JEE Advance
SEMICONDUCTOR & COMMUNICATION SYSTEM
Q.1
The battery is charged from full wave rectifier fed by a sinusoidal voltage (see figure). Ideal diodes,
ammeter and voltmeter show the time-average value.At idle with only key K1 closed, voltmeter shows
12 V, and current is then absent, ie, reading of ammeter is 0. If only the key K2 is closed, the voltmeter
shows battery voltage at 12.3 V. During charging, when the K1 and K2 are closed, the voltmeter shows
12.8 V and ammeter shows 5A. find the internal resistance of battery.
Q.2
Complete the following table of values for this diode circuit, assuming a typical forward voltage drop of
0.65 volts for the diode:
R1
I
P
Total
12V
V
R
D1
470
SEMICONDUCTOR & COMMUNICATION SYSTEM
Q.3
Describe the operation of this diode testing circuit:
Identify what the two light-emitting diodes (LEDs) will do when testing these three types of diodes:
(A) Good diode
(B) Diode failed shorted
(C) Diode failed open
Explain how can we use it to find polority of a good LED.
Q.4
Write a truth table for the circuit in figure. including the states at C, D, E, F and G..
A
E
C
D
G
F
B
Q.5
For the circuit shown in figure, find
i) the output voltage
ii) the voltage drop across series resistance
iii) the current through zener diode.
IL
15k
IZ
100V
10V
5k
SEMICONDUCTOR & COMMUNICATION SYSTEM
Q.6
Atechnician uses a multimeter’s ”diode check” function to identifythe terminals on a BJT.There are only
two places where a non-infinite reading is obtained, and they are as follows:
Black
Black
Red
Red
From these measurements, determine what type of BJT this is (PNP or NPN) and identify all three
terminals.
Q.7
Predict how all three transistor currents (IB, IC, and IE) will be affected as a result of the following faults.
Consider each fault independently (i.e. one at a time, no multiple faults):
(A) Base resistor RB fails open:
(B) Collector resistor RC fails open:
(C) Solder bridge (short) past base resistor RB:
(D) Solder bridge (short) past collector resistor RC:
For each of these conditions, explain why the resulting effects will occur.
Q.8
Complete the table of output voltages for several given values of input voltage in this common-collector
amplifier circuit.Assume that the transistor is a standard silicon NPN unit, with a nominal base-emitter
junction forward voltage of 0.7 volts:
Vin
0.0 V
0.5 V
1.0 V
1.5 V
5.0 V
7.8 V
Vout
SEMICONDUCTOR & COMMUNICATION SYSTEM
Q.9
Complete the table of output voltages, output currents, and input currents for several given values of
input voltage in this common-collector amplifier circuit.Assume that the transistor is a standard silicon
NPN unit, with a nominal base-emitter junction forward voltage of 0.7 volts:
Vin
0.0 V
0.4 V
1.2 V
3.4 V
7.1 V
10.8 V
Vout
Iin
Iout
Calculate the voltage and current gains of this circuit from the numerical values in the table:
Q.10
Avery important measure of a transistor’s behavior is its characteristic curves, a set of graphs showing
collector current over a wide range of collector-emitter voltage drops, for a given amount of base
current. The following plot is a typical curve for a bipolar transistor with a fixed value of base current:
A ”test circuit” for collecting data to make this graph looks like this:
Identify three different regions on this graph: saturation, active, and breakdown, and explain what each
of these terms mean. Also, identify which part of this curve the transistor acts most like a currentregulating device.
SEMICONDUCTOR & COMMUNICATION SYSTEM
Q.11
Is it necessary for a transmitting antenna to be at the same height as that of the receiving antenna for
line-of-sight communication? ATV transmitting antenna is 81m tall. How much service area can it
cover if the receiving antenna is at the ground level?
Q.12
A carrier wave of peak voltage 12V is used to transmit a message signal. What should be the peak
voltage of the modulating signal in order to have a modulation index of 75%?
Q.13
A modulating signal is a square wave, as shown in fig.
1
m(t) in volts
2 t in seconds
1
The carrier wave is given by c(t) = 2 sin (8t) volts.
(i) Sketch the amplitude modulated waveform
(ii) What is the modulation index?
Q.14
For an amplitude modulated wave, the maximum amplitude is found to be 10V while the minimum
amplitude is found to be 2V. Determine the modulation index .
