Semiconductors & Communication · JEE Main previous year questions

1. A.1/3
2. B.1/2
3. C.1/4
4. D.2/3

1. A.3.5 mA
2. B.1.5 mA
3. C.2.5 mA
4. D.7.5 mA

In an amplitude modulator circuit, the carrier wave is given by, $\text{C}(t)=4\sin (20000\pi t)$ while modulating signal is given by, $\text{m}(t)=2\sin (2000\pi t)$. The values of modulation index and lower side band frequency are :

1. A.0.3 and 9 kHz
2. B.0.5 and 10 kHz
3. C.0.4 and 10 kHz
4. D.0.5 and 9 kHz

A message signal of frequency 100 MHz and peak voltage 100 V is used to execute amplitude modulation on a carrier wave of frequency 300 GHz and peak voltage 400 V. The modulation index and difference between the two side band frequencies are :

1. A.0.25; $1\times 10^8\mathrm{Hz}$
2. B.4; $2\times 10^8\mathrm{Hz}$
3. C.4; $1\times 10^8\mathrm{Hz}$
4. D.0.25; $2\times 10^8\mathrm{Hz}$

An n-p-n transistor operates as a common emitter amplifier, with a power gain of 60 dB. The input circuit resistance is $100\Omega$ and the output load resistance is $10\mathrm{k\Omega }$. The common emitter current gain $\beta$ is :

1. A.$10^4$
2. B.$10^2$
3. C.$6\times 10^2$
4. D.60

An NPN transistor is used in common emitter configuration as an amplifier with $1\mathrm{k\Omega }$ load resistance. Signal voltage of 10 mV is applied across the base-emitter. This produces a 3 mA change in the collector current and $15\mu \mathrm{A}$ change in the base current of the amplifier. The input resistance and voltage gain are :

1. A.$0.67\mathrm{k\Omega }$, 200
2. B.$0.33\mathrm{k\Omega }$, 1.5
3. C.$0.67\mathrm{k\Omega }$, 300
4. D.$0.33\mathrm{k\Omega }$, 300

A signal $A\cos \omega t$ is transmitted using $v_0\sin {\omega }_{0}t$ as carrier wave. The correct amplitude modulated (AM) signal is

1. A.$v_0\sin {\omega }_{0}t+A\cos \omega t$
2. B.$(v_0+A)\cos \omega t\sin {\omega }_{0}t$
3. C.$v_0\sin [{\omega }_0(1+0.01A\sin \omega t)t]$
4. D.$v_0\sin {\omega }_{0}t+\frac{A}{2}\sin ({\omega }_0-\omega )t+\frac{A}{2}\sin ({\omega }_0+\omega )t$

1. A.NAND
2. B.AND
3. C.NOR
4. D.OR

The physical sizes of the transmitter and receiver antenna in a communication system are :-

1. A.proportional to carrier frequency
2. B.inversely proportional to carrier frequency
3. C.inversely proportional to modulation frequency
4. D.independent of both carrier and modulation frequency

A common emitter amplifier circuit, built using an npn transistor, is shown in the figure. Its dc current gain is 250, $R_C=1\mathrm{k\Omega }$ and $V_{CC}=10\mathrm{V}$. What is the minimum base current for $V_{CE}$ to reach saturation ?

1. A.$100\mu \mathrm{A}$
2. B.$7\mu \mathrm{A}$
3. C.$10\mu \mathrm{A}$
4. D.$40\mu \mathrm{A}$

In a line of sight radio communication, a distance of about 50 km is kept between the transmitting and receiving antennas. If the height of the receiving antenna is 70m, then the minimum height of the transmitting antenna should be : (Radius of the Earth = $6.4\times 10^6\mathrm{m}$).

1. A.40 m
2. B.20 m
3. C.51 m
4. D.32 m

A 100 V carrier wave is made to vary between 160 V and 40 V by a modulating signal. What is the modulation index ?

1. A.0.5
2. B.0.6
3. C.0.4
4. D.0.3

1. A.$20\mu \mathrm{A}$ and 2.8 V
2. B.$25\mu \mathrm{A}$ and 2.8 V
3. C.$20\mu \mathrm{A}$ and 3.5 V
4. D.$25\mu \mathrm{A}$ and 3.5 V

Mobility of electrons in a semiconductor is defined as the ratio of their drift velocity to the applied electric field. If, for an n-type semiconductor, the density of electrons is $10^{19}{\text{ m}}^{-3}$ and their mobility is $1.6{\text{ m}}^2/(\text{V.s})$ then the resistivity of the semiconductor (since it is an n-type semiconductor contribution of holes is ignored) is close to :

