Rr221401-electronic-devices-and-circuits

Set No. 1

Code No: RR221401

II B.Tech Supplimentary Examinations, Aug/Sep 2008 ELECTRONIC DEVICES AND CIRCUITS (Mechatronics) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) Draw the Energy Band Diagrams of i. conductor ii. semiconductor and iii. Insulator and discuss the nature of their conductivities (b) The voltage across a Silicon diode at room temperature of 3000 K is 0.7 volts, when 2ma of current flows through it. If the voltage increases to 0.75 volts, calculate the diode current when the voltage equivalent of temperature ‘VT ? =26mv=0.026 volts [10+6] 2. (a) Draw the full wave rectifier circuit with center tapped secondary power transformer. (b) With reference to the fullwave rectifier circuit, draw the input signal waveform, at the secondary winding and at the load, when the input voltage to the primary of the Transformer is VPr = 230Sin(ωt) and the secondary voltages are VS1 = 30Sin(ωt) and VS2 = 30Sin(ωt+Π). (c) Explain the working of Full Wave rectifier circuit using the signal waveforms. (d) Calculate the magnitude of the ripple factor for the given set of observations of VA.C = 24 Volts and VD.C = 50 volts. Identify the type of rectifier circuit based on the value of the ripple factor. [4x4] 3. (a) Draw a diagram showing Various currents in a PNP Transistor in CB mode. (b) Explain the phenomenon of Base width modulation in Transistor operation and discuss its influence on Base current ‘IB ’ in a Common Base operated Transistor. (c) Draw the output characteristics of a Common Base operated Transistor and discuss the role of ‘Early Effect’ on the CB Transistor output characteristics. (d) Explain the operation of a PNP Transistor in Common Base configuration. [3+3+4+6] 4. (a) Discuss the reasons for Transistor bias stabilization. (b) Discuss the importance of coupling and bypass capacitances in low frequency transistor circuits (c) Draw the low frequency small signal ac equivalent circuit of a Common Emitter Transistor and discuss the assumptions made in obtaining the circuit. (d) Discuss the phenomenon of ‘Thermal runaway’ in Transistor operation. [3+4+6+3] 1 of 2

Set No. 1

Code No: RR221401

5. (a) Explain the method of calculation of voltage and current gains for a single stage CE Transistor amplifier using CE Transistor output characteristics and load line (b) Draw the typical graph showing the variations of voltage gain of transistor amplifier with frequency. Show the various regions of operations on the graph. Also mark the method of calculation for amplifier Bandwidth. [8+8] 6. (a) An N-channel JFET having VP = - 4V and IDSS = 12 ma is used as common source FET amplifier with potential divider biasing circuit. The parameter values are VDD =24V. RS = 1KΩ ; RL = 1.25KΩ ; R1 = 450KΩ and R2 = 90KΩ . Draw the circuit with these parameters and explain the circuit working as amplifier (b) Draw the D.C equivalent circuit of the above amplifier circuit and determine the values of drain current ID and VDS . [8+8] 7. (a) Draw typical frequency responses of an amplifier with and without negative feedback. (b) Derive the expression for increase in bandwidth of an amplifier using negative feedback and substantiate for such feature. (c) R-C coupled amplifier has midband gain Am = 500; Low frequency Cut-off point fL = 30 Hz and High frequency cut-off point fH = 20 KHZ. Negative voltage Feedback is added such that β = 2x10−3 . Calculate the midband gain Amf and the bandwidth of the negative feedback amplifier. [4+6+6] 8. (a) State the requirements for oscillations as embodied in the Barkhausen criterion. (b) Draw a circuit of tuned-collector oscillator and explain its working with a clear discussion on positive feedback used in the circuit and oscillator frequency (c) Calculate the transistor Colpitts oscillator’s frequency f0 ; when its frequency determining network components are L= 100µ H; C1 =0.05µ f and C2 = 0.01µ f. [4+8+4] ⋆⋆⋆⋆⋆

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Set No. 2

Code No: RR221401

II B.Tech Supplimentary Examinations, Aug/Sep 2008 ELECTRONIC DEVICES AND CIRCUITS (Mechatronics) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) Explain about Drift and Diffusion currents in semiconductors.

