LOGIC GATES

It is an electronic circuit which makes logic decisions.
It has one or more input but one output.
Output consists of certain combinations of input signal.
Digital system builds on the basis of Logic gates.
Logic gates enables and implement the hardware logic functions on the basis of logical algebra developed by George Boole.
And that’s why this logical algebra is called Boolean algebra.
The main and unique characteristic of the Boolean algebra is that it has two variables and assumes only one of the two values i.e. either 0 or 1.

LOGIC CIRCUIT & IC’S

Logic circuit comes in the form of various IC families.
The most popular families are :
1. Transistor-transistor logic (TTL)
2. Emitter-coupled logic (ECL)
3. Metal-oxide-semiconductor (MOS) and complementary metal-oxide-semiconductor (CMOS)
By all these we get different logic gates i.e. OR, AND, NOT, NOR, NAND and exclusive OR (XOR) gates.
And from these we also get their truth tables accordingly.

POSITIVE & NEGATIVE LOGIC

So in computing system, the two variable 0 and 1 or the number symbols 0 and 1 represent the two possible states of a circuit or device.
From this we can easily represent 0 & 1 with the logic of ON and OFF, CLOSED and OPEN, HIGH and LOW, PLUS and MINUS, TRUE and FALSE, depending on the situations.
It means always there will be two opposite conditions prevail.

POSITIVE & NEGATIVE LOGIC

Now in a positive logic, 1 represents :

SO JUST OPPOSITE CONDITION PREVAIL

THE OR GATE

When both inputs (A, B) are 0 (switches are open) output C is 0 (lamp is OFF)
When A is in logic state 0 (switch A is OPEN) but B is in logic state 1 (switch B is CLOSED), the output C is in logic state 1(lamp is ON)
Lamp would be also ON when A is CLOSED and B is OPEN.
And again lamp would be ON when both switches are CLOSED.

IT IS SO BECAUSE AN OR GATE IS EQUIVALENT TO A PARALLEL CIRCUIT IN ITS LOGIC FUNCTION.

A + B = C , according to Boolean algebra, OR gate performs logical addition and its truth table can be written as
Now it is important to understand that ‘+’ sign in Boolean algebra does not stand for the addition as in case of the ordinary or numerical algebra.
So the ‘+’ sign indicates OR operation.
Then in logic algebra, A+B=C means that if A is true OR B is true, then C will be true.

IT DOES NOT MEAN HERE THAT SUM OF A AND B EQUALS C

The other equation could also be written as AUB= C or AVB= C
The meaning of last three logic additions is that output is 1 when either input A or B or both are 1
The first addition implies that output is 0 only when both inputs are 0.
The meaning of ‘+’ sign –
We can write the above OR laws in more general terms –

YOU CAN BE RICH OR YOU CAN BE POOR

In this gate, output is 1 if its either input but not both is 1.
And output is 0 only when inputs are the same.
This logic gate has output 0 when inputs are either all 0 or all 1.
This gate works on the Boolean equation – A ⊕ B = C
The previous switching circuit shows switch position A and B will individually light up the lamp but a combination of A and B is not possible.

YOU CAN BE RICH OR YOU CAN BE POOR – > Obviously you can’t be both at the same time.

This circuit is also called an inequality comparator or detector because it produces an output only when the two inputs are different.

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