Code: A-05 Subject: BASIC ELECTRONICS

Time: 3 Hours Max. Marks: 100

NOTE: There are 11 Questions in all.

Question 1 is compulsory and carries 16 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.

Answer any THREE Questions each from Part I and Part II. Each of these questions carries 14 marks.

Any required data not explicitly given, may be suitably assumed and stated.

Q.1 Choose the correct or best alternative in the following : (2x8)

a. n-type silicon is obtained by

(A) doping with tetravalent element.

(B) doping with pentavalent element.

(C) doping with trivalent element.

(D) doping with a mixture of trivalent and tetravalent elements.

b. The forward characteristic of a diode has a slope of approximately 50 mA/V at a desired point. The approximate incremental resistance of the diode is

(A) . (B) .

(C) . (D) .

c. Two stages of BJT amplifiers are cascaded by R-C coupling. The voltage gain of the first stage is 10 and that of the second stage is 20. The overall gain of the coupled amplifier is

(A) (B) 10 + 20.

(C) . (D) .

d. In the voltage range, of an ideal JFET or MOSFET

(A) the drain current varies linearly with .

(B) the drain current is constant.

(C) the drain current varies non linearly with .

(D) the drain current is cut off.

e. In a voltage shunt negative feedback amplifier system, the input resistance and the output resistance of the basic amplifier are modified as follows:

(A) is decreased and increased.

(B) Both and are decreased.

(C) Both and are increased.

(D) is increased and is decreased.

f. The use of crystal in a tunable oscillator

(A) improves frequency stability.

(B) increases the gain of the oscillator.

(C) helps to obtain optimum output impedance.

(D) facilitates generation of wide range of frequencies.

g. The large signal band-width of an Op-Amp is limited by its

(A) loop gain. (B) slew rate.

(C) output impedance. (D) input frequency.

h. Rectification efficiency of a full wave rectifier without filter is nearly equal to

(A) 51%. (B) 61%.

(C) 71%. (D) 81%.

PART I

Answer any THREE Questions. Each question carries 14 marks.

Q.2 a. What are intrinsic and extrinsic semiconductors? Comment on the conductivity of extrinsic semiconductor. (4)

b. Describe Hall effect. Mention at least three applications of Hall effect. (4)

c. A sample of pure silicon has electrical resistivity of . The free carrier density in it is . If the electron mobility is three times that of hole mobility, find electron mobility and hole mobility. The electronic charge = Coulomb. (6)

Q.3 a. Explain Zener breakdown. The Zener diode in the circuit shown below in Fig.1 regulates at 50 V, over a range of diode currents from 5 to 40 mA. The supply voltage V = 150 V. Compute the value of R to allow voltage regulation from a zero load current to a maximum load current . What is ? (8)

b. Draw a figure to show the output v i characteristic curves of a BJT in CE configuration. Indicate thereon, the saturation, active and cut off regions. Explain how, using these characteristics, one can determine the value of or . (6)

Q.4 a. Present a comparison of performance characteristics of the three BJT amplifier configurations, in qualitative terms, regarding their input, output impedances and voltage, current gains. (5)

b. Draw a small-signal h-parameter equivalent circuit for the CE amplifier shown below in Fig.2.

Find an expression for voltage gain of the amplifier. Compute the value of voltage gain, if and . (9)

Q.5 a. The circuit of a common-source FET amplifier is shown below in Fig.3. Find expressions for voltage gain and current gain for the circuit in mid frequency region where is bypassed by . Find also the input resistance for the amplifier.

if compute the values of . (8)

b. Explain the classification of power amplifiers according to operational modes. (6)

Q.6 a. Draw a block diagram of a single-loop feedback amplifier, and explain the function of each block. What are the characteristics of an amplifier that are modified by negative feedback? For the four topologies of negative feedback, indicate whether:

the input impedance and output impedance increase or decrease as a result of feedback. (9)

b. State Barkhausen criterion for sustained oscillations in a sinusoidal oscillator. The capacitance values of the two capacitors and of the resonant circuit of a Colpitt oscillator are and . The inductor has a value of 22 What is the operating frequency of oscillator? (5)

PART II

Answer any THREE Questions. Each question carries 14 marks.

Q.7 a. Draw the block schematic of a typical operational amplifier and briefly explain the function of each block. Also give the equivalent circuit of the Op-Amp. (9)

b. Illustrate how Op-Amp can be used as

(i) Summer (ii) Integrator

(iii) Differentiator. (5)

Q.8 a. Draw the circuit of a monostable multivibrator using Op-Amp. Describe its operation and derive an expression for its pulse period. (10)

b. Show how the monostable multivibrator circuit you have drawn can be converted to astable type by simple changes in the monostable circuit. (4)

Q.9 a. Sketch the circuit of a bridge rectifier and describe its operation. Derive expressions for rectification efficiency and ripple factor of the circuit. If a capacitor circuit is added to the circuit, show the output voltage waveform of the rectifier. (8)

b. Describe, using a neat circuit diagram, the operation of a transistor series voltage regulator. (6)

Q.10 a. Prove the following postulate of Boolean Algebra using truth tables.

(3)

b. Simplify the following Boolean function using Karnaught map.

Give the logical implementation of the simplified function in SOP form using suitable gates. (6)

c. Prepare the truth-table for the output F of the logic circuit shown below in Fig.4.

Write also the canonical SOP expression for F. (5)

Q.11 a. Write the logic diagram and truth-table of clocked S-R flip-flop and explain its operation. What are the drawbacks of S-R flip-flop? (6)

b. Construct a 4-bit binary ripple counter using suitable flip-flops and describe its working? (6)

c. Write the truth table for full adder. (2)

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