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Code No: R5100207
I B.Tech (R05) Supplementary Examinations, June 2009 ELECTRONIC DEVICES AND CIRCUITS (Common to Electrical & Electronics Engineering, Electronics & Communication Engineering, Computer Science & Engineering, Electronics & Instrumentation Engineering, Bio-Medical Engineering, Information Technology, Electronics & Control Engineering, Computer Science & Systems Engineering, Electronics & Computer Engineering and Instrumentation & Control Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Give the block diagram of CRO and explain about each block in detail? (b) In a electrostatic deflecting CRT the length of the deflection plats is 2cm, and spacing between deflecting is 0.5cm, The distance from the cenlve of the deflecting plate to the screen is 20cm, the deflecting voltage is 25V Find the deflecting sensitivity, the angle of diction and velocity of the beam. Assume final anode potential is 1000V [8+8] 2. (a) Explain the formation of depletion region in an open-circuited pn-junction with neat sketches. [8] (b) A pn-junction diode has a reverse saturation current of 30 µA at a temperature of 1250 C. At the same temperature find the dynamic resistance for 0.2V bias in forward and reverse direction. [8] 3. (a) Explain why a bridge rectifier is preferred over a centre-tap rectifier. [4] (b) Explain the necessity of a bleeder resistor. [4] (c) A diode has an internal resistance of 20Ω and 1000Ω load from a 110V rms source of supply. Calculate i. the efficiency of rectification ii. the percentage regulation from no load to full load. [8] 4. (a) Describe a set up to obtain the output characteristics of a transistor in CE configuration. Indicate the various regions of operation on the output characteristics. [8] (b) Explain the principle of MOSFET in depletion mode. With neat sketches and o/p characteristics. [8] 5. (a) Draw a BJT fixed bias circuit and derive the expression for the stability factor ‘S’. [3+5] (b) An NPN transistor with β = 50 is used in a common emitter circuit with VCC = 10V,RC = 2k. The bias is obtained by connecting a 100K resistance from collector to base. Assume VBE = 0.7 V. Find i. the quiescent point and [4+4] ii. the stability factor, S. 6. (a) Draw the circuit for darlington pair and derive the expressions for AI , AV , RI and R0 . [3+5] (b) The figure 1 shows a CE amplifier with collector to base bias. Calculate AI , AV , RI . The transistor parameters are hie =1.1K, hf e =50, hoe =25× 10−6 A/V, hre =2.5 × 10−4 . [8]
Figure 1: 7. (a) Describe with necessary derivations the effect of negative feedback on the bandwidth and distortion in an amplifier. [8] [4+4] (b) Explain the effect of input and output impedances on voltage series feedback amplifier. 8. (a) What type of feedback is employed in oscillators? And what are the advantages. Discuss the conditions for sustained oscillations. (b) Find the capacitor C and hf e for the transistor to provide a resonating frequency of 10KHZ of a phase-shift oscillator. Assume R1 =25kΩ, R2 =60kΩ, Rc =40kΩ, R=7.1kΩ and hie =1.8kΩ. [10+6] ?????
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Code No: R5100207
I B.Tech (R05) Supplementary Examinations, June 2009 ELECTRONIC DEVICES AND CIRCUITS (Common to Electrical & Electronics Engineering, Electronics & Communication Engineering, Computer Science & Engineering, Electronics & Instrumentation Engineering, Bio-Medical Engineering, Information Technology, Electronics & Control Engineering, Computer Science & Systems Engineering, Electronics & Computer Engineering and Instrumentation & Control Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. Compare the motion and trajectories of electron when placed (a) Only in electric fields (b) Only in magnetic fields (c) Combined electric & Magnetic fields.
[5+5+6]
2. (a) Explain the process of break down of a pn-junction diode due to i. Avalanche effect ii. Zever effect [8] 0 (b) Find the concentration of holes and electrones in a p-type silicon at 300 K assuming resistivity as 0.02 Ωcm. Assume µp =475m2 /v-sec, ni =1.45× 1010 per cm3 . [8] 3. (a) Explain why a bridge rectifier is preferred over a centre-tap rectifier. [4] [4] (b) Explain the necessity of a bleeder resistor. (c) A diode has an internal resistance of 20Ω and 1000Ω load from a 110V rms source of supply. Calculate i. the efficiency of rectification ii. the percentage regulation from no load to full load. [8] 4. (a) With neat diagram explain the various current components in an pnp transistor. (b) Explain the input and output characteristics of a transistor in CB configuration.
[8] [8]
5. (a) Explain how a FET is used as a voltage variable resistor. (b) A self biased p-channel JFET has a pinch off voltage VP =5V and IDSS =12mA. The supply voltage available is 12V. Determine the values of RD and RS so that ID =5mA and VDS =6V. [8+8] 6. (a) Draw the common base circuit and derive the expressions for voltage gain and current gain. [3+5] (b) The figure 1 shows the circuit of a common gate JFET amplifier. The JFET has gm =2500µS. Determine the voltage gain and input resistance. [4+4]
Figure 1: 7. (a) Explain the concept of feedback with block diagram. [4+4] (b) An Amplifier with negative feedback gives an output of 12.5V with an input of 1.5V. When feedback is removed, it requires 0.25V input for the same output. Find i. value of voltage gain without feedback ii. value of β, if the input and output are in phase and β is real. [3+5] 8. (a) Draw the circuit diagram of a RC phases shift oscillator using BJT. Derive the expression for frequency of oscillators. (b) Classify different type of oscillators based on frequency range. (c) Why RC oscillators are not suitable for high frequency applications. [8+4+4] ?????
