# WBUT Exam Papers

## Basic Electrical And Electronics Engineering B Tech 1st Sem 2011

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 :

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

a)         4 Q      b)  6 Q c) 8(2

d) equal in all resistors

GROUP -B ( Short Answer Type Questions )c) 5V.5Q                         d) 15 V, 15-ft.

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 •

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 : ,
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

Cast steel

Cast steel

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 :

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 ft                         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.

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

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 :

i)         Clipper circuit

ii)       Ripple factor

iii)      Varactor diode.