Physics of Semiconductors Test

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Physics of Semiconductors Test - 3

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Question 1
1 / -0

Electromagnetic waves of frequencies higher than 9MHz are found to be not reflected by the ionosphere on a particular day at a place. The maximum electron density in the ionosphere is

A
10¹² m^-3

B
10¹² m^-3

C
2 10¹² m^-3

D
5 10¹² m^-3

Solution

f = 9

f = 9 10⁶ = 9
N_max = 2 10¹² m^-3
Question 2
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Compared to CB amplifier, CE amplifier has

A
low input resistance

B
low output resistance

C
low current amplification

D
infinite current amplification
Solution
Among the different configurations of transistors CC (common collector), CE (common emitter) and CB (common base), the common emitter (CE) configuration transistor has the following advantages.
- High input resistance
- Low output resistance
- More efficiency
- More current amplification
- More voltage amplification
Question 3
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Which of the following statements is wrong?

A
Radio waves in the frequency range 30 MHz to 60 MHz are called sky waves.

B
Radio horizon of the transmitting antenna for space wave is d_T = (R = radius of earth, h_T = height of transmitting antenna).

C
Within the skip distance, neither the ground waves nor the sky waves are received.

D
The principle of fibre optical communication is total internal reflection.

Solution

Radio waves in the frequency range of a few MHz to 30 to 40 MHz are called sky waves.
Question 4
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In a common base circuit of a transistor, the current amplification factor is 0.95. What is the base current when emitter current is 2 mA?

A
0.1 mA

B
0.6 mA

C
1 mA

D
10 mA

Solution

Current gain of common base circuit,

A

A

A

A

Or, 0.1 mA
Question 5
1 / -0

In an optical communication system operating at 1200 nm, only 2% of the source frequency is available for TV transmission having a bandwidth of 5 MHz. The number of TV channels that can be transmitted is

A
2 million

B
10 million

C
0.1 million

D
1 million

Solution

=
= 25 10¹³ Hz
Frequency = 25 10¹³
= 5 10¹² Hz
Number of channels =
= 1 million
Question 6
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A transmitting antenna of height 20 m and a receiving antenna of height h are separated by a distance of 40 km for satisfactory communication in line-of-sight mode. The value of h (given radius of Earth is 6400 km) is

A
15 m

B
20 m

C
30 m

D
45 m

Solution
Question 7
1 / -0

Directions: The following question has four choices out of which ONLY ONE is correct.

If the critical frequency for sky wave propagation is 12 MHz, the maximum electron density in the ionosphere is

A
1.78 10¹²/m³

B
0.178 10¹⁰/m³

C
1.1.2 10¹²/m³

D
0.56 10¹²/m³

Solution

f = 9 (N_max)²
12 106 = 9(N_max)²
1.78 10¹²
Question 8
1 / -0

Directions: The following question has four choices, out of which ONLY ONE is correct.

In a broacasting studio, a 1000 kHz carrier wave is modulated by an audio signal of frequency range 100-5000 Hz. The width of the channel (in kHz) is

A
4900 Hz

B
4500 Hz

C
4000 Hz

D
3500 Hz

Solution

Frequency of sidebands = 5000 - 100 = 4900 Hz
Question 9
1 / -0

The truth table for the following logic circuit is:

A

B

C

D

Solution

The logic circuit is an XOR gate.
Equation of the circuit is given by:

So, the truth table of the circuit diagram is:
Question 10
1 / -0

Directions: The following question has four choices out of which ONLY ONE is correct.

In the diagram, the input ac is across the terminals A and C. The output across B and D is

A
same as input

B
half wave rectified

C
zero

D
full wave rectified
Question 11
1 / -0

In the following circuit, the current flowing through 1 kresistor is

A
0 mA

B
5 mA

C
10 mA

D
15 mA

Solution

I = = 5 mA
Question 12
1 / -0

^{_{In a common emitter configuration of a transistor, the voltage drops across a 500} _{resistor in the collector circuit of 0.5 V. If the current gain in the common base mode is 0.96, then the base current is}}

A
μA

B
μA

C
mA

D
mA

Solution

We have I_E = I_B+ I_C

β = = I_B = ,where I_C = I_B = A
I_B =mA
Question 13
1 / -0

In the given circuit, the current through the resistor of 2 k is

A
2 mA

B
4 mA

C
6 mA

D
1 mA

Solution

The current through the resistor of 2 k^{can be calculated as follows:}

I = 6 mA

As the voltage across the 2k = Voltage across the zener diode
Question 14
1 / -0

The graph of input characteristics of common emitter amplifier is

A

B

C

D

Solution
Question 15
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A p-n junction diode is connected to a battery of emf 5.5 V and external resistance of 5.1 k. The barrier potential in the diode is 0.4 V. The current in the circuit is

A
1.08 mA

B
0.08 mA

C
1 mA

D
1 A

Solution

The potential difference across the resistance = 5.5 - 0.4 = 5.1 V.

The current through the resistance is I.

So, V = IR

I = 1 mA
Question 16
1 / -0

If current gain of common emitter n-p-n transistor is 100, its emitter current is

A
slightly more than the collector current

B
slightly less than the collector current

C
equal to the collector current

D
equal to the base current

Solution

Current gain of common emitter n-p-n transistor is 100. Its emitter current is slightly more than the collector current.
Question 17
1 / -0

The current gain of a transistor in common emitter mode is 40. To change the collector current by 160 mA at constant V_CE, the necessary change in the base current is

A
0.25 μA

B
4 μA

C
4 mA

D
40 mA

Solution

The current gain in the common emitter configuration is

mA
Question 18
1 / -0

Two diodes have a resistance of 20 ohm and are centre-tapped with rms secondary voltage from centre tap to each end of secondary voltage of 50 V. If the external resistance is 980 ohm, then what is the mean load?

A
4.5 A

B
45 mA

C
0.25 A

D
25 mA

Solution
Question 19
1 / -0

In a p-n junction diode, a square input signal of 10 V is applied as shown in the figure. Then, the output signal across R_L will be

A

B

C

D

Solution

The junction diode is forward biased. The amplitude of the input signal is 5 V. The amplitude of the output signal remains the same but the negative part of the signal is removed as the diode does not conduct the negative voltage.
Question 20
1 / -0

Which of the following configurations has the highest voltage gain?

A
CB configuration

B
CE configuration

C
CC configuration

D
Same in all cases

Solution

CB transistor configuration has the highest voltage gain.