WBUT Exam Papers CS Digital Electronics And Logic Design B Tech Third Sem Dec 2006

WBUT Exam Papers CS Digital Electronics And Logic Design

B Tech Third Sem  Dec 2006


( Full Marks : 70Group – A (Multiple Choice Questions)

Choose the correct alternatives for any ten of the following questions :

a)              A full adder circuit can be designed using 1) a half adder & an OR gate

ii)              a half adder & an AND gate 111) two half adders & an OR gate iv) none of these.

b)               The fastest logic family is

1) TTL                   ii) CMOS

Hi) RTL                  iv) ECL.

c)               For a parallel in parallel out shift register we need I) 1 ii) n

ill) 2n + 1              iv) n + 1

clock pulse/pulses.

d)              The number of Flip-Flops required to design a MOD-18 counter is i) 3                         ii) 4

iii)             5                    iv) 6.




The maximum positive number that can be represented in l’s complement representation is


2 n_1 – 1 ii) 1 -2n_1 UU _ (2 1 – l)                                           tv)

n- 1




f) Flip-Flop has

i) one stable state

111) no stable state The fastest ADC is

1)               dual slope 110 parallel comparator

14 ■ two stable states Iv) none of these.

11)        successive approximation iv) none of these


The minimum number of NAND gates required to implement an EX-OR gate is 1) 2                        ID 3

ill) 4                   iv) 5.                      1

According to Boolean algebra which of the following relations is not valid ?

I)                X ( Y, Z) = (XY) Z   U) X ( Y + Z ) = XY + XZ ill) X + XZ = X iv) X (X + Y) = 1.

The figure of merit of a logic family is given by ^ gain bandwidth product

II)               propagation delay time x power dissipation Hi) fan-out x propagation delay time tv) noise margin x power dissipation.

PROMs are used primarily for

I)                 data storage

II)               temporaiy program and data storage

ill) they are inexpensive

iv)             permanent program & data storage.

The decimal 37 is represented in BCD by 1) 100101 U) 00111011

ill) 00100101            iv) none of these.


Group -B (Short Answer Questions)

Answer any three questions.          3×5*15

  1. Perform the arithmetic operation ( – 25 ) l0 + ( – 15 ) 10 in sign 2’s complement method, Assume 1-bit sign and 6-bit information.
  2. Minimize the following expression using K-map and realize the simplified expression using NAND gates only.

G (A, B, C, D ) = X ( 1, 2, 3, 5, 6. 11, 12 ) + D ( 7, 8, 10, 14 )

  1. Implement a full adder circuit using decoder.
  2. Implement a clocked JK flip-flop using NAND gates only.
  3. Implement the following function using 8 : 1 MUX :

F (A, B, C, D ) = I ( 0, 2, 4, 8. 9, ( 10, 11, 12, 13, 14, 15 )

Group – C (Long Answer Questions)

Note: Answer any three of the following questions.

a)              Distinguish between ROM, PLA and PLD’s as elements realising Boolean functions. q

b)              Design a combinational circuit using an 8 x 4 ROM that accepts a 3-bit number and generates an output binary number equal to the square of the Input number.


c)               Draw logic diagram of Master/Slave JK flip-flop. Why is it called so ?                      3

a)              Write down the excitation table of JK and D flip-flops. Derive the excitation equations for these two flip-flops.                                                                                                                3 + 3

b)              Design a 4-bit Up/Down asynchronous counter using all JK flip-flops and other necessary logic gates. Use one direction control input M. If M = 0, the counter will count up and for M = 1 the counter will count down.                                 9

  1. a) Describe the operation of successive approximation type A to D converter. How

many dock pulses are required |n worst case for conversion for an 8-bit SAR ? Define quantizing error for an ADC. 6+1 + 1

b) Draw a neat diagram for a R-2R ladder type DAC. What is linearity error and offset error in a DAC ? ,      5+1 + 1

  1. a) Draw the circuit for a four-bit Johnson counter using D flip-flops and explain its L

operation. Draw the timing diagram for this 4-bit Johnson counter. How does this timing diagram differ from that of a Ring counter ?                                     8 + 2 + 2

b)                                                                                                                                                                                                                                                                                                             Perform the conversion from D flip-flop to JK flip-flop.                 3

  1. Write short notes on any three from the following :     3×5=15

a)               Content Addressed Memoiy

b)               Tri-state gates in TTL family

c)                BCD-to-7 segment decoder/driver

d)               Data lock-out in a counter.

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