Logic Gates

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          Logic Gates

A gate is a digital circuit that is designed for performing a particular logical operation. 
There are 3 basic Gates:

(1). "OR" Gate
(2). "AND" Gate
(3). "NOT" Gate

(1). 'OR' Gate with explanations

         An "OR" Gate have more than 2 inputs but having only 1 output. 

Bullian expression

If 'A' and 'B' is the inputs and 'Y' is the output then

    Y = A + B

Truth table



Symbol



Circuit diagram



Working

When A = 0 & B = 0 diode remain unbiased and there is no current obtained in output Y = 0.

When A = 1,  D1 become forward biased and
When, B = 0, D2 become reverse biased 
Then, output Y = 1

When A = 0 and B = 1, D1 become reversed biased and D2 become forward biased then output Y = 1.

When A = 1 & B = 1, D1 & D2 become forward biased then we get output Y = 1.

Question - Draw output wave for OR Gate. 

Solution. 



(2). 'AND' Gate

           It is a Gate having more than 2 input and 1 output. 

Bullian expression

Y = A.B

Symbol


Truth table




Circuit diagram




When A = 0,B = 0, both unbiased so output Y = 0.
When A = 1,B = 0, output Y = 0.
When A = 0, B = 1, output Y = 0.
When A = 1,B = 1, output Y = 1.


Question - Draw output wave for AND Gate. 

Solution.  



(3). 'NOT' Gate

             In it, it is having only one input and one output. 

Both input and output are reciprocal of each other. 

It means, when switch is close the bulb doesn't glow and when switch is open bulb starts to glow. 

Bullian expression



Symbol




Truth table



Circuit diagram



Working

It works as the next transistor as an amplifier with common emitter configuration. 
In it when first point "A" = 0 or switch is open, we will get an output Y = 1.
When A = 1 or switch is closed we will get an Y = 0.

Question - Draw output wave for NOT Gate. 

Solution. 




Combination of Gates

There are two combination of Gates

(1). "NOR" Gate

        It is the combination of OR and NOT Gate. 

Symbol



Truth table



Circuit diagram



Question - Draw output wave for NOR Gate. 

Solution. 




(2). NAND Gate

        It is the combination of AND and NOT Gate. 

       Symbol



Truth table



Circuit Diagram



Question - Draw output wave for NAND Gate. 

Solution.   



































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