Digital Logic Gate

First, we need to know about the logic gates “A logic gate is an electronic circuit or device which makes logic decisions. In other words, a logic gate is an electronic circuit having one or more than one inputs and only one output which will be either high or low (depends on the input status). Logic gates are widely used in digital computers and different types of digital systems and circuits.

As logic gates are the building blocks of different digital systems they can be used to make many digital systems. In 1854 George Boole introduced Boolean algebra for calculations. Digital logic gates basic functions are Boolean algebra/logical algebra. These logic gates perform calculation functions in no time, which made the calculation very easy nowadays.

Positive and Negative logic

    Zeros and ones are very important in computer systems, which illustrate two different possible states of a circuit or device. These two states are can be identified with different names. In positive logic symbol 1 can be called as ON, Closed, High, Plus, True, Up while the symbol 0 can be known as Off, Open, low, Minus, False, Down. In negative logic, the situation will be opposite. For example, if a digital system is working it will be identified as 1 in positive logic but it will be called as 0 in negative logic.

Types of Logic Gates

          1) OR gat 2) AND Gate     3) NOT Gate       4) NOR Gate 5) NAND Gate

          6) XOR Gate 7) XNOR Gate

Here we are going to discuss only the basic logic gates.

How OR Gate Work?  

That type of logic gate which is consists of two or more than two inputs and a single output. It gives high output when one of its input is high is called OR gate. It is also called all or any gate. In other words the logic gate it gives high output when any input is high.
In the above figure (a) OR gate symbol with Boolean expression shown. As we can see that the OR gate has two inputs A, B, and Y output. In the other figure (b) truth table is given for OR gate. In which we can see that in the first option when both inputs are at 0 level then an output is also 0 but when any of the input goes high then output Y is 1. The logic operation says it all that OR logic gate is adding all his inputs among itself. Figure (c) illustrates the equivalent circuit of OR gate. In which two switches are connected in parallel to each other and connected in series to the lamp. When both switches are open means both inputs are at level 0 then the lamp will be off. But if any of the switches is closed then the battery will in connected in the circuit and lamp will glow.

How AND Gate Work?

That type of logic gate which is consists of two or more than two inputs. It gives low output when any of its input is low. It can only give output high when all its inputs are at high level. It multiplies it inputs among each other.

From the above figures, in the first figure, it is AND gate symbol and Boolean expression shown. Which clearly demonstrate that AND gate has two inputs X, Y and Z as output. In b/w the inputs there is nothing means both inputs multiplies with each other. Truth table gives the possible logical operation options. On the other hand in the figure ©, an equivalent circuit is shown. Which are consists of two switches connected in series with each other and also with the lamp? When any of the switch A or b is open the circuit will be incomplete hence the lamp will not provide light but when both the switches get closed then circuit will be complete and the light will start to glow.

How NOT Gate Work?

Another type of logic gate which is also called inverter. We can define the logic gate which always gives complement output of its input Is called NOT gate. This logic gate is consists of one input and one output. NOT gate symbol is given below in the figure.

In the above figure (a) NOT gate symbol is shown. NOT gate symbol is triangular in shape with a bubble on the end. NOT gate has only one input A and only one output Z which is always complementing of the input. A truth table of NOT gate is given in the next figure which gives all possible options. Transistor not gate is the equivalent circuit for this particular logic gate. When no voltage is provided on the base input of the transistor then transistor remains off and the +Vcc voltages flow towards the output Ā. But when some voltage is provided on the base of the transistor than it turns ON and the current flow towards the earth. The common point from which the output is taken remains at zero potential. This in result output is zero.