[3+3]

(b) Explain about cut-in voltage of forward characteristic of semiconductor diode [4] (c) Calculate the magnitude of maximum electric field at 3000 K for P-N junction with NA =1020 /cm3 on P-side and ND =1015 /cm3 on N-side of a Silicon semiconductor diode.Intrinsic carrier concentration ni =2.5x1010 /cm3 and. VT =25 mv at 3000 K [3] (d) Explain about the significance of built-in voltage

[3]

2. (a) Mention the parameters that cause for fluctuations in the output voltage of a regulated power supply circuit. (b) Draw the equivalent circuit of a Tunnel Diode. (c) Explain the significance of the ratio of peak current to valley currents on the Tunnel Diode characteristic and its relation to the Figure of merit of Tunnel Diodes. Explain its significance as the device to operate as a high speed switch. [4+6+6] 3. (a) Draw a diagram showing the structural details of UJT device. (b) Draw a diagram showing the equivalent circuit of an Unijunction Transistor. (c) Define Intrinsic stand-off ratio ‘η ? from the equivalent circuit of UJT device (d) Obtain the relation between peak-point voltage ‘VP ’ on the UJT Characteristics, supply voltage ‘VBB ’, Intrinsic stand-off ratio ‘η ’ and the barrier potential of P-N junction. Explain the significance of peak-point voltage on switching action of UJT device. [3+4+3+6] 4. (a) Draw the small signal Low Frequency equivalent circuit of CE Transistor using the Transistor h-parameters and explain the various parameters used in the circuit. (b) From the CE Transistor h-parameter equivalent circuit derive the expressions for calculating voltage and current gains of Transistor amplifier circuits. (c) Discuss the concept of DC bias and its stability for linear Transistor amplifier operation [6+6+4] 5. (a) State and explain the parameters fα , f β , f T and Gain-Bandwidth product of Transistor amplifiers. 1 of 2

Set No. 2

Code No: RR221401

(b) A transistor with Alpha Cut-Off Frequency fα = 6 MHZ and hfe or β = 60 is used in a common Emitter transistor amplifier circuit. It has CSh = 100pf at the output terminals. Calculate the values of fβ and the upper cutoff frequency when RL = 5KΩ. [8+8] 6. (a) An N-channel JFET having VP = - 4V and IDSS = 12 ma is used as common source FET amplifier with potential divider biasing circuit. The parameter values are VDD =24V. RS = 1KΩ ; RL = 1.25KΩ ; R1 = 450KΩ and R2 = 90KΩ . Draw the circuit with these parameters and explain the circuit working as amplifier (b) Draw the D.C equivalent circuit of the above amplifier circuit and determine the values of drain current ID and VDS . [8+8] 7. (a) State the assumptions made in investigating the effect of negative feedback on the amplifier characteristics so that expression for voltage gain AV f with feed back is correct. (b) Mention the five characteristics of an amplifier that are effected by negative feedback (c) A negative feedback of β = 0.004 is applied to an amplifier of gain AV = 1000. Calculate the change in overall gain of the feedback amplifier, if the internal amplifier has undergone a gain reduction of 25%. [3+5+8] 8. (a) Draw the circuit of R-C phase shift oscillator circuit using JFET as the active device and discuss the nature of feed back used in the feedback path. (b) In the R-C phase shift oscillator circuit, discuss the passive of part of the circuit that is responsible to get the 1800 phase shift. (c) Calculate the value of ‘C’ in the frequency-determining network of a FET ? RC phase shift oscillator circuit having R = 2.5 KΩ ; assuming frequency of oscillation f = 1.625 KHZ. (d) Repeat (c) if it is a BJT ? RC phase shift oscillator with RC = 4kΩ [5+5+3+3] ⋆⋆⋆⋆⋆

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Set No. 3

Code No: RR221401

II B.Tech Supplimentary Examinations, Aug/Sep 2008 ELECTRONIC DEVICES AND CIRCUITS (Mechatronics) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) Derive the expression for the barrier potential Vo at the junction of a P-N diode (b) Explain the situation for the occurrence of the phenomenon of Avalanche breakdown in semiconductor diodes and discuss the consequences. (c) Draw the equivalent circuit of a Semiconductor diode and briefly explain, how the diode acts as a switch. [6+5+5] 2. (a) Explain the application of a semiconductor diode as a rectifier. (b) Draw the input signal and output signal of a Half Wave rectifier circuit. (c) Derive the expression for VDC and VR.M.S of H W R output signal. (d) Derive the expression for ripple factor ‘γ ’ of a Half Wave rectifier circuit. [3+3+4+6] 3. (a) Draw the circuit diagram with biasing voltages to obtain UJT device characteristics. (b) Draw the UJT device characteristics and show the various regions with necessary explanation. (c) With suitable circuit diagram, explain how UJT can be used as relaxation oscillator. [4+6+6] 4. (a) Why biasing is necessary for a Transistor circuit in a given configuration. Mention the three different types of biasing a Bipolar Junction Transistor. (b) Draw the Transistor biasing circuit using fixed bias arrangement and explain its principle with suitable analysis. (c) Mention the DC load line equation for CE Transistor fixed bias circuit and describe the method of drawing the DC load line on the CE Transistor output characteristics. [6+6+4] 5. (a) Draw the practical circuit of a single stage Common Base Transistor Amplifier with potential divider biasing. (b) Assuming sinusoidal input signal to the above CB Transistor amplifier, explain the working of the amplifier with necessary waveforms. (c) Draw the A C equivalent circuit of the CB Transistor amplifier and explain the concept of amplification and mention some practical applications of it. [4+4+8] 1 of 2

