# May/Jun-2008 Set-II

1. (a) An electron is moving perpendicular to magnetic field ‘B’. Derive the expression for radius ‘R’ of the trajectory and period of rotation T.
(b) Derive the expression for the electro magnetic deflection sensitivity in the case of hte CRT.
2. (a) How does the reverse sturation current of diode varies with temperature. Explain.
(b) Draw the energy band diagram of p-n diode for no bias, forward bias and reverse bias and explain.
3. (a) Define the following terms of a half wave rectifier with resistive load:
i. Ripple factor
ii. Peak inverse voltage
iii. Rectification efficiency.
(b) A 230 V, 60Hz voltage is applied to the primary of a 5 : 1 step down, center tapped transformer used in a full wave rectifier having a load of 900?. If the diode resistance and the secondary coil resistance together has a resistance of 100?, determine:
i. dc voltage across the load
ii. dc current flowing through the load
iii. dc power delivered to the load
iv. PIV across each diode.
v. Ripple voltage and its frequency.
4. (a) For a small signal JFET iD = f(VGS, VDS). Obtain expressions for iD and hence define gm, rd and ?.
(b) From the definition of gm and rd obtain expression for ?.
(c) For an n-channel silicon FET with a = 3×10?4cm and ND = 1015 electrons/cm3. Find the pinch off voltage.

5. (a) Prove that stability factor S?? = (IC1?ICO1)S2/ ?1(? 2+1) where S2 is the value of stabilizing factor S when ? = ?2
(b) Draw a circuit employing a sensistor compensation and explain its working.

6. (a) Draw the equivalent circuit for the CE and CC configurations subject to the restriction that RL=0. Show that the input impedance of the two circuits are identical.
(b) For any single-transistor amplifier prove that Ri = hi /1?hrAv

7. (a) An amplifier has a gain of -100 and a distortion of 8%. What is the effect of introducing negative feedback with feedback factor of 0.05?
(b) Find Af for a CE stage with an un bypassed emitter resistor.

8. (a) Draw the circuit diagram of a transistor phase-shift oscillator and discuss its operation. Obtain an expression for its frequency of oscillation.
(b) What is the range of frequencies over which a crystal oscillator may be normally used and why?