T 8L52

B.E./B.Tech. DEGREE EXAMINATION, NOVEMBER/DECEMBER 2006.

Fourth Semester

Electronics and Communication Engineering

EC 1251- ELECTRONIC CIRCUITS - II

(Common to B.E. (Part-Time) Third Semester - Regulation 2005)

(Regulation 2004)

Time : Three hours Maximum: 100 marks

Answer ALL questions.

PARTA- (10 x2=20 marks )

Mention two advantages of negative feedback.

A voltage series feedback amplifier has a voltage gain with feedback as 83.33and feedback ratio as 0.01. Calculate the voltage gain of the amplifier withoutfeedback.

Mention the essential conditions to be satisfied bv an oscillator circuit.

Give the equivalent circuit of quartz crystal and mention its series and parallelresonance frequency.

A tuned amplifier has its maximum gain at a frequency of 2 MHz and has abandwidth of 50 kHz. Calculate the Q-factor.

Derive the resonance frequency for the tank circuit shown :

1 .

2 .

3 .

4.

5 .

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7.

9.

10.

Give the circuit of an integrator using R and C as circuit elements and mentionits output voltage V", if Vr is the input voltage.

8. Give the circuit of combinational clipper. Draw its input-output waveform forsinusoidal input.

Define sweeptime and flyback time with respect to a voltage time base signal.

Draw a simple current time base circuit and give its expression relatingcurrent and time.

1 1 . ( a )

12. (a)

(b)

PARTB- (5x16=80marks )

(i) Give the block diagram of feedback amplifier and discuss the effectof negative feedback with respect to closed loop gain, bandwidthand distortion. (10 )

(ii) An amplifier has a midband gain of 125 and a bandwidth 250 kHz.

(1) If 4Vo negative feedback is introduced, find the new bandwidthand gain.

(2) If the bandwidth is to be restricted to 1 MHz frnd the feedbackratio. (6)

Or

(i) Draw the block diagram of four types of feedback topologies andcompare them with respect to Rr and R"r. (10)

(ii) Explain Nyquist criterion to analyze the stability of feedbackamplifiers. (6)

Explain RC phase shift oscillator with neat circuit diagram. Derive itsfrequency of oscillation. Give the amplifier gain and feedback network

v

v

(b) Explain Colpitt's and Hartley oscillator and derive their frequency ofoscillation. (16)

13. (a) (i) Explain single tuned voltage amplifier and discuss its frequency(10 )response.

(ii) Explain Hazeltinetuned amplifiers.

gain to sustain oscillator operation.

Or

(16 )

neutralization method to maintain stability in(6)

Or

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(b) ( i)

(ii)

Define Class-C amplifier. Sketch a tuned Class-C amplifier with anLC tank circuit as load. Derive its efficiencv. (10)

A Class-C amplifier has a base bias voltage of -5V and Vcc = B0 V.It is determined that a peak input voltage of 9.8 V at 1 MHz isrequired to drive the transistor to its saturation current of 1.8 A.

(1) Find the conduction angle

(2) Find the output power at 1 MHz

(3) Find the efficiencv.

14. (a) (i) With neat circuit diagram explain the working of astablemultivibrator. Derive its time period. Draw the base and collectorwaveforms. (I2)

(ii) Determine the value of capacitors to be used in an astablemultivibrator to provide a train of pulse 2 p s wide at a repetitionrate of 100 kHz if Rl = Rz = 20kg-. (4)

Or

(b) Design a schmitt trigger circuit to have vcc = 12 v, urp = b v,LTp = B vand Ic = 2 mA using two silicon NPN transistors with hpe (min) = 100 andIz = 0 .1 I c , .

(6)

(16 )

(8)

15' (a) (i) Draw the circuit for generating sweep using UJT switch andexplain its operation. Give expression for sweep period andfrequency. (g)

(ii) A uni-junction transistor with q = 0.62 (intrinsic stand-off ratio)is used in a relaxation oscillator circuit with R = 5 kO andC = 0.0b pf .

(1) Determine the period and frequency of oscillation

(2) Determine the new value of R, which must be chansed inorder to obtain a frequency of oscillation of 50 Hz.

(3) IfC is increased by a factor of 10, how the value ofR changes,if the frequency is to be 50 Hz.

Or

(b) Explain Miller and Bootstrap sweep generators with necessary circuitsand waveforms. (16)

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