r09220203-electroniccircuits

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Code No: R09220203 R09 Set No. 2

II B.Tech II Semester Examinations,December-January, 2011-2012

ELECTRONIC CIRCUITS

Electrical And Electronics Engineering

Time: 3 hours Max Marks: 75

Answer any FIVE Questions

All Questions carry equal marks

1. (a) Show that the gain of wein bridge oscillator using BJT must be at least 3 foroscillations to occur.

(b) A crystal has L = 2H, C = 0.01pF, R = 2k, its mounting capacitance is 2pF

calculate series and parallel resonating frequencies. [7+8]2. (a) State and prove clamping-circuit theorem

(b) A square wave input as shown in Figure 1 is applied to the clamping circuit.Sketch the steady-state output waveform and derive the necessary expressions.

[5+10]

Figure 1:

3. (a) Explain the transistor switch in saturation region.

(b) Explain the diode switch. [8+7]

4. A BJT has hie=6 K , hfe = 224, at IC = 1mA with fT = 80MHz and Cbc = 12pF.Determine gm, rbb at room temperature and a collector current of 1 mA? [15]

5. (a) Design a collector coupled astable multivibrator for the following specifica-tions with silicon transistor. IC(sat) = 10mA; hfe(min) = 20; VCC =10V; pulsewidth=10sec; duty cycle = 40%.

(b) Draw the circuit diagram of a fixed bias binary with speed up capacitors andexplain. [10+5]

6. (a) Define thermal resistance of a power BJT and derive expression for the same.

(b) A transistor with a maximum junction temperature specification of 1500Cdissipates a maximum power of 40 watts at a case temperature of 250C and 2watts at an ambient temperature of 250C. Find

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i. The thermal resistance between the junction and the case.

ii. The thermal resistance between the junction and ambient.iii. Maximum power dissipation capability for safe operation in free space ata temp. of 500C. [3+12]

7. (a) With the help of necessary equations discuss the variation ofAi, Av, Ri andRo with Rs and RL in CE configuration.

(b) Derive the condition to obtain Av=1 in CC single stage amplifier. [7+8]

8. (a) Draw the circuit diagram of current series feedback circuit and derive expres-sions for voltage gain, input and output resistances?

(b) Explain the concept of feedback as applied to electronic amplifier circuit.What are the advantages and disadvantages of positive and negative feed-back? [7+8]

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ELECTRONIC CIRCUITS





1. (a) Derive an expression for lower cut off frequency of feedback amplifier.

(b) Show that for current series feedback amplifier input and output resistancesare increased by a factor (1+A) with feedback. [7+8]

2. Draw and explain the circuit of Monostable Multivibrator with necessary waveformsand also derive the expression for pulse width. [15]

3. A transformer coupled class-A power amplifier drives an 8 loud speaker througha transformer with turns ratio 20:1. The supply voltage is 24V, and the circuitdelivers 4W to the load. Find

(a) Primary input power

(b) RMS value of secondary voltage and current

(c) Efficiency, if the operating collector current is 400mA. [15]

4. (a) State and prove clamping circuit theorem.

(b) A clamping circuit and square wave input with T1 = 100sec & T2 = 1000secis shown in Figure 2. Calculate and plot to scale the steady state output.Assume diode forward resistance, Rf = 100. [5+10]

Figure 2:

5. (a) Discuss the factors influencing the low frequency response of a common sourceampifier.

(b) Determine the lower cut off frequency for the circuit given below in Figure 3using the following parameters, Cs = 10F, Ce = 20 F, CC = 1F, RS = 1k, R1 = 40 k, R2 = 10 k, Re = 2 k, RC = 4k, RL = 2.2k, = 100,VCC = 20V. Assume any parameter values required. [7+8]

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Figure 3:

6. (a) Explain how transistor can be used as a switch in the circuit, under whatcondition a transistor is said to be OFF and ON respectively.

(b) A germanium transistor is operated at room temperature in the CE config-uration. The supply voltage is 6 V, the collector-circuit resistance is 200 and the base current is 20 percent higher than the minimum value required todrive the transistor into saturation. Assume the following transistor param-eters: Ico = -5A, IEO = -2A, hFE = 100, and rbb = 250 . Find VBE(Sat)and VCE(Sat). [7+8]

7. (a) Define

i. Damped oscillationsii. Undamped oscillations

(b) Why an LC tank circuit once excited does not produce sustained oscillations?

(c) What are the factors which effect frequency stability of an oscillator?

(d) Give two Barkhausen conditions required for sinusoidal oscillations to be sus-tained. [15]

8. For the circuit shown below in Figure 4, find Zi, Zo, Av and Ai with h-parametersas hie = 2.7K, hre = 2104 , hfe = 180 and hoe = 25s. [15]

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figure 4

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Code No: R09220203 R09 Set No. 1Figure 4:


ELECTRONIC CIRCUITS





1. (a) What is meant by clipping in wave shaping?