1. A.$2\Omega \text{m}$
2. B.$4\Omega \text{m}$
3. C.$0.4\Omega \text{m}$
4. D.$0.2\Omega \text{m}$

1. A.5.5 mA
2. B.6.7 mA
3. C.2.5 mA
4. D.3.3 mA

A carrier wave of peak voltage 14 V is used for transmitting a message signal. The peak voltage of modulating signal given to achieve a modulation index of 80% will be :

1. A.7 V
2. B.28 V
3. C.11.2 V
4. D.22.4 V

Light of wavelength $550nm$ falls normally on a slit of width $22.0\times 10^{-5}cm$. The angular position of the second minima from the central maximum will be (in radians) :

1. A.$\frac{\pi }{12}$
2. B.$\frac{\pi }{8}$
3. C.$\frac{\pi }{6}$
4. D.$\frac{\pi }{4}$

1. A.13.5 mA
2. B.0
3. C.15 mA
4. D.11.5 mA

A telephonic communication service is working at carrier frequency of 10 GHz. Only 10% of it is utilized for transmission. How many telephonic channels can be transmitted simultaneously if each channel requires a bandwidth of 5 kHz ?

1. A.$2\times 10^6$
2. B.$2\times 10^3$
3. C.$2\times 10^5$
4. D.$2\times 10^4$

The current gain of a common emitter amplifier is 69. If the emitter current is 7.0 mA, collector current is :

1. A.9.6 mA
2. B.6.9 mA
3. C.0.69 mA
4. D.69 mA

A signal is to be transmitted through a wave of wavelength $\lambda$, using a linear antenna. The length l of the antenna and effective power radiated ${\mathrm{P}}_{\mathrm{eff}}$ will be given respectively as : (K is a constant of proportionality)

1. A.$\lambda$, ${\mathrm{P}}_{\mathrm{eff}}=\mathrm{K}{\left(\frac{l}{\lambda }\right)}^2$
2. B.$\frac{\lambda }{8}$, ${\mathrm{P}}_{\mathrm{eff}}=\mathrm{K}\left(\frac{l}{\lambda }\right)$
3. C.$\frac{\lambda }{16}$, ${\mathrm{P}}_{\mathrm{eff}}=\mathrm{K}{\left(\frac{l}{\lambda }\right)}^3$
4. D.$\frac{\lambda }{5}$, ${\mathrm{P}}_{\mathrm{eff}}=\mathrm{K}\left(\frac{l}{\lambda }\right){\left(\frac{l}{\lambda }\right)}^{\frac{1}{2}}$

What is the conductivity of a semiconductor sample having electron concentration of $5\times 10^{18}{m}^{-3}$, hole concentration of $5\times 10^{19}{m}^{-3}$, electron mobility of $2.0{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ and hole mobility of $0.01{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ ? (Take charge of electron as $1.6\times 10^{-19}\mathrm{C}$)

1. A.$1.68(\Omega \text{-}\mathrm{m}{)}^{-1}$
2. B.$1.83(\Omega \text{-}\mathrm{m}{)}^{-1}$
3. C.$0.59(\Omega \text{-}\mathrm{m}{)}^{-1}$
4. D.$1.20(\Omega \text{-}\mathrm{m}{)}^{-1}$

A signal of frequency 20 kHz and peak voltage of 5 Volt is used to modulate a carrier wave of frequency 1.2 MHz and peak voltage 25 Volts. Choose the correct statement.

1. A.Modulation index = 5, side frequency bands are at 1400 kHz and 1000 kHz
2. B.Modulation index = 5, side frequency bands are at 21.2 kHz and 18.8 kHz
3. C.Modulation index = 0.8, side frequency bands are at 1180 kHz and 1220 kHz
4. D.Modulation index = 0.2, side frequency bands are at 1220 kHz and 1180 kHz

A red $LED$ emits light at $0.1\mathrm{watt}$ uniformly around it. The amplitude of the electric field of the light at a distance of $1\mathrm{m}$ from the diode is :

1. A.$5.48V/m$
2. B.$7.75V/m$
3. C.$1.73V/m$
4. D.$2.45V/m$

A signal of $5\mathrm{kHz}$ frequency is amplitude modulated on a carrier wave of frequency $2\mathrm{MHz}$. The frequencies of the resultant signal is/are :

1. A.$2005\mathrm{kHz}$, $2000\mathrm{kHz}$ and $1995\mathrm{kHz}$
2. B.$2000\mathrm{kHz}$ and $1995\mathrm{kHz}$
3. C.$2\mathrm{MHz}$ only
4. D.$2005\mathrm{kHz}$ and $1995\mathrm{kHz}$