Classical

Digital Logic - Combinational Circuits

1:  

In which of the following gates, the output is 1, if and only if at least one input is 1?

A.

NOR

B.

AND

C.

OR

D.

NAND

 
 

Option: C

Explanation :

In OR gate  we need atleast one bit to be equal to 1 to generate the output as 1 because OR means  any of the condition out of two is equal to 1  which means  if atleast one input is 1 then it shows output as 1 . Number of 1's in input may be more than one but the output will always be 1 in OR gate. So the answer is 'C'.


2:   The time required for a gate or inverter to change its state is called
A. Rise time
B. Decay time
C. Propagation time
D. Charging time
 
 

Option: C

Explanation :


3:   The time required for a pulse to change from 10 to 90 percent of its maximum value is called
A. Rise time
B. Decay time
C. Propagation time
D. Operating speed
 
 

Option: A

Explanation :


4:   The maximum frequency at which digital data can be applied to gate is caled
A. Operating speed
B. Propagation speed
C. Binary level transaction period
D. Charging time
 
 

Option: A

Explanation :


5:  

What is the minimum number of two-input NAND gates used to perform the function of two input OR gate ?

A.

one

B.

two

C.

three

D.

four

 
 

Option: C

Explanation :

Y=A+B. This is the equation of OR gate. We require 3 NAND gates to create  OR gate.  We can also write

After 1st NAND operation
Y = (A AND B)' 
Y=  A'  + B'  (Demorgan's Law)
After 2nd NAND operation
Y= ( A'  + B')'
Y=  A . B  (Demorgan's Law)
After 3rd  NAND operation
Y= ( A . B )'
Y=  A' +  B '  (Demorgan's Law)

So we need 3 NAND gates.




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Syllabus covered in this section is-

  • Logic functions, Minimization,
  • Design and synthesis of combinational and sequential circuits
  • Number representation and computer arithmetic (fixed and floating point)
  • Propositional (Boolean) Logic
  • Satisfiability and Tautology
  • Logic Families: TTL, ECL and C-MOS gates.
  • Boolean algebra and Minimization of Boolean functions
  • Flip-flops-types, race condition and comparison.
  • Design of combinational and sequential Circuits
  • Representation of Integers: Octal, Hex. Decimal and Binary.
  •  2's complement and 1 's complement arithmetic
  • Floating point representation.

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