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**Question Paper Code: 11283**

**B.E./B.Tech.Degree Examinations, April/May 2011**

**Regulations 2008**

**Fourth Semester**

**Electronics and Communication Engineering**

**EC 2251 Electronics Circuits II**

**( Common to PTEC 2251 Electronics Circuits II for B.E.(Part -Time) Third**

**Semester ECE - Regulations 2009)**

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**Part A - (10 x 2 = 20 marks)**

1. What i s `return ratio' of a feedback amplifier?

2. State the e®ect on output resistance and on input resistance of amplifier when current shunt feedback i s employed.

3. A Weinbridge oscillator is used for operations at 9 kHz. If the value of the resistance R is 100 kohm, what i s the value of C required?

4. A tuned collector oscillator in a radio receiver has a fixed inductance of 60 ¹H and has to be tunable over the frequency band of 400 KHz to 1200 KHz. Find the range of variable capacitor to be used.

5. What i s the e®ect of cascading n stages of identical single tuned amplifiers (synchronously tuned) on the overall 3 dB bandwidth?

6. What is narrow band neutralization?

7. What is meant by clipper circuit?

8. What are the applications of bistable multivibrator?

9. What are the applications of Blocking oscillator?

10. Sketch and define the `slope error' of a voltage sweep waveform.

**Part B - (5 x 16 = 80 marks)**

11. (a) (i) Sketch the block diagram of a feedback amplifier and derive the expressions for gain

(1) with positive feedback and (2) with negative feedback. State the advantages of negative feedback. (6)

(ii) An amplifier, without feedback, has a voltage gain of 400, lower cut-off frequency f1 = 50 Hz, upper cut-ff frequency f2 = 200 KHz and a distortion of 10%.

Determine the amplifier voltage gain, lower cut-ff frequency and upper cut-off frequency and distortion, when a negative feedback i s applied with feedback ratio of 0.01. (5)

(iii) An amplifier, with feedback, has voltage gain of 100. When the gain without feedback changes by 20% and the gain with feedback should not vary more than 2%. If so, determine the values of open loop Gain A and feedback ratio fi. (5)

OR

11. (b) (i) Draw the circuits of voltage shunt and current series feedback amplifiers and derive the expressions for input impedance Rif . (10)

(ii) Discuss Nyquist criterion for stability of feedback amplifiers, with the help of Nyquist plot and Bode plot. (6)

12. (a) (i) Sketch the basic block diagram of an oscillator and explain how it works. If the gain of the amplifier is A and the feedback factor i s fi, sketch the output waveforms for the three cases (1) jAfij > 1, (2) jAfij = 1 and (3) jAfij < 1.

Derive the conditions of sustained oscillations. (10)

(ii) Make a table of comparison of RC phase shift oscillator and Wien-bridge oscillator bringing out the similarities and di®erences. (6)

OR

12. (b) (i) Explain the working of a Colpitts oscillator with a neat circuit diagram and derive the frequency of oscillation. (8)

(ii) In a Colpitt's oscillator, the value of the inductor and capacitors in the tank circuit are L = 40 mH, C1 = 100 pF and C2 = 500 pF. (8)

(1) Find the frequency of oscillation.

(2) If the output voltage is 10 V, find the feedback voltage at the input side of the amplifier.

(3) Find the minimum gain, if the frequency is changed by charging `L' alone.

(4) Find the value of C1 for a gain of 10 if C2 i s kept constant as 500 pF. Also find the resulting new frequency.

13. (a) (i) Draw the circuit diagram and the equivalent circuit of a capacitor-coupled single tuned amplifier and explain its operation. Derive the equations for voltage gain and for 3-dB bandwidth. Sketch also the frequency response of the amplifier.

(12)

(ii) A single tuned transistor amplifier is used to amplify modulated RF carrier of 600 kHz and bandwidth of 15 kHz. The circuit has total output resistance Rt = 20 k and output-capacitance C0 = 50 pF. Calculate the values of inductance and capacitance of tuned circuit. (4)

OR

13. (b) (i) Explain, with suitable circuit diagrams, Hazeltine neutralization and coil neutralization techniques. (8)

(ii) A Class C tuned amplifier has RL = 6 k and the tank circuit is required to have QL = 80. Calculate the values of L and C of the tank circuit. Assume Vcc = 20V, resonant frequency = 5 MHz and worst case power dissipation = 20 mW. (8)

14. (a) (i) With necessary circuit diagrams and waveforms, explain the operation of the following: (1) Positive clipper, (2) Negative clipper, (3) Biased clipper and (4) Combinational (Two-way) clipper. (12)

(ii) Mention the applications of clamping circuits. (4)

OR

14. (b) (i) With neat circuit diagram and suitable wave forms, explain the operation of a collector coupled transistor monostable multivibrator. (8)

(ii) Design a Schmitt trigger circuit to have VCC = 12 V, UTP = 5 V, LTP = 3V and IC = 2 mA using two silicon NPN transistors with hfe(min) = 100 and I2 = 0:1IC2. (8)

15. (a) (i) Draw and explain the triggering circuit used in monostable blocking oscillator.(8)

(ii) Explain, with the help of circuits and wave-forms, the operation of RC-controlled push-pull astable blocking oscillator with emitter timing. (8)

OR

15. (b) (i) Design a UJT relaxation oscillator to generate a saw tooth wave form at a frequency of 500 Hz.

Assume the supply voltage VBB = 20 V, VP = 2:9 V, VV = 1:118 V, IP = 1:6 mA and IV = 3:5 mA. State further assumptions made,if any. Sketch the circuit designed. (8)

(ii) Sketch a current time base circuit and explain its working with the help of relevant waveforms. (8)

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