# WBUT Question Papers IT

## Basic Electrical And Electronics Engineering-I B Tech Sem Ist 2010-11

Time Allotted : 3 Hours

Full Marks : 70

The figures in the margin indicate full marks.

Candidates are required to give their answers in their own words

as far as practicable.

PART – I ( Marks : 35 )

GROUP – A ( Multiple Choice Type Questions )

1. Choose the correct alternatives for any five of the following :

5×1=5

i)       The form factor of a wave is 1. Its shape is

a) sinusoidal         b) triangular

c) square                d) sawtooth.

ii)       The admittance of a parallel circuit is 0-5 Z – 30°. The circuit is

a) inductive                       b) capacitive

c) resistive                         d) in resonance.

iii)     The force experienced by a small conductor of length L, carrying a current I, placed in a magnetic field B* at an angle 0 with respect to B* is given by

a)                                                               BIL    b) BIL sin 0

c) BIL cos 0                        d) zero.

iv)      The mutual inductance between two coupled coils is 10 mH. If turns of one coil are doubled and that in other are halved, the mutual inductance will be

a)                                                               5 mH b) 10 mH

c) 14 mH                           d) 20 mH.

v)        Three resistors of 4Q, 6 Q and 8 £2 are connected in

parallel. The maximum power dissipation will occur in

– i

a)        4 Q      b) ■ 6 Q

c) 8(2                                 d) equal in all resistors

5 V, lOfl  b) 10 V, 10 ft

c) 5V.5Q                         d) 15 V, 15-ft.

GROUP -B ( Short Answer Type Questions )

Answer any two of the following. 2×5= 10

1. State and prove maximum power transfer theorem.
2. Compare electric and magnetic circuits with respect to their similarities and dissimilarities.
3. What is resonance ? Deduce the expression of frequency in a series RLC circuit at resonance.
4. At t = 0, the instantaneous value of a 50 Hz, sinusoidal current is 5 Amp and increases in magnitude further. Its R.M.S. value is 10 Amp.

a)       Write the expression for its instantaneous value

• i

b)        Find the current at t= 0-01 and t = 0-015 sec

c)        Sketch the waveform indicating these values.

GROUP -C ( Long Answer Type Questions )

Answer any two of the following. 2×10 = 20

1. a) For the circuit shown below, determine the current 11 , l2 , l3 using nodal analysis : ,

TVVW

as/u

b)        For the circuit shown below, find the potential difference between a and d :

 S’S*

6 + 4

Explain what are meant by phase and phase difference of sinusoidal waves.

A coil of resistance 30 Q and inductance 320 mH is connected in parallel to a circuit consisting of 75 Q in series with 150 pF capacitor. The circuit is connected to a 200 volt, 50 Hz supply. Determine supply current and circuit power factor.          2 + 8

1. £0 State and explain Biot-Savart law.

b)        A ring having a mean diameter of 21 cm and a cross­section of 10 cm 2 is made of two semicircular sections of cast iron and cast steel respectively with each joint having reluctance equal to air gap of 0-2 mm as shown in figure. Determine the ampere turns required to produce a flux of 0-8 mWb. The relative permeabilities of cast iron and cast steel are 166 and 800 respectively. Neglect fringing and leakage effects. 4 + 6

9. a) Prove that current in purely resistive circuit is in phase with applied A.C. voltage and current in purely capacitive circuit leads applied voltage by 90° and draw their waveforms.

b)        A circuit consists of series combination of elements as reistance of 6 £2, inductance of 0-4 M and a variable capacitor across 100 V, 50 Hz supply. Calculate (i) value of capacitance at resonance, (ii) voltage drop across capacitor and (iii) Q factor of coil.                                                                          5 + 5

USB SEPARATE ANSWER-BOOK TO ANSWER PART-II QUESTIONS. PART – II ( Marks : 35 ) GROUP – A ( Multiple Choice Type Questions )

1. Choose the correct alternatives for any five of the following :5×1=5

i)         Barrier potential of Ge diode is

a) 0-3 V                             b) 0-7 V

c)        04 V                           d) 0 V.

ii)       With both junctions      reverse biased the transistor operates in

a) active region                    b) cut-off region

c)  saturation region   d)       inverted region.

iii)      If a resistor has the colour code ( brown-black-red ), the value of the resistor equals

a)       1000 Q                       b) 10 k£2

c)        110 Q                         d) 100 Q.

iv)      For full-wave rectifier

a)        one centre-tapped transformer is required

b)        two centre-tapped transformers are required

c)        more than two centre-tapped transformers are required

d)        centre-tapped transformer is not required.

v)  X has high current, voltage, power gain. X is

a) CE amplifier b) CB amplifier

 d)

GROUP -B ( Short Answer Type Questions )

Answer any two of the following. 2×5= 10

1. Differentiate between Avalanche and Zener breakdowns.
2. a) Explain with appropriate diagram why a semiconductor

acts as an insulator at about O K and why its conductivity increases with increasing temperature.

b)        If a donor type of impurity is added to the extent of one

atom in 10 million Ge atoms, calculate the resistivity and conductivity of the N-type material so formed. What is the percentage of increase in the conductivity compared to the intrinsic Ge at 399 K. Given at 300 K, atoms/m 3 of Ge = 4-4 x 10 28 , Ni = 2 5 x 10 19, p = 0-38 m 2/V-s, p = 0-18 m 2/V-s.                 2 + 3

1. Compare two types of full-wave recitifier :

a)        Centre tapped transformer

b)        Bridge type.

1. Find the values of (i) IB , (ii) IE , (iii) V CE , (iv) V E and V B for the following circuit. Assume (3 = 49 and VBE = 0-7 V.
GROUP-C ( Long Answer Type Questions )
Answer any two of tJie following. 2 x 10 = 20
1. a) What is thermal runaway ? Can we interchange the

emitter and collector of a transistor ? In what region of the characteristic curve does a transistor operate when it is used as a switch.  4 + 3+1

b)        What do you mean by load line for a transistor circuit ?

2

1. Define h-parameters used in hybrid model of transistor with diagram. Compare the characteristics of CE, CC and CB transistors.      6 + 4
2. a) Explain the principle of n-channel depletion MOSFET.
3.  b)  Write a short note on CMOS. 6 + 4
1. Write short notes oh any two of the following :        2×5

i)         Clipper circuit

ii)       Ripple factor                       1

iii)      Varactor diode.