- 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 :

- 1. an ON circuit
- 2. a CLOSED switch
- 3. a HIGH voltage
- 4. a PLUS sign
- 5. a TRUE statement
- And in a negative logic, 0 represents :
- 1. an OFF circuit
- 2. an OPEN switch
- 3. a LOW voltage
- 4. a MINUS sign
- 5. a FALSE statement

__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.