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Code No: R7100207
I B.Tech (R07) Regular & Supplementary Examinations, June 2009 BASIC ELECTRONIC DEVICES AND CIRCUITS (Electrical & Electronics 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. (b) Two parallel plates of a capacitor are separated by 4 cms. An electron is at rest initially at the bottom plate. Voltage is applied between the plates, which increases linearly from 0V to 8V in 0.1 m.sec. [8+8] 2. (a) Derive the expression for contact difference of potential V0 in the case of an open circuited p-n junction. (b) What is the ratio of current for a forward bias of 0.08 V to the current for the same magnitude of reverse bias for the Germanium diode. [8+8] 3. (a) Discuss a full wave rectifier with π-filter. (b) Compare the performance of L-section and π-filters.
[8+8]
4. (a) Compare JFET and MOSFET with respect to various features. (b) Draw the biasing circuit suitable for JFET and if the JFET is replaced by a MOSFET for what mode of operation it is valid and explain about the function of each component used in the circuit. [6+10] 5. (a) Draw and explain a fixed bias circuit. Explain why the circuit is unsatisfactory if the transistor is replaced by another of the same type. (b) Find the value of thermal resistance required for the Ge transistor for self bias circuit with the collector current of 1.5mA at 25 0 C and it increases by 0.131mA over a temperature range of 25 to 75 0 C in order for the circuit to be thermally stable. Assume Vcc =30V and Rc =2.0K and Re =4.7K. [8+8] 6. (a) Explain the method of evaluating h parameters for a transistor in CC configuration. (b) A CB amplifier is driven by a voltage source of internal resistance Rs =1kΩ. The load impedance is RL =1K. The transistor parameters are hib = 22Ω, hf b = -0.98, hrb =2.9×10−4 , hob = 0.5µA/V. Compute current gain, voltage gain, input and output impedance of the amplifier. [8+8] 7. (a) List the steps required to carry out the analysis of a feedback amplifier. (b) Calculate voltage gain, input impedance and output impedance of a CE amplifier with voltage-shunt negative feedback. [6+10] 8. (a) Explain why in every practical oscillator the loop gain is slightly larger than unity? (b) The RC network of a Wien-bridge oscillator consists of resistors of values R1 =R2 =220KΩ and capacitors of values C1 =C2 =250pF. Calculate the frequency of oscillation. [8+8] ?????
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Code No: R7100207
I B.Tech (R07) Regular & Supplementary Examinations, June 2009 BASIC ELECTRONIC DEVICES AND CIRCUITS (Electrical & Electronics 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) Derive the expressions for diffusion capacitance of PN juncion Diode. (b) An ideal Ge p-n junction diode has reverse saturation current of 30 µA at a temperature of 1250 C. Find the dynamic resistance for a 0.2V bias: i. in the forward direction ii. in the reverse direction. [8+8] 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 900Ω. If the diode resistance and the secondary coil resistance together has a resistance of 100Ω, 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. [6+10] 4. (a) Compare JFET and MOSFET with respect to various features. (b) Draw the biasing circuit suitable for JFET and if the JFET is replaced by a MOSFET for what mode of operation it is valid and explain about the function of each component used in the circuit. [6+10] 5. (a) What is temperature compensation? Explain any temperature compensation method using active components. (b) Calculate the values of resistors in a collector to base bias circuit using the following specifications. Vcc =9V, β =115, Vbe =0.7V, ICQ =9.2mA and VceQ =4.4V and also calculate its stability factor. [8+8] 6. Obtain CB h parameters in terms of CE h parameters.
[16]
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? [8+8] (b) Find Af for a CE stage with an un bypassed emitter resistor. 8. (a) Sketch the circuit of a phase shift oscillator using a bipolar junction transistor and explain its operation. (b) Draw the circuit diagram of a crystal oscillator. What are the advantages of a crystal oscillator over an other oscillators? [8+8] ?????
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Code No: R7100207
I B.Tech (R07) Regular & Supplementary Examinations, June 2009 BASIC ELECTRONIC DEVICES AND CIRCUITS (Electrical & Electronics 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. (b) Two parallel plates of a capacitor are separated by 4 cms. An electron is at rest initially at the bottom plate. Voltage is applied between the plates, which increases linearly from 0V to 8V in 0.1 m.sec. [8+8] 2. (a) Derive diode equation and discuss about various parameters involved in the equation. (b) Explain the working principle of an LED with its characteristics.
[10+6]
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) The reverse saturation current of the Ge transistor is 2µA at room temperature of 25 0 C and increases by a factor of 2 for each temperature increase of 10 0 C. Find the reverse saturation current of the transistor at a temperature of 75 0 C. (b) Explain the working principle of MOSFET in enhancement and depletion modes. [6+10] 5. Explain how to minimize the percentage variations in IC due to variations in ICO , VBE and β with suitable circuit diagrams. [16] 6. (a) The hybrid parameters of a transistor used as an amplifier in the CE configuration are hie =800Ω, hf e = 46, hoe = 80×10−4 Ω−1 . If RL =5KΩ, Rs =500Ω. Calculate Ai , Ri , Av and Pi . (b) Which of the transistor configurations has the highest Ri , Ro , Av , AI .
[10+6]
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+8]
8. (a) Prove that the amplitude of the oscillations is limited by the onset of nonlinearity. (b) Design a phase-shift oscillator to operate at a frequency of 5kHz. Use a MOSFET with µ=55 and rd =5.5K. The phase shift network is not to load down the amplifier. Find the minimum value of the drain circuit resistance Rd for which the circuit will oscillate. [8+8] ?????
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Code No: R7100207
I B.Tech (R07) Regular & Supplementary Examinations, June 2009 BASIC ELECTRONIC DEVICES AND CIRCUITS (Electrical & Electronics 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 plates is 2cm, and spacing between deflecting plates is 0.5cm, The distance from the centre of the deflecting plate to the screen is 20cm, the deflecting voltage is 25V. Find hte deflecting sensitivity, the angle of deflection and velocity of the beam. Assume final anode potential is 1000V. [8+8] 2. (a) Draw the band diagrams of PN junction under open circuit conditions and explain. (b) Sketch charge density, electric field intensity and potential energy barriers for electrons and holes. [8+8] 3. (a) Explain the circuit diagram of a single phase full-wave bridge rectifier and sketch the input and output wave forms. (b) Define percentage regulation and prove that the regulation of both half wave and full wave rectifier is given by percentage regulation equal to Rf × 100%. Rl [8+8] 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 , explain how UJT works for these conditions. 5. (a) Prove that stability factor S 00 =
[6+5+5]
(IC −ICO1 )S β(β+1)
(b) Draw a circuit employing a thermistor compensation and explain its working. [8+8] 6. (a) Draw the small signal model of common source MOSFET amplifier and define all parameters. (b) Consider an emitter follower and show that as Re → ∞ then Ri 1/hob .
[8+8]
7. (a) Define Desensitivity D. What is the significance of this? (b) An amplifier without feedback gives a fundamental output of 36V with 7 percent secondharmonic distortion when the input is 0.028V. If 1.2 percent of the output is fed back into the input in a negative voltage series feedback circuit, what is the output voltage? If the fundamental output is maintained at 36V but the second-harmonic distortion is reduced to 1 percent, what is the input voltage? [6+10] 8. (a) What is an oscillator? Obtain an expression for the Barkhausen criterion for oscillation. (b) A phase shift oscillator uses three identical RC sections. The values of the components are R=100KΩ and C=0.01µF. Determine its frequency of oscillation. [8+8] ?????