A SKILL BASED PRESENTATION ON

ACTIVITY BASED LEARNING

TOPIC: “LOGIC GATE IMPLEMENTATION WITHOUT USING

ICS”

By

Mr. Nimay Giri

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CONTENTS Objective Introduction Computer & electricity Basic concept of Logic Gates Constructing Gates Logic chips or ICs Logic circuit design process Deriving Logical Expressions Conclusion

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OBJECTIVE

Identify the basic gates and describe the behavior of each.

Describe how gates are implemented using transistors.

Combine basic gates into circuits.

Describe the behavior of a gate or circuit using Boolean expressions, truth tables, and logic diagrams.

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INTRODUCTION A Logic gate is an idealized or physical device implementing a

Boolean function (i.e. it performs a logic operation on one or more logic inputs & produces a single logic outputs).

A Logic gate primarily implemented using diodes or transistor.

Logic is the branch of mathematics deals with True (‘1s’) & False (‘0s’).

A gate is a device that performs a basic operation on electrical signals.

Gates are combined into circuits to perform more complicated tasks.

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COMPUTER & ELECTRICITY

There are three different, but equally powerful, notational methods for describing the behavior of gates and circuits

Boolean expressions

logic diagrams

truth tables

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BOOLEAN ALGEBRA Boolean algebra was introduced by George Boole in his first

book The Mathematical Analysis of Logic  in 1847. Boolean algebra: expressions in this algebraic notation (True

=‘1s’ & False=‘0s’) are an elegant and powerful way to demonstrate the activity of electrical circuits.

Many computer functions defined in terms of Boolean equationsExample: sum of 2 single binary digit numbers

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LOGIC DIAGRAM & TRUTH TABLE

Logic diagram: a graphical representation of a circuit

Each type of gate is represented by a specific graphical symbol

Truth table: defines the function of a gate by listing all possible input combinations that the gate could encounter, and the corresponding output.

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GATES OR LOGIC GATES Integrated circuits (ICs) constructed from transistor switches

and other electronic components.

Hardware consists of a few simple building blocks

These are called logic gates

AND, OR, NOT, … Simple logic gatesNAND, NOR, XOR, XNOR …

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BASIC CONCEPTS OF SIMPLE GATES Functionality of Gates can be

expressed by a truth table A truth table lists output for

each possible input combination.

If the two input values for an AND gate are both 1, the output is 1; otherwise, the output is 0.

F=A.B If the two input values for an OR

gate are both 0, the output value is 0; otherwise, the output is 1.

F=A+B A NOT gate is sometimes referred

to as an inverter because it inverts the input value. F=A’

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NAND, NOR XOR… NAND = AND + NOT NOR = OR + NOT XOR implements exclusive-OR

function NAND and NOR gates (Universal

Gate) require only 2 transistorsAND and OR need 3 transistors!

The NAND and NOR gates are essentially the opposite of the AND and OR gates, respectively.

An XOR gate produces 0 if its two inputs are the same, and a 1 otherwise.

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CONSTRUCTING GATES A diode is a two-terminal electronic component with

asymmetric conductance; it has low (ideally zero) resistance to current in one direction, and high (ideally infinite) resistance in the other.

A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals.

Semiconductor diodes always include special marks on the packages.

It is a Active device, which is given below (Ex-1N4007);

Any part number beginning with “1N” is always a diode. However, not all diode part numbers begin with “1N”!

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CONT… Light emitting diodes (LEDs) are diodes made with direct

band gap semiconductors (GaAs, GaN, etc.)

LEDs are designed to generate light when enough current passes through the device

LEDs are functional similar to any diode, but they typically have a much larger threshold voltage ~2V for red and green LEDs~4V for blue and white LEDs

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CONT… A transistor is a semiconductor device used

to amplify and switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit.

It is developed in 1947 by American physicists John Bardeen, Walter Brattain, and William Shockley.

A voltage or current applied to one pair of the transistor's terminals changes the current through another pair of terminals.

It is made of a semiconductor material, which is neither a particularly good conductor of electricity, such as Gold, silver, copper…, nor a particularly good insulator, such as rubber.

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LOGIC CHIPS OR LOGIC ICS

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CONSTRUCTING GATES WITH ELECTRONICS COMPONENTS

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CONT…

T1/T2= BC547, R= 1K/10K

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GATE CONSTRUCTING VIEW

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LOGIC CIRCUIT DESIGN PROCESS A simple logic design process involves

Problem specificationTruth table derivationDerivation of logical expressionSimplification of logical expression Implementation

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DERIVING LOGICAL EXPRESSIONS Derivation of logical expressions from truth tables

sum-of-products (SOP) formproduct-of-sums (POS) form

SOP form Write an AND term for each input combination that

produces a 1 outputWrite the variable if its value is 1; complement otherwise

OR the AND terms to get the final expression

Example: F = A B C + A B C + A B C + A B C POS form

Dual of the SOP form

Example: F = (A + B + C) (A + B + C)(A + B + C) (A + B + C)

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INTEGRATED CIRCUITS An integrated circuit (also called a chip) is a piece of silicon on which multiple

gates have been embedded. These silicon pieces are mounted on a plastic or ceramic package with pins along

the edges that can be soldered onto circuit boards or inserted into appropriate socket.

Integrated circuits (IC) are classified by the number of gates contained in them Integration levels

SSI (small scale integration) Introduced in late 1960s 1-10 gates (previous examples)

MSI (medium scale integration) Introduced in late 1960s 10-100 gates

LSI (large scale integration) Introduced in early 1970s 100-10,000 gates

VLSI (very large scale integration) Introduced in late 1970s More than 10,000 gates

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CPU CHIPS The most important integrated circuit

in any computer is the Central Processing Unit, or CPU. Each CPU chip has a large number

of pins through which essentially all communication in a computer system occurs.

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CONCLUSION

We conclude that, Logic gates or circuits can be implemented by using electronics components (diodes, transistors & resistors etc) and verified by the helps of Boolean expressions, truth tables, and logic diagrams.

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