f_{OH}$ with a high pass second order section having cut-off frequency $f_{OL}$ provided

• A. $f_{OH}>f_{OL}$
• B. $f_{OH}<f_{OL}$
• C. $f_{OH}=f_{OL}$
• D. $f_{OH}\le\frac{1}{2}f_{OL}$

Answer: A. $f_{OH}>f_{OL}$

Explanation: To form a pass band, the higher cut-off frequency (from the low-pass filter, $f_{OH}$) must be greater than the lower cut-off frequency (from the high-pass filter, $f_{OL}$).

Question 60

Topic: Electronic Devices

If $\alpha=0.995$, $I_{E}=10$ mA and $I_{CO}=0.5~\mu A$ then $I_{CEO}$ will be

• A. $100~\mu A$
• B. 10.1 mA
• C. $25~\mu A$
• D. 10.5 mA

Answer: A. $100~\mu A$

Explanation: $I_{CEO}=\frac{I_{CO}}{1-\alpha}=\frac{0.5}{1-0.995}\mu A=100~\mu A.$

Question 61

Topic: Computer Programming

A programme having features such as data abstraction, encapsulation and data hiding, polymorphism inheritance is called

• A. Structured programme
• B. Object oriented programme
• C. Open source programme
• D. Real time programme

Answer: B. Object oriented programme

Question 62

Topic: Computer Programming

The library function exit () causes as exit from

• A. the loop in which it occurs
• B. the block in which it occurs
• C. the functions in which it occurs
• D. the programme in which it occurs

Answer: D. the programme in which it occurs

Explanation: exit (): This is a C library function, exit () that can be used to cause a program to end. The exit () function is defined in header file, stdlib.h, you have to include the header file at the beginning of your program. A non zero argument to the exit() function tells the operating system that the program has terminated abnormally.

Question 63

Topic: Computer Programming

The "go to statement" causes control to go to

• A. An operator
• B. A label
• C. A variable
• D. A function

Answer: B. A label

Question 64

Topic: Engineering Mathematics

The differential equation satisfying $y=Ae^{3x}+Be^{2x}$ is

• A. $\frac{d^{2}y}{dx^{2}}+5\frac{dy}{dx}-6y=0$
• B. $\frac{d^{2}y}{dx^{2}}-5\frac{dy}{dx}+6y=0$
• C. $\frac{d^{2}y}{dx^{2}}+5\frac{dy}{dx}+6y=0$
• D. $\frac{d^{2}y}{dx^{2}}-5\frac{dy}{dx}-6y=0$

Answer: B. $\frac{d^{2}y}{dx^{2}}-5\frac{dy}{dx}+6y=0$

Explanation: $D^{2}-5D+6=0$\n$D^{2}-3D-2D+6=0$\n$D(D-3)-2(D-3)=0$\n$D=2,3$\nThe general solution is:\n$y=Ae^{3x}+Be^{2x}$

Question 65

Topic: Signals and Systems

For an energy signal $g(t)$ and its time-shifted version $f(t-T)$, the

• A. amplitude spectra are identical and their phase spectra are also identical
• B. amplitude spectra are identical but their phase spectra are different
• C. amplitude spectra are different but their phase spectra are identical
• D. amplitude spectra and their phase spectra are different

Answer: B. amplitude spectra are identical but their phase spectra are different

Question 66

Topic: Communication Systems

As the modulation index of an FM signal with sinusoidal modulation is increased from zero to two, the power in the carrier component will

• A. Decrease continuously
• B. Increase continuously
• C. First increase, becomes zero and then decrease
• D. First decrease, become zero and then increase

Answer: A. Decrease continuously

Question 67

Topic: Electromagnetics

For a short wave radio link between two stations via the ionosphere, the ratio of the maximum usable frequency to the critical frequency

• A. is always less than 1
• B. May be $\le1$ depending on the distance between the two stations
• C. is always greater than 1
• D. Does not depend on the distance between the two stations

Answer: C. is always greater than 1

Explanation: $f_{muf}\ge f_{c}$\n$\frac{f_{muf}}{f_{c}}\ge1$

Question 68

Topic: Communication Systems

A carrier voltage is simultaneously modulated by two sine waves causing modulation indices of 0.4 and 0.3. The overall modulation index is

• A. 0.35
• B. 0.7
• C. 0.5
• D. Can not be calculated unless the phase relations are known

Answer: C. 0.5

Explanation: $m=\sqrt{(m_{1})^{2}+(m_{2})^{2}}=\sqrt{(0.4)^{2}+(0.3)^{2}}=0.5$

Question 69

Topic: Communication Systems

If each stage has a gain of 10 dB, and noise figure of 10 dB, then the overall noise figure of a two-stage cascade amplifier will be

• A. 10
• B. 1.09
• C. 1.0
• D. 10.9

Answer: D. 10.9

Explanation: $F=F_{1}+\frac{F_{1}-1}{G_{1}}=10+\frac{10-1}{10}=10+0.9=10.9$

Question 70

Topic: Communication Systems

De-emphasis circuit is used

• A. To attenuate high frequencies
• B. To attenuate low frequencies
• C. To attenuate midband frequencies
• D. None of the above