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Code No: R5100207
I B.Tech (R05) Supplementary Examinations, June 2009 ELECTRONIC DEVICES AND CIRCUITS (Common to Electrical & Electronics Engineering, Electronics & Communication Engineering, Computer Science & Engineering, Electronics & Instrumentation Engineering, Bio-Medical Engineering, Information Technology, Electronics & Control Engineering, Computer Science & Systems Engineering, Electronics & Computer Engineering and Instrumentation & Control Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 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 [8+8] (b) Derive the expression for the electro magnetic deflection sensitivity in the case of the CRT. 2. (a) Explain why a semi conductor acts as an insulator at 00 K and its conductivity increase with temperature. (b) A step graded P-n junction has ND = 103 .NA and NA corresponds to 1 atom per 108 germanium atoms. Calculate the junction potential. Assume ni = 2.5 × 103 per cm3 , atom density of Ge = 4.4 × 1022 atoms per cm3 . [8+8] 3. (a) Draw the circuit diagram of full-wave rectifier with inductor filter. (b) A full-wave rectified voltage of 18V peak is applied across a 500µF filter capacitor. Calculate the ripple and d.c. voltages if the load takes a current of 100mA. [8+8] 4. (a) With neat diagram explain the various current components in an pnp transistor. (b) Explain the input and output characteristics of a transistor in CB configuration.
[8] [8]
5. (a) List the three sources of instability of collector current and define three stability factors. (b) For the circuit shown (figure 1), calculate IB , VC and VCE .
[4+4] [8]
Figure 1: 6. (a) Discuss the effect of emitter bypass capacitor on low frequency response of CE amplifier. [6] (b) A CE amplifier is drawn by a voltage source of internal resistance rs = 800 Ω and the load impedance is a resistance RL = 1000 Ω. The h- parameters are hie =1K, hf e =50, hoe = 25 µA/V, hre = 2 × 10−4 . Calculate AV , AI , RI and RO . 1 1 1 1 [2 /2 ]+[2 /2 ]+[2 /2 ]+[2 /2 ] 7. (a) Draw the circuit diagram of voltage series feedback amplifier and derive expressions for voltage gain and feedback factor. [2+3+2] (b) An amplifier has a voltage gain of 400, f1 =50Hz, f2 =200KHz and a distortion of 10% without feedback. Determine the amplifier voltage gain f1f , f2f and Df when a negative feedback is applied with feedback ratio of 0.01. [8] 8. (a) Draw the circuit diagram of a RC phases shift oscillator using BJT. Derive the expression for frequency of oscillators. (b) Classify different type of oscillators based on frequency range. (c) Why RC oscillators are not suitable for high frequency applications. [8+4+4] ?????
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Code No: R5100207
I B.Tech (R05) Supplementary Examinations, June 2009 ELECTRONIC DEVICES AND CIRCUITS (Common to Electrical & Electronics Engineering, Electronics & Communication Engineering, Computer Science & Engineering, Electronics & Instrumentation Engineering, Bio-Medical Engineering, Information Technology, Electronics & Control Engineering, Computer Science & Systems Engineering, Electronics & Computer Engineering and Instrumentation & Control Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Derive the expression for trajectory of an electron placed in combined electric(E) and magnetic fields(B). Both the fields are perpendicular to each other and the initial velocity is zero (b) The magnetic flux density B = 0.02wb/m2 and electric field strength E = 105 v/m are uniform fields, perpendicular to each other.A pure source of an electron is placed in a field. Determine the minimum distance from the source at which an electron with 0v will again have 0v in its trajectory under the influence of combined Electric and magnetic fields [8+8] 2. (a) Explain the formation of depletion region in an open-circuited pn-junction with neat sketches. [8] (b) A pn-junction diode has a reverse saturation current of 30 µA at a temperature of 1250 C. At the same temperature find the dynamic resistance for 0.2V bias in forward and reverse direction. [8] 3. (a) Draw the circuit diagram of bridge rectifier with L-section L-C filter and explain its working in brief. [8] (b) A bridge rectifier with capacitor filter in fed from 230V to 50V step down transformer. If average d.c. current is load in 1 AMP and capacitor filter of 1000µF. Calculate load regulation and ripple factor. Assume power line frequency of 50Hz. Neglect diode forward resistance and d.c resistance of secondary of transformer. [8] 4. (a) What is early effect? How does it modify the V-I characteristics of a BJT. (b) Define αdc and βdc of a transistor. Derive the relation between them. (c) Give reason for cut off conditions for Si and Ge transistors are different. 5. (a) Explain in detail about thermal runaway and thermal resistance. (b) For the circuit shown figure 1, determine IE , VC and VCE . Assume VBE =0.7V
[6] [6] [4]
[8+8]
Figure 1: 6. (a) Discuss the effect of emitter bypass capacitor on low frequency response of CE amplifier. [6] (b) A CE amplifier is drawn by a voltage source of internal resistance rs = 800 Ω and the load impedance is a resistance RL = 1000 Ω. The h- parameters are hie =1K, hf e =50, hoe = 25 µA/V, hre = 2 × 10−4 . Calculate AV , AI , RI and RO . 1 1 1 1 [2 /2 ]+[2 /2 ]+[2 /2 ]+[2 /2 ] 7. (a) Explain the concept of feedback as applied to electronic amplifier circuit. What are the advantages and disadvantages of positive and negative feedback? [4+3+3] (b) An Amplifier has a voltage gain of 1000. With negative feedback, the voltage gain reduces to 10. Calculate the fraction of the output that is fedback to the input. [6] 8. (a) Draw the circuit diagram of a RC phases shift oscillator using BJT. Derive the expression for frequency of oscillators. (b) Classify different type of oscillators based on frequency range. [8+4+4] (c) Why RC oscillators are not suitable for high frequency applications. ?????