R09 - December, 2011 - Regular Examinations - Set - 1.

B.Tech II Year - I Semester Examinations, December 2011
ELECTRONIC DEVICES AND CIRCUITS
(COMMON TO EEE, ECE, CSE, EIE, BME, IT, MCT, E.COMP.E, ETM, ICE)
Time: 3 hours Max. Marks: 75
Answer any five questions
All questions carry equal marks
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1.a) What is Fermi level? By indicating the position of Fermi level in intrinsic, n-type and p- type semiconductor, explain its significance in semiconductors?
b) Sketch V-I characteristics of a PN diode for the following conditions:
i) Rf = 0, V? = 0, Rr = 8
ii) Rf = 0, V?= 0.6V, Rr = 8
iii) Rf = Non-zero, fixed value, V? = 0, Rr = 8
iv) Rf = Non-zero, fixed value, V? = 0. 6V, Rr = 8
Where V? is the cut-in voltage, Rf is the forward dynamic resistance & Rr is the reverse dynamic resistance of the diode. [7 8]
2.a) Draw the block diagram of a regulated power supply and explain its operation.
b) A full wave bridge rectifier having load resistance of 100O is fed with 220V, 50Hz through a step-down transformer of turns ratio 11:1. Assuming the diodes ideal, find
i) DC output voltage
ii) Peak inverse voltage
iii) Rectifier efficiency. [9 6]
3.a) With a neat diagram explain the various current components in an NPN bipolar junction transistor & hence derive general equation for collector current, IC?
b) What is Early-effect; explain why it is called as base-width modulation? Discuss its consequences in transistors in detail? [9 6]
4.a) What is ‘Thermal Runaway’ in transistors? Derive the condition to prevent ‘Thermal Runaway’ in Bipolar Junction Transistors.
b) A silicon NPN transistor has Ico = 20nA and ß=150, Vbe = 0.7V. It is operated in Common Emitter configuration (as shown in Figure.1) having Vbb = 4.5V,Rb= 150K,Rc = 3K, Vcc = 12V. Find the emitter, base and collector currents and also verify in which region does the transistor operate. What will happen if the value of the collector resistance is increased to very high values? [5 10]
Figure.1

5.a) Draw the circuit diagram & small signal equivalent of CB amplifier using accurate h-parameter model. Derive expressions for AV, AI, Ri and R0.
b) Draw small signal equivalent circuit of Emitter Follower using accurate h-parameter model. For the emitter follower circuit with RS = 0.5K and RL = 5K, calculate Ri, AV and RO. Assume, hfe = 50, hie =1K, hoe = 25 µA/V. [8 7]
6.a) Explain the construction & operation of a P-channel MOSFET in enhancement and depletion modes with the help of static drain characteristics and transfer characteristics?
b) “A depletion mode MOSFET can also be operated in enhancement mode but an enhancement mode MOSFET cannot be operated in depletion mode”. Justify?
[10 5]
7.a) Explain the need of biasing a Field Effect Transistor. With necessary equations and valid reasons explain why a simple ‘fixed bias’ arrangement for FETs is not used in practical applications?
b) A Common Source FET amplifier circuit shown in Figure.2 with un-bypassed RS has the following circuit parameters: Rd = 15K, RS = 0.5K, Rg = 1M, rd = 5K, gm= 5mS and VDD = 20 V. Calculate AV, AI, Ri and R0. [7 8]
Figure.2
8.a) With neat energy band diagrams, explain the V-I characteristics of Tunnel diode. Also discuss the negative resistance property of tunnel diode.
b) With neat sketches explain the operation of Schottky Barrier Diode. [10 5]
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