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Code No: 210551 II B.Tech I- Semester Examination November/December 2002. ELECTRONICS DEVICES AND CIRCUITS (Computer Science & Engineering)

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4 a)

Explain how reverse biased diodes are used as varactor diodes. Sketch its equivalent circuit and explain. Explain hall effect and how it can be applied to measure the charge density ‘e’ and mobility ‘’ of change carrier Derive an expression for the electro-magnetic defection sensitivity of a C.R.T. Explain the working of a PNP transistor and derive expressive for its ‘emitter injection efficiency” and “base-transport factor” in terms of the physical parameters of the transistor. A Ge transistor with the 0 = 0.98 gives a reverse saturation current Ico = 10 A in CB configuration. When the transistor is used in CE configuration with a base current of 0.22 m A, calculate the collector current. Derive the formula used. Sketch the static drain and transfer characteristics of a JFET and justify their nature. What is pendi-off voltage and mark it on the characteristics. Explain how do you obtain JFET parameters from the above characteristics. AJFET has Vp = -4.5v, IDSS=16 mA and IDS=4 mA. Determine Vas required and value of “Im” at this value of vas.

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Max. Marks: 70 Answer any Five questions All questions carry equal marks ---Explain the generation and recombination of charges with regard to semiconductors. Draw the V-I characteristics of a PN junction diode and label the important parameters on it. Explain the temperature dependence of these V-I characteristics. What is the transition capacitance of a PN junction diode? How does it vary with the applied volluge . What is the order of its magnitude?

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Time: 3 hours

Explain the basic construction of an SCR. Explain its principle of operation using two transistor model. Sketch its V-I characteristics. Define ‘Hold current ‘ and latch current’ and mark them on the characteristics. Explain its turn-off mechanism. Describe the fabrication of (i) Enhancement move MOSFET aml (ii) MOS capacitor with neat cross-sectional diagrams in monolithic Integrated circuits. Contd…2.

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5 a)

b)

Code No: 210551

c)

An amplifies uses a cascade of fan BJT stages suggest suitable choice of configuration for each stage, if it is to be a high gain amplifier with high input impedance and low output impedance. Justify an answer. In a self biased circuit comprising R2 = 90 kr, R1 = 10 kr, Vcc= 22.5v, RC = 5.6 kr and RE = 1.0 kr and a BJT with 0 = 55 and VBE = 0.6v. Find the operating point and ‘S’ of the circuit. Prove that for any amplifier configuration using a BJT the input impedance Ri is given by hi Ri = __________ 1-hr AV

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Explain how a Zener diode can be used as voltage regulator. Design a power supply for a local that requires 30 walts of power. The load voltage is 300V and the ripple voltage in the load should be less that 0.2v. Use an L-section fitter and critical value of inductance. Specify the required element values and input r.m.s voltage using a single phase bridge rectifier. Assume supply frequency of 504t.

8

Write short notes on the following: a) Back ward diodes b) Biasusing circuits for MOS FETS. c) Emitter follower amplifier d) Thermal run away in BJT amplifiers

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Semester Examination November/December 2002 ...

b) Explain hall effect and how it can be applied to measure the charge density 'e' · and mobility 'μ' of change carrier · c) Derive an expression for the electro-magnetic defection sensitivity of a C.R.T. · 3 a) Explain the working of a PNP transistor and derive expressive for its 'emitter · injection efficiency” and “base-transport ...

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