WBUT Exam Papers IT Basic Electrical And Electronics EngineeringI B Tech Sem Ist 2011
WBUT Exam Papers IT
Basic Electrical And Electronics EngineeringI B Tech Sem Ist 2011^{ }
Time Allotted : 3 Hours
Full Marks : 7
The figures in the margin indicate full marks.
Candidates are required to give their answers in their own words
as far as practicable.
PARTI { Marks : 35 ) , GROUP – A ( Multiple Choice Type Questions )
Choose the correct alternatives for any Jive of the following :
i) The conductance G of a series r r r•* * ^{1 = 5} resistance R and inductive reactance X_{L} is glven^ ^{&}
3) G = — ^ ~ R
_{R} O) G =
c) G ——— 5________________________ „ R^{2}
p2 y2 G ~ – 7T——
R +X_{L R}2_{+X}2
[ Turn over
ii) Inductive reactance of a coil of inductance 02 H at 50 Hz is
a) 628 Q b) 628 Q
c) 02 Q d) 20 Q.
iii) For a coil with Nturns, the self inductance will be proportional to
1
a) N b)
c) N^{2} d) .
N’
iv) Area of hysteresis loop is a measure of ,
a) retentivity
b) coercivity
c) saturated flux density ‘
d) energy loss.
v) The power factor of a purely inductive circuit is
a) zero b) one
c) infinity d) 05.
vi) The form factor of a current waveform is 1, its shape is
a) sinusoidal b) triangular
c) square d) sawtooth.
GROUP B
I
f‘
( Short Answer Type Questions )
Answer any two of the following. 2×5= 10
2. A network of resistances is formed as shown in Figure 1. Compute the resistance between the points A and B.

 Derive a mathematical expression for r.m.s. value of a sinusoidal voltage v = V_{m} sin tot.
 Two coils have self inductances L_{x} and and mutual inductance between them is M. Derive a mathematical expression for coefficient of coupling k for these coils.
 Determine the value of R in Figure 2 such that 4 Q resistor consumes maximum power
GROUP C ( Long Answer Type Questions )
Answer any two of the following. 2 x 10 = 2
 a) What is meant by the term “resonance” in a series R.L.C. circuit ? 3
b) A 20 £2 resistor, a choke coil having some inductance and some resistance and a capacitor are connected in series across a 25 V variable frequency source. When frequency is 400 Hz, the current is maximum and its value is 05 A and the potential difference across the capacitor is 150 V. Calculate the resistance and the inductance of the choke coil and the capacitance of the capacitor. 7
7 a) A flux of 0 0006 Wb is required in the airgap of an iron
ring of crosssection 5 0 cm^{2} and mean length 27 m
with an airgap of 45 mm. Determine the ampere turns
required. Six H values and corresponding B values are
noted from the magnetisation curve of iron and given
H ( AT/m )  200  400  500  600  800  ‘————
1000 
B ( Wb/m^{2} )  04  08  10  109  117  119 
b) 
A circuit receives du a cun cut aiu,—— — –
lag from a 250 V, 50 Hz, 1ph A.C. supply. Calculate the capacitance of the capacitor which is required to be
connected across the circuit to make the power factor
State and explain Thevenins theorem. ^{3}
Find the Thevenin equivalent of the circuit of Figure 3 as shown at terminal XY.
Derive a mathematical expression for the average real power delivered by a single phase a.c. source with an e.m.J. of e = V2E_{m}sin(01 when the source current is
i = V2 I_{m} sin (wiB).
Define power factor of an a.c. circuit. State the major disadvantages of poor power factor.
PART – II ( Marks : 35 )
GROUP – A ( Multiple Choice Type Questions )
 Choose the correct alternatives for any Jive of the following :5×1=5
i) Fermi level of an ntype semiconductor lies
a) near the conduction band edge
b) near the valence band edge
c) at the middle of the band gap
d) none of these.
ii) For an npn transistor, I_{CBO} approximately doubles for
temperature rise of every
%
a) 5°C b) 7°C
c) 10°C # d) none of these.
iii) If a of a BJT is 098, then the value of p is
a) 099 b) 99
c) 50 d) 49.
iv) The power rating of a BJT is determined by which of the following ?
a) Collector base junction area
b) Base width
c) Heat sink
d) Emitter base junction area.
v) It is easy to break the covalent bond by thermal energy in case of
a) Carbon b) Germanium
c) Arsenic d) Silicon.
vi) The temperature coefficient of Zener breakdown voltage is
a) positive
b) negative
c) zero
d) either positive or negative.
GROUP B ( Short Answer Type Questions )
Answer any two of the following. 2×5= 10
 What do you mean by an intrinsic semiconductor ? Will it
behave as an insulator at any temperature ? Explain. 2 + 3
 Explain the mechanism of Zener breakdown in pn junction and write how it differs from avalanche breakdown. 3 + 2
 What is meant by d.c. operating point or Q point in the context of transistor characteristics ? What is load line ? Why is transistor biasing necessary ? 2+1+2
 Explain the principle of operation of a varactor diode. Mention one application. 4 + 1
GROUP C ( Long Answer Type Questions )
Answer any two of the following. 2 x 10 = 20
 Consider an intrinsic silicon bar of crosssection 5 cm and length 05 cm at room temperature 300K. An average field of 20 V/cm is applied across the ends of the silicon bar.
a) Calculate —
i) electron and hole component of current density
ii) total current in the bar
iii) resistivity of the bar. 6o
b) If now donor impurity to the extent of 1 part in 10 atoms of Si is added, find the density of minority carriers and the resistivity. 4
Given :
Electron mobility = 1400 cm^{2} /V – s ‘
Hole mobilitv = 450 cm^{2}/V s
Intrinsic carrier concentration of Si
At room temperature ( 300K )= 15×10^{10} /cm^{J}
No. of Si atoms / m^{3} = 4 • 99 x 10^{28}.
 a) Explain drift and diffusion of charge carrier in semi
conductors. Derive the expression for electric current due to drift and diffusion. 4 + 3
b) With the help of energyband diagram, differentiate among conductor semiconductor and insulator. 3
 a)
What is ripple factor ? Evaluate the ripple factor and efficiency of a fullwave bridge rectifier. 2 + 3
A silicon diode with internal resistance R_{F} – 25 Q is
used for halfwave rectification. The input a.c. voltage is Vj =20 sin tot and the load resistance is 500 Q.
Find,
i) d.c. output voltage
ii) a.c. input power and
iii) efficiency of the rectifier. 2 + 2+1
Draw the circuit diagram for selfbiased configuration considering an npn transistor in CE configuration. Derive the expression for its stability factors. 3 + 2
Calculate V_{CE} and I_{c} in the circuit below. Assume V_{B£}=07V. _{4}
c) What is the voltage gain of a transmitter circuit in CC configuration ? 2