(b) Classify different types of clipper circuits. Give their circuits and explain theiroperation with the aid of transfer characteristics. [5+10]

2. (a) With the help of a diagram explain the working of a fixed Bias transistorbinary.

(b) The fixed bias binary uses n-p-n silicon transistors with hfe = 20. Assumethat VCC = 12V, VBB = 3V, RC = 1k, R1=10k & R2 = 20k. Find the stablestate currents and voltages if VCE(sat) = 0 and VBE(sat) = 0. [5+10]

3. (a) Draw circuit of Quartz crystal?

(b) What makes a Quartzcrystal produce stable oscillations?

(c) What are the advantages of crystal oscillator? [5+5+5]

4. (a) Classify amplifiers depending on conduction period and sketch the input andoutput waveforms for each classification.

(b) Discuss the design comsiderations for a maximum undistorted peak - to - peakoutput from a single stage BJT amplifier. [7+8]

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5. (a) The open loop gain of an amplifier is 50 and its bandwidth is 20 kHz. Whena negative feedback is applied the bandwidth is increased to 25 kHz. Whatwill be the required feedback?

(b) Derive an expression for upper cutoff frequency of feedback amplifier? [7+8]

6. (a) Explain the phenomenon of latching in a transistor switch.

(b) A transistor has fT = 50 MHz, hFE = 40, Cbc = 3 PF and operates with

VCC = 12 V and RC = 500. The transistor is operating initially in theneighborhood of the cut-in point. What base current must be applied to drivethe transistor into saturation in 1 sec? [5+10]

7. (a) Explain why the complimentary symmetry power amplifier has become more

popular in modern circuits.(b) A series-fed class-A power amplifier shown in Figure 5, uses a silicon transistor

and operates from 20V DC source. If the applied sinusoidal input signalgenerates a peak base current of 9mA, calculate:

i. DC input power

ii. AC output power

iii. Efficiency [5+10]

Figure 5:

8. (a) Discuss the influence of coupling capacitance Cc on low frequency response ofan amplifier.

(b) Explain how the frequency response can be determined experimentally by

applying a square wave to the amplifier. [7+8]

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ELECTRONIC CIRCUITS





1. (a) Explain the principle of clamping. Discuss the effect of source impedance,shunt resistance and cut in voltage.

(b) For the Figure 1bshown below, Vs is a sinusoidal voltage of peak 100 volts.

Assume ideal diodes & RL = . Sketch the output voltage and transfercharacteristic. [5+10]

Figure 1b:

2. In an RC phase shift oscillator if the frequency of oscillations is 955 Hz and R1=R2= R3= 680 k find the value of capacitance in the feedback path. [15]

3. (a) Describe with necessary derivations, the effect of negative feedback on thebandwidth and distortion in an amplifier.

(b) What do you mean by feedback in amplifier? Define negative feedback andpositive feedback. State the merits and demerits of negative feedback in anamplifier? [7+8]

4. (a) Draw the circuit diagram of single stage CE amplifier, Discuss the effect ofemitter bypass capacitor on low frequency response.

(b) The following low frequency parameters are known for a given transistor at Ic= 10mA, VCE = 10V and room temperature hie = 500, hoe = 4 105A/V,hfe = 100, hre = 10. At the same operating frequency fT = 500MHz and Ce= 3pF compute the values of all hybrid parameters. [7+8]

5. Find lower and upper threshold voltage for Schmitt trigger circuits, with followingdata. Assume transistors with hFE = 30.VCC = 12V; RC1 = 4K; RC2 = 1K; R1 = 2K; RS = 1K; R2 = 6K; RE = 3K. [15]

7

10 k

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6. (a) Define and explain the effects of Non linear Distortion, frequency distortionand phase distortion on the performance of Amplifiers.

(b) For a Single Stage Transistor amplifier, Calculate Ai, Av, Ri and Ro for CCconfiguration with Rs= RL=10k, by Assuming typical values of h parame-ters. [7+8]

7. (a) Consider the transistor switch in CE configuration shown in Figure 7, operatedwith Vcc = 12V and Vbb = 0V. It is given that R2 = 2R1 = 68K, Rc =2.2k.Determine the values of IB and IC of the transistor. What is the minimumvalue of hFE for the transistor to operate in saturation when it is in ON state?

Figure 7:

(b) Explain Zener & Avalanche breakdown mechanisms in diodes. [10+5]

8. (a) Explain why a power amplifier is always preceded by a voltage amplifier?

(b) A 16 speaker is driven by a transformer coupled class-A amplifier througha

15 : 1 transformer. The circuit delivers 2W to the load. Vcc = 36V.Calculate:

i. AC power across the transformer secondary

ii. RMS value of load voltage

iii. RMS value of primary voltage

iv. RMS value of load and primary currents. [3+12]

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