Q.15
If the sum of the heights of transmitting and receiving antennas in line of sight of communication is fixed
at h, show that the range is maximum when the two antennas have a height
Q.16
h
each.
2
Figure shows a communication system. What is the output power when input signal is of 1.01mW? (gain
in dB = 10 log10 (Po/Pi ).
10dB
20dB
Loss 2dB km
Input
–1
Output
5.0 km
Amplifier
Q.17
Amplifier
The maximum frequencyfor reflection of skywaves from a certain layer of the ionosphere is found to be
fmax = 9(Nmax)1/2, where Nmax is the maximum electron density at that layer of the ionosphere. On a
certain day it is observed that signals of frequencies higher than 5MHz are not received by reflection
from the F1 layer of the ionosphere while signals of frequencies higher than 8MHz are not received by
reflection from the F2 layer of the inonosphere. Estimate the maximum electron densities of the F1 and F2
layers on that day.
SEMICONDUCTOR & COMMUNICATION SYSTEM
Q.18
(i)
(ii)
The intensity of a light pulse travelling along a communication channel decreases exponentially
with distance x according to the relation I = I0e–x ,where I0 is the intensity at x = 0 and is the
attenuation constant.
ln 4
.
Show that the intensity reduces by 75 per cent after a distance of
Attenuation of a signal can be expressed in decibel (dB) according to the relation
I
dB =10 log10 . What is the attenuation in dB/km for an optical fibre in which the intensity
I0
falls by 50 per cent over a distance of 50 km?
Q.19
A 50 MHz space wave takes 4.04 ms to reach a receiver via re-transmission from a satellite 600 km
above earth’s surface.Assuming re-transmission time by satellite negligible, find the distance between
source and receiver. If communication between the two was to be done by Line of Sight (LOS) method,
what should size and placement of receiving and transmitting antenna be?
SEMICONDUCTOR & COMMUNICATION SYSTEM
Q.1
Q.2
0.1
R1
D1
Total
V
11.35V
0.65V
I
24.15 mA
24.15mA
12V
24.15mA
R
470
274.1 mW
15.7mW
289.8mW
P
Q.3
Q.4
Q.5
Q.6
Q.7
Q.8
Q.9
(A)AOneBLEDCis litD
E(B) Both
F LED
G are lit
0
0
1
1
0
0
1
0
1
1
0
0
1
0
1
0
0
1
1
0
0
1
1
0
0
0
0
1
(C) Neither LED is lit.
(i) 10 V (ii) 90 V (iii) 4 mA
PNP transistor left terminal emitter, middle terminal collector and right terminal base.
(A) All 3 currents stop
(B) Base current uncharged collector current stop, emitter current decreases = IB
(C)All 3 currents greatly increase
(D) Base curent uncharged collector current charges slightly.
Vin
0.0 V
0.5 V
1.0 V
1.5 V
5.0 V
7.8 V
Vout
0V
0V
0.3 V
0.8 V
4.3 V
7.1 V
Vin
0.0 V
0.4 V
1.2 V
3.4 V
7.1 V
10.8 V
Vout
0V
0V
0.5 V
2.7 V
6.4 V
10.1 V
Iin
0µA
0µA
2.5µA
13.5µA
32µA
50.5µA
Iout
0 mA
0 mA
0.23 mA
1.23 mA , AV = 1
2.91 mA
4.59 mA
AI = 91
SEMICONDUCTOR & COMMUNICATION SYSTEM
Q.10
Left: Saturation
Middle :Active
Right : Breakdown
In the active region, it acts like current regulating device.
Q.11
No. Service area will be A d T
Q.12
0.75
2
22
1626.410 6 3258 km 2 .
7
Am
Ac
Am = 0.75 × 12 = 9V
(b) = 0.5
Q.13
(a)
Q.14
Since the AM wave is given by (Ac + Am sin mt) cos ct, the
maximum amplitude is M1 = Ac +Am. Hence the modulation index is
m
Am 8 2
A c 12 3
with M2 = 0, clearly, m = 1 irrespective of M1.
Q.15
Q.16
Q.17
Q.18
Q.19
101 mW
For F1 layer = 3.086 × 1011 m–3
For F2 layer = 7.9 × 1011 m–3
0.0602 dB/km
606 km, Distance between source and receiver = 170 km, 565 = height
0
