# I B.Tech Regular Examinations, May/Jun 2008

# BASIC ELECTRONIC DEVICES AND CIRCUITS

# SET-4

1. (a) Draw the schematic diagram of a CRT and explain about the various sections and the materials used.

(b) In a CRT, the electrons emitted are accelerated by a potential of 500V. The length of the deflecting plates is 1.3 cm. Distance between the deflecting plates is 0.5 cm. The distance between the centre of the deflecting plates and the screen is 20 cm. Determine the value of electrostatic deflection sensitivity.

2. (a) Draw the basic structure of a varactor diode and explain its characteristics.

(b) Write the diode equation and discuss the effect of temperature on diode current.

3. (a) Discuss the operation of HW recitifier with and without capacitor filter.

(b) Draw the Half wave rectifier circuit using a step down Transformer with Vs = 46 sin(100? t) and a semiconductor diode. Calculate the turns ratio of the Transformer windings when the primary voltage of the Transformer is 230 volts.

4. (a) Give the UJT symbol and simplified equivalent circuit with external resistors included.

(b) Draw UJT emitter characteristics and mention various regions.

(c) If VE<VP and VE>VP , explain how UJT works for these conditions.

5. (a) What is the necessity to stabilize the operating point of transistor amplifier?

(b) What is thermal runaway?

(c) For a fixed bias configuration determine Ic, Rc, Rb and Vce using the following specifications: Vcc=12V, Vc=6V, ? =80, Ib = 40 ?A.

6. (a) For a CB configuration, what is the maximum value of RL for which Ri doesnot exceed 50 ? Transistor parameters are hib=21.6

, hrb = 2.9×10?4, hfb=- 0.98 and hob=0.49? A/V.

(b) Sketch the circuit of a CD amplifier. Derive the expression for the voltage gain at low frequencies and what is the order of magnitude of the output impedance.

7. (a) Draw a feedback amplifier in block-diagram form. Identify each block and state its funcetion.

(b) An amplifier has a voltage gain of -100. The feedback ratio if 0.01. Find the voltage gain with feedback, the output voltage of the feedback amplifier for an input voltage of 1mv, feedback factor, and feedback voltage?

8. (a) Prove that the frequency stability improves as d?/dw increases in oscillators.

(b) Draw the FET crystal oscillator and derive the equation for frequency of oscillations.