Set No. 3

Code No: RR221401

6. (a) Draw the potential divider bias circuit for P-Channel JFET and explain the function of each component in the circuit. (b) Derive the expression for voltage gain of JFET model for self bias configuration. [8+8] 7. (a) Draw the four topologies of negative feedback amplifiers; clearly showing the types of sampling and mixing of signals for each type of feedback amplifiers. (b) Calculate the gain of a negative feedback amplifier having A = 450 and β = 0.02. (c) If an amplifier with gain of - 500 and feedback factor β = -0.2 has a gain change of 25% due to changes in temperatures. Calculate the change in gain of he feedback amplifier. [8+4+4] 8. (a) Draw two circuit diagrams of crystal oscillators for the two cases of the crystal operating at i. series resonant frequency and ii. at parallel resonant frequency and also explain the behavior of the crystals under such cases. (b) Calculate oscillation frequency and feedback function b for transistor Hartley oscillator with circuit components L1 = 650µH; L2 = 650µH; M = 150µH and C = 160 pf. (c) Explain piezoelectric effect.

[8+4+4] ⋆⋆⋆⋆⋆

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Set No. 4

Code No: RR221401

II B.Tech Supplimentary Examinations, Aug/Sep 2008 ELECTRONIC DEVICES AND CIRCUITS (Mechatronics) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) Draw the simple voltage regulator circuit using a Zener diode and explain the working of the circuit. (b) Discuss the reasons for a semiconductor diode to act as an open switch for a diode with reverse bias and as a closed switch with forward bias. (c) Explain the concepts for junction capacitances of a semiconductor diode and explain their impact on practical applications. [6+4+6] 2. The Transformer of a Half Wave rectifier has a secondary voltage of 30 volts (VRMS ) with winding resistance ‘rS ′ of 10 ohms. The semiconductor diode in the circuit has a forward resistance ‘rf ′ of 100 ohms. Calculate the following: [8] (a) No load DC voltage. (b) DC output voltage when the load current ‘IL ’ = 25 ma. (c) Percentage regulation at the Load current ‘IL ’ of 25 ma. (d) Ripple voltage across the load (e) Ripple frequency ‘fr ’ (f) Ripple factor ‘γ’ (g) D.C. power output and (h) Peak Inverse Voltage (PIV) of the semiconductor diode.

[2+2+4]

3. (a) Draw a diagram showing various currents in a PNP Transistor in Common Collector mode. (b) Draw a circuit to obtain the PNP Transistor characteristics in CC configuration. (c) Draw the Common Collector Transistor characteristics. (d) Explain the operation of a PNP Bipolar Junction Transistor in CC configuration. [3+3+4+6] 4. (a) Draw the circuit diagrams showing the three configurations of Transistor amplifiers. (b) Draw the Transistor biasing circuit using Collector-to-base bias arrangement. Explain the concept of providing proper bias for the Transistor to act as amplifying device

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Set No. 4

Code No: RR221401

(c) Mention the DC load line equation for CE Transistor Collector to base bias circuit and describe the method of drawing the DC load line on the CE Transistor output characteristics. [6+6+4] 5. (a) For the following Common Emitter Amplifier Circuit, reorient the necessary circuit components and locations of Vin and Vout and draw CB and CC amplifier circuits. (figure 5a)

Figure 5a (b) Discuss the relative values of Voltage gain, Current gain, input impedance and output impedances for CE, CB, CC Transistor Amplifier circuits. [8+8] 6. (a) Draw the three types of JFET biasing circuits and compare them with reference to the stability of the amplifier circuits (b) Design a fixed bias circuit using JFET for the following specifications: VDD = 12V; VDS = 5V; IDSS = 10mA; VP = −8V; VGS = −2V. [10+6] 7. (a) Enumerate the various reasons for changes in gains of electronic amplifiers. (b) Derive the expression for the reduction of relative change in gain in negative feedback amplifier compared to that without feedback. (c) If an amplifier with a gain AV of ?1000 and feedback factor β = -0.2 has again change of 25% due to changes in temperature. Calculate the change in gain of the feedback amplifier. (d) With suitable expressions for voltage gain, describe the role of by-pass capacitor in a conventional CE amplifier using BJT. Also mention the type of feedback. [4x4] 8. (a) What are the conditions required for an electronic circuit to oscillate? (b) Explain the concept of positive feedback used in sinusoidal oscillators. (c) Discuss the conditions on absolute voltage gain for oscillator start up and for amplitude stability of oscillations. (d) Classify the oscillator circuits based on oscillation frequency and wave shape of the output voltages. [4x4] ⋆⋆⋆⋆⋆ 2 of 2