1 Basic Logic Gate

DIGITAL ELECTRONICS (331102)

1 | P a g e SOHIL VOHRA (LECTURER - SHRI K.J. POLYTECHNIC COLLEGE, BHARUCH (C.E. DEPTT))

PRACTICAL: 01

REALIZING BASIC LOGIC GATE

1.0 INTRODUCTION

A logic gate is an electronic circuit which makes logical decision.

To arrive these decisions, the most common logic gates used are OR, AND, NOT,

NAND and NOR gates.

The NAND and NOR gates are called Universal gates.

The Exclusive – OR (EX-OR) gate is another logic gate that can be constructed

using basic gates such as AND, OR and NOT.

2.0 CONSTRUCTION OF GATES

Logic Gates have two or more inputs and one output, except for NOT gate which

have one input and one output.

The output signal appears only for certain input combinations.

The manipulation of the binary information is done by the gates.

The Logic Gates are the basic building blocks of the hardware which is a available

in the form of various IC families.

Each gate has a distinct logic symbol and its operation can be described by the

means of an algebraic function.

This relation between input and output variable of each gate can be presented in

a tabular form called a TRUTH TABLE.

3.0 ‘OR’ GATE

OR gate performs logical

addition, commonly known

as OR function.

The OR gate has two or more

inputs and one output.

The operation of OR gate is

such that a HIGH (1) on the

output is produced when any

of the input is HIGH (1). The

output is LOW (0) only when



all the inputs are LOW (0).

Here is shown the graphic symbol and the truth table of OR gate.

If A & B are the input variables of OR gate, then the output will be :

o Y = A + B

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

o Y = A + B + C + D + ......

The OR gate using Diodes can be as shown. Here A & B are the inputs to the

gate, Y is input and resistance R is

the Load Resistance.

Working of the gate is as follows :

o If A = 0 and B = 0, both the

diodes will not conduct and

hence, the output will be Y =

0.

o If A = 1 and B = 0, diode D1

will conduct and hence V0 ≈

5V and so Y = 1.


will conduct and hence V0 ≈ 5V and so Y = 1.

o If A = 1 and B = 1, both diodes will conduct and hence V0 ≈ 5V and so Y

= 1.

The electrical equivalent circuit of

an OR gate is as shown, where

switches A and B are connected in

parallel.

If either A, B or both are closed,

then the output is HIGH (1).

4.0 ‘AND’ GATE

AND gate performs logical

multiplication, commonly known

as AND function.

The AND gate has two or more


The operation of AND gate is such

that a HIGH (1) on the output is

produced when both the inputs are

HIGH (1). The output is LOW (0)



when any of the inputs is LOW (0).

Here is shown the graphic symbol and the truth table of AND gate.

If A & B are the input variables of AND gate, then the output will be :

o Y = A · B

Similarly, for more than two variables,

the OR function can be expressed as :

o Y = A · B · C · D · ......

The AND gate using Diodes can be as

shown. Here A & B are the inputs to

the gate, Y is input and resistance R is

the Load Resistance.

Working of the gate is as follows :

o If A = 0 and B = 0, both the

diodes will conduct as both the

diodes are forward biased and

hence, the output will be Y =

0.


will conduct (being forward

biased) and D2 will not conduct

as it is reversed biased hence

and so Y = 0.

o If A = 0 and B = 1, diode D2 will conduct (being forward biased) and D1

will not conduct as it is reversed biased hence and so Y = 0.

o If A = 1 and B = 1, both diodes will not conduct as both are reversed

biased and hence V0 ≈ 5V and so Y = 1.

The electrical equivalent of the above circuit is as shown.

Here two switches A & B are connected in series. If both A & B are closed, then

the output is HIGH, else in any other combination, output will be LOW.

For an AND gate, when there are more inputs, all the inputs should be HIGH to

get a HIGH output. For this reason, AND gate is also called ALL Gate.

5.0 ‘NOT’ GATE

The NOT gate performs the basic logic function called inversion or complement.

The purpose of this gate is to convert one logic level into the opposite logic level.

It has one input and one output.



When HIGH level is applied to the inverter,

a LOW level appears at its output and vice

versa.

Here is shown the graphic symbol and the

truth table of AND gate.

A NOT gate using a transistor is as shown

in which A denotes the input and Y

represents the output i.e. Y = A’.

