Semiconductors & Communication · JEE Main previous year questions

In amplitude modulation, the message signal $V_m(t)=10\sin (2\pi \times 10^{5}t)$ volts and Carrier signal $V_C(t)=20\sin (2\pi \times 10^{7}t)$ volts The modulated signal now contains the message signal with lower side band and upper side band frequency, therefore the bandwidth of modulated signal is $\alpha$ kHz. The value of $\alpha$ is :

1. A.200 kHz
2. B.50 kHz
3. C.100 kHz
4. D.0

The instantaneous velocity of a particle moving in a straight line is given as $V=\alpha t+\beta t^2$, where $\alpha$ and $\beta$ are constants. The distance travelled by the particle between 1s and 2s is :

1. A.$3\alpha +7\beta$
2. B.$\frac{3}{2}\alpha +\frac{7}{3}\beta$
3. C.$\frac{\alpha }{2}+\frac{\beta }{3}$
4. D.$\frac{3}{2}\alpha +\frac{7}{2}\beta$

In a semiconductor, the number density of intrinsic charge carries at $27^{\circ }\mathrm{C}$ is $1.5\times 10^{16}/{\mathrm{m}}^3$. If the semiconductor is doped with impurity atom, the hole density increases to $4.5\times 10^{22}/{\mathrm{m}}^3$. The electron density in the doped semiconductor is ____ $\times 10^9/{\mathrm{m}}^3$.

A message signal of frequency 20 kHz and peak voltage of 20 volt is used to modulate a carrier wave of frequency 1 MHz and peak voltage of 20 volt. The modulation index will be :

Consider a situation in which reverse biased current of a particular P-N junction increases when it is exposed to a light of wavelength $\le 621\text{ nm}$. During this process, enhancement in carrier concentration takes place due to generation of hole-electron pairs. The value of band gap is nearly.

1. A.$1\text{ eV}$
2. B.$4\text{ eV}$
3. C.$0.5\text{ eV}$
4. D.$2\text{ eV}$

What should be the height of transmitting antenna and the population covered if the television telecast is to cover a radius of $150\text{ km}$? The average population density around the tower is $2000/{\text{km}}^2$ and the value of $R_e=6.5\times 10^6\text{ m}$.

1. A.$\text{Height}=1241\text{ m},\text{ Population covered}=7\times 10^5$
2. B.$\text{Height}=1731\text{ m},\text{ Population covered}=1413\times 10^5$
3. C.$\text{Height}=1800\text{ m},\text{ Population covered}=1413\times 10^8$
4. D.$\text{Height}=1600\text{ m},\text{ Population covered}=2\times 10^5$

Three particles P, Q and R are moving along the vectors $\vec{A}=\hat{i}+\hat{j}$, $\vec{B}=\hat{j}+\hat{k}$ and $\vec{C}=-\hat{i}+\hat{j}$ respectively. They strike on a point and start to move in different directions. Now particle P is moving normal to the plane which contains vector $\vec{A}$ and $\vec{B}$. Similarly particle Q is moving normal to the plane which contains vector $\vec{A}$ and $\vec{C}$. The angle between the direction of motion of P and Q is ${\cos }^{-1}\left(\frac{1}{\sqrt{x}}\right)$. Then the value of x is ________.

The normal reaction 'N' for a vehicle of $800\text{ kg}$ mass, negotiating a turn on a $30^{\circ }$ banked road at maximum possible speed without skidding is ________ $\times 10^3{\text{ kg m/s}}^2$. [Given $\cos 30^{\circ }=0.87,{\mu }_s=0.2$]

1. A.$12.4$
2. B.$7.2$
3. C.$6.96$
4. D.$10.2$

A carrier wave $V_c(t)=160\sin (2\pi \times 10^{6}t)\text{ volts}$ is made to vary between $V_{\max }=200\text{ V}$ and $V_{\min }=120\text{ V}$ by a message signal $V_m(t)=A_m\sin (2\pi \times 10^{3}t)\text{ volts}$. The peak voltage $A_m$ of the modulating signal is ________.

The correct relation between $\alpha$ (ratio of collector current to emitter current) and $\beta$ (ratio of collector current to base current) of a transistor is :

1. A.$\beta =\frac{\alpha }{1+\alpha }$
2. B.$\alpha =\frac{\beta }{1-\alpha }$
3. C.$\alpha =\frac{\beta }{1+\beta }$
4. D.$\beta =\frac{1}{1-\alpha }$

A TV transmission tower antenna is at a height of 20 m. Suppose that the receiving antenna is at : (i) ground level (ii) a height of 5 m. The increase in antenna range in case (ii) relative to case (i) is n%. The value of n, to the nearest integer, is ___________.

A block of 200 g mass moves with a uniform speed in a horizontal circular groove, with vertical side walls of radius 20 cm. If the block takes 40 s to complete one round, the normal force by the side walls of the groove is :

1. A.$9.859\times 10^{-2}\mathrm{N}$
2. B.$0.0314\mathrm{N}$
3. C.$9.859\times 10^{-4}\mathrm{N}$
4. D.$6.28\times 10^{-3}\mathrm{N}$

A 25 m long antenna is mounted on an antenna tower. The height of the antenna tower is 75 m. The wavelength (in meter) of the signal transmitted by this antenna would be :

1. A.200
2. B.400
3. C.100
4. D.300

LED is constructed from Ga-As-P semiconducting material. The energy gap of this LED is 1.9 eV. Calculate the wavelength of light emitted and its colour. $[h=6.63\times 10^{-34}\mathrm{Js}\text{ and }c=3\times 10^8\mathrm{m}{\mathrm{s}}^{-1}]$

1. A.654 nm and orange colour
2. B.654 nm and red colour
3. C.1046 nm and red colour
4. D.1046 nm and blue colour

The maximum and minimum amplitude of an amplitude modulated wave is 16V and 8V respectively. The modulation index for this amplitude modulated wave is $x\times 10^{-2}$. The value of x is _________.

If the highest frequency modulating a carrier is 5 kHz, then the number of AM broadcast stations accommodated in a 90 kHz bandwidth are _________.

For extrinsic semiconductors; when doping level is increased;

1. A.Fermi-level of both p-type and n-type semiconductors will go upward for $T>T_F\text{K}$ and downward for $T<T_F\text{K}$, where $T_F$ is Fermi temperature.
2. B.Fermi-level of p-type semiconductor will go upward and Fermi-level of n-type semiconductors will go downward
3. C.Fermi-level of p and n-type semiconductors will not be affected.
4. D.Fermi-level of p-type semiconductors will go downward and Fermi-level of n-type semiconductor will go upward.

If a message signal of frequency '$f_m$' is amplitude modulated with a carrier signal of frequency '$f_c$' and radiated through an antenna, the wavelength of the corresponding signal in air is :

1. A.$\frac{c}{f_c+f_m}$
2. B.$\frac{c}{f_c}$
3. C.$\frac{c}{f_c-f_m}$
4. D.$\frac{c}{f_m}$

A balloon filled with helium ($32^{\circ }\mathrm{C}$ and 1.7 atm.) bursts. Immediately afterwards the expansion of helium can be considered as

1. A.Irreversible adiabatic
2. B.Reversible adiabatic
3. C.Irreversible isothermal
4. D.Reversible isothermal

If a semiconductor photodiode can detect a photon with a maximum wavelength of 400 nm, then its band gap energy is : Planck's constant $h=6.63\times 10^{-34}\mathrm{J}\cdot \mathrm{s}$. Speed of light $c=3\times 10^{8}m/s$

1. A.1.5 eV
2. B.2.0 eV
3. C.3.1 eV
4. D.1.1 eV