Answer: A. To attenuate high frequencies

Question 71

Topic: Communication Systems

An amplifier has an input power of 2 microwatts. The power gain of the amplifier is 60 dB. The output power will be

• A. 6 microwatts
• B. 120 microwatts
• C. 2 milliwatts
• D. 2 watts

Answer: D. 2 watts

Explanation: Power gain (dB) = $10 \log \frac{P_0}{P_{in}}$\n$60=10 \log\frac{P_{0}}{P_{in}}$\n$\frac{P_{0}}{P_{in}}=(10)^{6}$\n$P_{0}=P_{in}\times(10)^{6}=2\times10^{-6}\times10^{6}=2~Watt$

Question 72

Topic: Electromagnetics

A transmission line has a VSWR of 2, the reflection coefficient is

• A. $1/3$
• B. 0
• C. $1/4$
• D. $1/2$

Answer: A. $1/3$

Explanation: $VSWR=\frac{1+|\rho|}{1-|\rho|}$\n$2=\frac{1+|\rho|}{1-|\rho|}$\n$\Rightarrow 2-2|\rho|=1+|\rho|$\n$|\rho|=\frac{1}{3}$

Question 73

Topic: Communication Systems

Which vacuum based device is widely used as a power amplifier in satellite communication system?

• A. Amplitron
• B. Klystron
• C. Magnetron
• D. TWT

Answer: D. TWT

Question 74

Topic: Electromagnetics

Maxwell's equations establish the relationship between E,D,H and $\overline{B}$ any point in a continuous medium. When we move from one medium to another, then which of the following statement is correct

• A. At the boundary separating the two media, the tangential components of E and H are continuous
• B. At the boundary separating the two media the tangential components of only $\overline{E}$ are continuous
• C. If there are surface charges present on the boundary then normal component of $\overline{D}$ continuous
• D. If there are no surface charges then normal component of $\overline{D}$ is discontinuous

Answer: B. At the boundary separating the two media the tangential components of only $\overline{E}$ are continuous

Question 75

Topic: Electromagnetics

The input impedance of short circuited lossless line of length less than a quarter wavelength is

• A. Purely resistive
• B. Purely inductive
• C. Purely capacitive
• D. Complex

Answer: B. Purely inductive

Question 76

Topic: Electromagnetics

The intrinsic impedance of free space is 377 $\Omega$. The approximate intrinsic impedance of a medium with relative permittivity and permeability of 4 and 1 respectively will be

• A. 75 $\Omega$
• B. 94 $\Omega$
• C. 188 $\Omega$
• D. 377 $\Omega$

Answer: C. 188 $\Omega$

Explanation: $\eta=\sqrt{\frac{\mu_{0}}{\epsilon_{0}}}\cdot\sqrt{\frac{\mu_{r}}{\epsilon_{r}}}=377\cdot\sqrt{\frac{1}{4}}=188~\Omega$

Question 77

Topic: Communication Systems

A 8 kHz communication channel has an SNR of 30 dB. If the channel bandwidth is doubled, keeping the signal power constant, the SNR for the modified channel will be

• A. 27 dB
• B. 30 dB
• C. 33 dB
• D. 60 dB

Answer: A. 27 dB

Explanation: $\frac{S}{N}=1000$\nNew $S/N$ ratio is here signal power is constant but by doubling channel B.W. noise will double, so\n$(\frac{S}{N})_{new}=(\frac{S}{2N})=\frac{1000}{2}=500$\n$(\frac{S}{N})_{new}=10 \log 500=26.98~dB\simeq27~dB$

Question 78

Topic: Signals and Systems

Two orthogonal signals $S_{1}(t)$ and $s_{2}(t)$ satisfy the following relation

• A. $\int_{0}^{T}S_{1}(t)s_{2}(t)dt=0$
• B. $\int_{0}^{T}S_{1}(t)s_{2}(t)dt=1$
• C. $\int_{0}^{T}S_{1}(t)s_{2}(t)dt=\infty$
• D. $\int_{0}^{T}S_{1}(t)s_{2}(t)dt=\pi$

Answer: A. $\int_{0}^{T}S_{1}(t)s_{2}(t)dt=0$

Question 79

Topic: Communication Systems

In which modulation system, when modulating frequency is doubled, the modulation index also becomes double

• A. AM
• B. FM
• C. PM
• D. None of the above

Answer: D. None of the above

Question 80

Topic: Engineering Mathematics

The matrix $A=\begin{bmatrix}-4&-3&-3\\ 1&0&1\\ 4&4&x\end{bmatrix}$ is its own adjoint. The value of x will be

• A. 5
• B. 3
• C. -3
• D. -5

Answer: B. 3

Explanation: Compare original matrix A with Adj A, then $x=3$