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.       When reverse bias is applied to p-n junction, the depletion region width

                   (A)  increases w.r.t. unbiased value.    

                   (B)  remains the same as at unbiased value.

(C)     oscillates about a mean value.      

(D)    decreases w.r.t. unbiased value.

             b.   An ohmic contact

(A)  provides rectification in  a direction depending on the valency of the contact metal.      

(B)  is intended to create potential drop.

(C)  has no potential drop across it under any condition.     

(D) has constant contact potential irrespective of the direction of current.

             c.   Consider the clipping circuit shown in Fig.1    select the correct output waveform for this circuit 

(A)                                                        (B) 

(C)                                                      (D) 

             d.   The transconductance and output resistance of a MOSFET are given as          2 mA/V and 60,000  respectively.  The amplification factor of the device is 

(A)     30.                                               (B)  120.

(C)  .                                     (D) .

e.         In a Wien bridge oscillator, sometimes zener diodes are used to

(A)     increase gain of the basic amplifier.    

(B)  decrease the frequency of oscillation.

(C)  provide temperature compensation.   

(D)  stabilize the amplitude of the oscillations.

             f.    The input resistance of a typical op-amp is

(A)     zero.                                           

(B)     of the order of a few multiples of ten of ohms.

(C)  of the order of a few multiples of ten of . 

(D)  of the order of a few multiples of ten of M.

             g.   A voltage ramp is fed to a differentiating circuit.  The output of the circuit is

(A)     constant voltage.                          (B)  sinusoidal voltage.

(C)  triangular voltage wave.                (D)  exponential voltage.

             h.   Which of the following is called an universal gate?

(A)     AND                                           (B)  NAND

(C)  OR                                              (D)  NOT

PART I

Answer any THREE Questions. Each question carries 14 marks.

  Q.2     a.   Explain the physical mechanisms by which avalanche and zener breakdowns of junctions are given rise to.                                                           (7)

b.     

Determine the bias values  and  for the transistor having  in the circuit shown in Fig.2.                                                                (7)

  Q.3     a.   Draw the circuit of a typical current mirror used for biasing a BJT in an IC.  Explain how it provides bias stability against a spread of -values.   (10)

             b.   A sinusoidal voltage is to be clamped so that its negative peak is at  volts.  Assuming the amplitude of the input wave is sufficiently larger than , suggest a clamping circuit and explain its operation.      (4)

  Q.4     a.   Draw the circuit of BJT RC-coupled amplifier.  Explain the role of each component in the circuit.  Draw its h-parameter equivalent circuit applicable to mid frequency region and analyse the same to determine an expression for mid-frequency voltage gain A_{v mid}.                                              (10)          

             b.   Explain by using circuit diagram, a Darlington pair. What are its advantages?                    (4)

  Q.5     a.   Draw a neat sketch to show the structure of n-channel depletion MOSFET.  Also draw a family of drain characteristic curves  and explain the shape of the curves.                              (7)

             b. A hybrid cascade amplifier circuit is shown in Fig.3.  Show that if , the voltage gain of the amplifier can be fairly approximated and given by , where  is the FET trans- conductance.                                                                                                     (7)

  Q.6     a.   Draw the circuit of Clapp oscillator.  Derive an expression for its frequency of oscillation.  What is the advantage of Clapp oscillator over Colpitt oscillator?                                                          (7)

             b.   Give a circuit example of current-series feedback and analyse it to find an expression for voltage gain with feedback.                                            (7)

PART II

Answer any THREE Questions. Each question carries 14 marks.

  Q.7     a.   What are the characteristics of an ideal op-amp?  Explain how an op-amp can be used as

                   (i)    non-inverting amplifier.                  (ii)  Differentiator.

                   (iii)  integrator.                                                                                                   (7)                                                                                             

             b.   Define the following terms as applied to op-amps. 

                   (i)   input bias current.                          (ii)  input offset current.

                   (iii) input offset voltage.                        (iv) output offset voltage.

                   Also describe how output offset voltage can be balanced.                                   (7)                      

  Q.8     a.   In the Schmitt trigger circuit shown below, the zener voltage , opamp gain, .  The threshold voltages desired are, .  Find (i)   and (ii)  loop gain for the circuit to function as desired.                                               (7)

            b.   Draw the internal block schematic of the 555 timer.  Propose a circuit based on the timer to generate ramp and explain the operation of the circuit.                                                               (7)

  Q.9     a.   Draw the circuit of a half-wave rectifier and derive expressions for its rectification efficiency and ripple factor.  Explain the role of capacitor input filter in a rectifier.                                                           (7)

             b.   Sketch the circuit of transistor shunt voltage regulator and describe its operation.  Compare its merits and limitations with those of series voltage regulator.                                                                 (7)

Q.10           a.                                                        State and prove DeMorgans theorems.  (4)                      

             b.   Find the simplified expression in  SOP form for the Boolean function.

                   Give logical implementation of the simplified expression using only NAND gates. (5)

             c.   Draw the logic diagram of a four-to-one multiplexer and explain its operation.     (5)

Q.11           a.  Show how a J-K flip-flop can be constructed using an S-R flip-flop.  Construct its truth-table and explain its operation. What is race-around in J-K flip-flop?  How may the race around be overcome?    (7)

             b.   Construct a 4-bit synchronous counter using J-K flip-flops and series carry.  Explain the operation of the counter.                                                                                                             (7)