When the input is HIGH, the transistor is

in ON state and the output (VC = VCE(sat)) is

LOW.

If the input is LOW, the transistor is in OFF

state and the output (VC = VCC) is HIGH.

6.0 ‘NAND’ GATE

NAND is the contraction of AND – NOT

gates.

It has two or more inputs and only one

output i.e. Y = A · B.

When all the inputs are HIGH, the output

is LOW. If any one or both the inputs are

LOW, then the output is HIGH.

The Logic symbol and the truth table of

NAND gate is as shown here.

The small circle (or bubble) represents the operation of inversion.

The NAND gate is equivalent to an OR gate with the bubble at its inputs which are

as shown.



7.0 ‘NOR’ GATE

NOR is the contraction of OR – NOT

gates.

It has two or more inputs and only

one output i.e. Y = A + B.

When all the inputs are LOW, the

output is HIGH. If any one or both

the inputs are HIGH, then the

output is LOW.

The Logic symbol and the truth table of NOR gate is as shown here.

The small circle (or bubble) represents the operation of inversion.

The NOR gate is equivalent to an AND gate with the bubble at its inputs which are

as shown.

8.0 ‘EX - OR’ GATE

An Exclusive – OR gate is a gate with

two or more inputs and one output.

The output of two input Ex-OR gate

assumes as a HIGH state if one and

only one input assumes HIGH state.

This is equivalent of saying that the

output is HIGH if either input A or input

B is HIGH exclusively and LOW when

both are 1 or 0 simultaneously.

The logic symbol of Ex-OR and its truth table is as shown.



The truth table of Ex-OR gate shows that the output is HIGH when any one, but

not all, of the input is at 1.

This exclusive feature eliminates the similarity to the OR gate.

The Ex-OR gate responds with a HIGH output only when an odd number of inputs

is HIGH, when there is an even number of HIGH inputs, the output will always be

LOW.

9.0 ‘EX - NOR’ GATE

The Exclusive – NOR gate,

abbreviated as EX-NOR, is an EX-OR

gate followed by NOT gate.

An Ex- NOR gate has two or more


The output of two-input Ex-NOR gate

assumes a HIGH state if both the

inputs assumes the same logic state

or have even number of 1’s, and its

output is LOW when the inputs

assume different logic states or have

an odd number of 1’s.

The logic symbol and truth table of Ex-NOR are as shown. From the truth table, it

is clear that the Ex-NOR output is tje complement of Ex-OR gate.

The most important property of Ex-NOR gate is that it can be used for bit

comparison.



The output of Ex-NOR gate is 1 if both the inputs are similar i.e. both are 0 or 1,

otherwise its output is 0.

Hence, it can be used as one bit comparator. It is also called Coincidence

circuit.

Another property of Ex-NOR gate is that it can be used as an even-parity

checker. The output of Ex-NOR gate is 1 if the number of 1s in its inputs is even,

if the number of 1s is odd, the output is 0. Hence, it can be used as an even-odd

parity checker.

Hence the two – input Ex-NOR gate is immensely useful for bit comparison and

parity checking.

10.0 EXERCISE

10.1 What is the only condition under which an OR gate output will be ‘0’? Ans :

10.2 Under what conditions, will the output of an Ex-OR gate will be 1? Ans :

10.3 Under what conditions, will the output of an Ex-NOR gate will be 0? Ans :

10.4 Under what conditions, will the output of an NAND gate will be 0? Ans :

10.5 Under what conditions, will the output of an NOR gate will be 0? Ans :



10.6 Is there such thing as a ‘3 – input NOT gate’? Ans :

10.7 Which gate is called Coincidence Circuit? Why? Ans :

10.8 Which gate is called ‘ALL Gate’? Why? Ans :

11.0 ASSIGNMENTS

11.1 Write the truth table and logic symbol of three-input OR gate.

11.2 Define the NAND and NOR gates through their truth tables.

11.3 Explain the function of OR and AND gates using the diodes.

11.4 Draw the Logic diagram of Ex-OR and Ex-NOR gate and discuss their

operation with their truth tables.

11.5 What is Logic Gate? Also explain the Positive and Negative Logic

Designation.

11.6 Explain the function of an Inverter.

Grades for Exercise: .................................................

Grades for Assignment: .................................................

Signature of Lab Co-ordinators: .................................................

1 Basic Logic Gate

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