Discuss about Electronic Logic Gates.

Introduction:

          A logic gate is an idealized or physical device implementing a Boolean function, that is, it performs a logical operation on one or more logic inputs and produces a single logic output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and unlimited fan-out, or it may refer to a non-ideal physical device (see Ideal and real op-amps for comparison).

Logic gates are primarily implemented using diodes or transistors acting as electronic switches, but can also be constructed using electromagnetic relays (relay logic), fluidic logic, pneumatic logic, optics, molecules, or even mechanical elements. With amplification, logic gates can be cascaded in the same way that Boolean functions can be composed, allowing the construction of a physical model of all of Boolean logic, and therefore, all of the algorithms and mathematics that can be described with Boolean logic.

Logic circuits include such devices as multiplexers, registers, arithmetic logic units (ALUs), and computer memory, all the way up through complete microprocessors, which may contain more than 100 million gates. In practice, the gates are made from field-effect transistors (FETs), particularly MOSFETs (metal-oxide-semiconductor field-effect transistors).

Logic Gate:

Logic Gates perform basic logical functions and are the fundamental building blocks of digital integrated circuits. These are process signals which represent true or false. Normally the positive supply voltage +Vs represents true and 0V (Zero) represents false. Other terms which are used for the true and false states are shown in the table on the right. It is best to be familiar with them all.

Gates are identified by their function: NOT, AND, NAND, OR, NOR, EX-OR and EX-NOR. Capital letters are normally used to make it clear that the term refers to a logic gate.

Logic states
True	False
1	0
High	Low
+Vs	0V
On	Off

Note that logic gates are not always required because simple logic functions can be performed with switches or diodes:
• Switches in series (AND function)
• Switches in parallel (OR function)
• Combining IC outputs with diodes (OR function)

Logic Gate Symbols:

There are two series of symbols for logic gates:
1. Traditional Symbols &
2. IEC (International Electrotechnical Commission) Symbols.

The Traditional Symbols have distinctive shapes making them easy to recognize so they are widely used in industry and education.

The IEC (International Electrotechnical Commission) symbols are rectangles with a symbol inside to show the gate function. They are rarely used despite their official status, but you may need to know them for an examination.

Inputs and Outputs:

Gates have two or more inputs, except a NOT gate which has only one input. All gates have only one output. Usually the letters A, B, C and so on are used to label inputs, and Q is used to label the output. On this page the inputs are shown on the left and the output on the right.

The Inverting Circle (o):

Some gate symbols have a circle on their output which means that their function includes inverting of the output. It is equivalent to feeding the output through a NOT gate. For example the NAND (Not AND) gate symbol shown on the right is the same as an AND gate symbol but with the addition of an inverting circle on the output.

Truth Tables:

A truth table is a good way to show the function of a logic gate. It shows the output states for every possible combination of input states. The symbols 0 (false) and 1 (true) are usually used in truth tables. The example truth table bellow shows the inputs and output of an AND gate.

Input A	Input B	Output Q
0	0	0
0	1	0
1	0	0
1	1	1

There are summary truth tables below showing the output states for all types of 2-input and 3-input gates. These can be helpful if you are trying to select a suitable gate.

NOT Gate (inverter):

The output Q is true when the input A is NOT true, the output is the inverse of the
input: Q = NOT A

A NOT gate can only have one input. A NOT gate is also called an inverter.

(a) Traditional NOT Gate (Inverter) symbol

(b) International Electrotechnical

Commission NOT Gate (Inverter) symbol

Truth Table

Input A	Output Q
0	1
1	0

AND Gate:

The output Q is true or high if input A AND input B are both true or high:
Q = A AND B
or, Q = A.B

An AND gate can have two or more inputs. A dot (.) is used to show the AND operation i.e. A.B.
Keep in mind that this dot is sometimes omitted i.e. AB

(a) Traditional AND gate Symbol

(b) IEC AND gate Symbol

AND gate Truth Table

Input A	Input B	Output Q
0	0	0
0	1	0
1	0	0
1	1	1

OR Gate:

The output Q is true if input A OR input B is true (or both of them are true):
Q = A OR B
or, Q = A+B
An OR gate can have two or more inputs, its output is true if at least one input is true.

(a) Traditional (ANSI) OR Gate Symbol

(b) IEC OR Gate Symbol

OR Gate Truth Table

Input A	Input B	Output Q
0	0	0
0	1	1
1	0	1
1	1	1

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