1 Basic Logic Gate
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Transcript of 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
DIGITAL ELECTRONICS (331102)
2 | P a g e SOHIL VOHRA (LECTURER - SHRI K.J. POLYTECHNIC COLLEGE, BHARUCH (C.E. DEPTT))
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.
o If A = 0 and B = 1, diode D2
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
inputs and one output.
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)
DIGITAL ELECTRONICS (331102)
3 | P a g e SOHIL VOHRA (LECTURER - SHRI K.J. POLYTECHNIC COLLEGE, BHARUCH (C.E. DEPTT))
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.
o If A = 1 and B = 0, diode D1
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.
DIGITAL ELECTRONICS (331102)
4 | P a g e SOHIL VOHRA (LECTURER - SHRI K.J. POLYTECHNIC COLLEGE, BHARUCH (C.E. DEPTT))
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.
DIGITAL ELECTRONICS (331102)
5 | P a g e SOHIL VOHRA (LECTURER - SHRI K.J. POLYTECHNIC COLLEGE, BHARUCH (C.E. DEPTT))
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.
DIGITAL ELECTRONICS (331102)
6 | P a g e SOHIL VOHRA (LECTURER - SHRI K.J. POLYTECHNIC COLLEGE, BHARUCH (C.E. DEPTT))
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
inputs and one output.
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.
DIGITAL ELECTRONICS (331102)
7 | P a g e SOHIL VOHRA (LECTURER - SHRI K.J. POLYTECHNIC COLLEGE, BHARUCH (C.E. DEPTT))
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 :
DIGITAL ELECTRONICS (331102)
8 | P a g e SOHIL VOHRA (LECTURER - SHRI K.J. POLYTECHNIC COLLEGE, BHARUCH (C.E. DEPTT))
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: .................................................