# Logic Gates Types

#### Introduction of Logic Gates Types

A logic gates is an electronic circuit, which makes logical decision. To arrive at these decisions, the most common logic gates used are OR, AND, NOT, NAND and NOR gates. The NAND and NOR gates are called as the Universal gates. The exclusive OR gate is another logic gate which can be constructed using basic gates such as AND, OR and NOT gates.

Logic gates have two or more inputs and only one output except for the NOT gate, which has only one input. The output signal appears only for certain combinations of the input signals. The gates do the manipulation of binary information. The logic gates are the building blocks of hardware, which are available in the form of various IC families. Each gate has a distinct logic symbol and its operation can be described by means of an algebraic function. The relationship between input and output variables of each gate can be represented in a tabular form called a truth table.

## Logic Gates Types

#### These are basic logic gate types

##### 1. OR Gate

The OR gate perform logical addition, commonly known as OR function. The OR gate has two or more input and only one output. The operation of OR gate is such that a HIGH (1). On the output is produced when any of the inputs is HIGH (1). The output is LOW (0) only when all the input are LOW (0).

If A and B are the input variables of an OR gate and Y is its output, then

Y= A +B

Similarly, for more than two variables, the OR function can be expressed as

Y= A+B+C+D+….

If A=0 & B=0

If A=1 & B=0

If A=0 & B=1

If A=0 & B=1

##### 2. AND Gate

The AND gate perform logical multiplication, commonly known as AND function. The AND gate has two or more inputs and a single output. The output of an AND gate is HIGH only when all the inputs is LOW, the output will be LOW. It is basic logic gates types.

If A and B are the input variables of an AND gate and Y is its output, the      Y=   A B.

#### 3. NOT Gate (Inverter).

The NOT gate performs the basic logical function called inversion or complementation. The purpose of this gate is to convert one logic level into the opposite logic level. It is also basic logic gates types.

It has one input and one output. When a HIGH level is applied to an inverter, a LOW level appears at its output and vice versa.

#### These are Universal Gates types

##### 1. NAND Gate .

NAND is a contraction of the NOT-AND gates. It has two or more inputs and only one output, i.e. Y= A B. when all the input are HIGH, the output is LOW. If any one or both the are LOW, then the output is HIGH. it is universal Gate Types.

The logic symbol for the NAND. The small circle or bubble represents the operation of inverse.

##### 2.  NOR Gate

NOR is a contraction of NOT-OR gates. It has two or more inputs and only one output, i.e. Y=A+B. the output is HIGH, only when all the inputs are LOW. If any one or both the inputs are HIGH, the output is LOW. The logic symbol for the NOR gate. The small circle or bubble represents the operation of inversion.

##### Combinational Circuit Working.

A combinational circuit consists of input variables, logic gates and output variables.

The design of combinational circuits starts from the verbal outline of the problem or from a set of Boolean functions, ands in a logic circuit diagram. The steps involved in the design of combinational circuits are as follows:

• State the problem in words.
• Find the number of input and output variables.
• Assign letter symbol to the input and output variables.
• Obtain the truth table using the word statement.
• Obtain Boolean expressions for each output from the truth table.
• Simplify the Boolean expression to minimize the number of variables by using laws of Boolean algebra or Karnaugh map method or McCluskey’s method.
• Draw the logic circuit diagram corresponding to the simplified Boolean expression.
##### Exclusive-OR (Ex-OR) gate

An exclusive-OR gate is a gate two or more inputs and one output. The output of  a two-input Ex-OR gate assumes a HIGH state if one and only one input assumes a HIGH state.