# VTU Previous Question Papers BE EC 3rd Semester

# Electronic Circuits July 2008

**Note : Answer any FIVE fall questions.**

1 a. For the sketch shown in fig.Ql(a) below,Vj varies from 0 to 150V. Sketch the output voltage V_{0} to the same time scale as the input voltage. Assume diodes to be ideal.

b. Explain the operation of full wave voltage doubler circuit.

c. A full wave rectifier circuit is fed from a secondary center tapped transformer. The rms voltage from either end of secondary to center tap is 30V if diode resistance Rf = 2Ω, half secondary resistance R_{s}=8Ω and load R_{L}= 1kΩ, calculate efficiency of rectification.

2 a. Discuss the causes for bias instability in a transistor,

b. With neat circuit diagram, explain how compensation for Vbe can be obtained using diode in emitter circuit of a transistor.

c. A Ge transistor used in self biased circuit has Vcc ^{=} 20V, Rc ” 2k, operating point V_{C}e ^{=} 10V and Ic — 4mA, and p – 50. Calculate Ri, R_{2} and R_{E} if stability factor S = 5I_{C} /5Ico=10 is desired.

3 a. Discuss the biasing techniques used for linear integrated circuits.

b. Derive the expression for Ay Ai Rj_{n} and Ro of an emitter follower.

4 a. Derive the expression for transistor transcon ductance g_{m} and input conductance, gbc in the case of transistor.

b. State and prove Miller’s theorem.

5 a. Derive an expression for input and output resistance of voltage series feedback amplifier.

b. An amplifier with negative feedback has a voltage gain of 120. It is found that without feedback, an input signal of 60mV is required to produce a particular output, where as with feedback the input signal must be 0.5V to get the same output. Find the Av and J3 of the amplifier.

c. Mention the advantages of negative feedback.

6 a. Derive an expression for the maximum conversion efficiency of a class B push – pull amplifier.

b. Calculate the input power, output power and efficiency of the amplifier in the figure Q6(b) shown for an input voltage in a base current of 10mA peak. Also calculate the power dissipated by the transistor.

**Fig-6(b)**

7 a. Draw the circuit and explain how to measure the CMRR of an OP-amp.

b. Find output voltage V_{0} in terms of Vi and V_{2} for the op-amp circuit shown in fig. 7(b). Assume ideal op-amps.

**Fig. 7(b)**

c. Design a Schmitt trigger whose Vlt and Vur are ± 5V. Draw waveforms. Take LM741 with V_{sat} = ± 13.5V and supply voltage ± 15V.

8 a. Draw and explain the working of positive clamper.

b. Explain the working of D/A converter [Binary weighted resistors] with neat sketch.

c. Give the circuit schematic off 555 timer connected as an a stable multivibrator. Describe its operation.