# WBUT Exam Papers EC

## Digital Electronic Circuits B Tech Sem Forth 2010

Time Allotted : 3 Hours

Full Marks : 70

The figures in the margin indicate full marks.

Candidates are required to give their answers in their own words

as far as practicable.

GROUP-A

(Multiple Choice Type Questions ) r

1. Choose the correct alternatives for any ten of the following : 10 x 1 = 10

i)         A message bit is 010101, We are using even parity generator. So the parity bit added to the message bit is

a) 0      /          b) 1

c) 0 & 1            d) none of these.

ii)       What is the decimal equivalent code for the following canonical ?

. p (x, y, z) = ex + y + z1) (X + y‘ ± z1) (X + y + z)(x> + y + z1)

(x’ +y’+z’)

. . / .

a)        mr + m2 + m3 + m4 b) mx + m2 + m4 + m5

c)    mj + m3 + m4 + m5 d) none of these.

4053                                (Turnover

111) If (128)I0 = (1003)fc, the possible base b Is

a) 3                                             b) 4

c> 5                                    d) 6.

iv) The logical expression y = Jm(0,3,6,7, 10, 12, 15 ) is equivalent to

a)        y * [I M IO, 3, 6, 7, 10. 12, 15 )

b)        n ( 1, 2, 4, 5, 8, 9, 11, 13, 14 )

c)        S m( 1.2, 4, 5, 8, 9, 11, 13, 14)

d)        y-Zm(0.3.6. 7, 10. 12. 15).

v)       The Sop form of logical expression is most suitable for designing logic circuits using only

a) XOR gates         b) AND gates

c) NAND gates        d) NOR gates.

vi)      Which of the following flip-flops is used as latch ? a) JK flip-flop     b) D flip-flop

c) RS flip-flop      d) T flip-flop.

vii)    The fraction 0 -6810 is equal to 1

a) OOlOlOla •          b) OIOlj

c) OlOlOLj                d) O’lOl 112.

4053                                                  «

viii)   The resolution of a 12-bit D/A converter using a binary ladder with + 10V as the full scale output will be

a)    2-44 mV b) 3-50 mV

c) 4-32 mV          d) 512 mV.      ‘■

ix)      The initial state of MOD 16 down counter is 0110. The state after 37 clock pulse will be

a)    0000    b) 0110

c) 0101             d) 0001.

x)        In a D type latch Enable i/p is HIGH, D = 1. The o/p will be

a)         0                      b) 1

c) don’t care       d) blocked.

xi)      The frequency of the pulse at point A is

a)       10 kHz           b) 31-25 Hz

c)        50 Hz            d) 5 kHz.

[ Turn over

The value of F is

a)       AXORB

b)       AXNORC

c)       AXORC

d)       B XNOR C.

xill) Largest negative number that can be represented by 8- blt word length in 2’s complement system is

a) -255                             b) -128.

c) -127                             d) -256.

xiv) The logic family that gives fastest switching is

a)     CMOS                  b) ECL

c) Schottky TTL     d) DTL.

_     Answer any three of the following. 3×5=15

1. Draw a BCD adder circuit to add two BCD lumbers maximum up to 9. The output of this adder should also be in BCD.
2. Design a full subtractor circuit using multiplexer.
3. Construct a 2-bit comparator using only decoder.
1. a) Define the following terms related with digital IC :

ij Noise margin         \

ii)       Propagation delay

ill) Fan-In & Fan-out.

b)       Write down the characteristic equation of JK & D Flip-flops.     /                           3 + 2

1. What is the main difference between a latch and a flip-flop ?
1. a) Write down the differences between combinational logic

circuit & sequential logic circuit.

b)        Design a MOD 14 asynchronous UP/DOWN counter with JK flip-flop.  3 +12

1. a) With the help of necessary circuit diagram explain the

b)       A 6-bit R-2R ladder type DAC has reference voltage of 6-5 V.

Find :

i)           resolution in % & volt

ii)         the full scale voltage *

iii)    the o/p for the 1/p 011100.  10 + 2+1 + 2

1. a) Discuss the totem pole output configuration of TTL logic

family.   ,

b)        Design a combinational circuit th^t accepts a BCD as i/p and generates XS3 as an o/p using ROM.

c)        Design & explain the operation of a 4-bit universal register.    5 + 5 + 5

1. a) Write down the excitation table of JK and D flip-flops

and derive the excitation equation for these two flip- flops.

b)        Design a full subtractor using full adder module and NOT gates.

c  Design a full subtractor using 4 to 1 MUX. 6 + 3 + 6

1. Write short notes on any three of the following :  3×5

a)        Priority encoder

b)        Even parity generator and checker

c)        PLD ‘

d)        Johnson counter

e)        Parallel in serial out ( PISO ) shift register.