- Boolean algebra was developed by George Boole in 1847 and applied to switching circuits by Claude Shannon in 1939.
- It defines basic logic operations like inverse, AND, and OR and how they relate to switching circuits. Boolean expressions correspond directly to logic gate circuits.
- Truth tables specify the output values of a Boolean expression for all possible combinations of its variables. Theorems like distribution and DeMorgan's laws allow simplifying expressions and optimizing equivalent circuits.
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Unit 02
1. Department of Communication Engineering, NCTU 1
Unit 2 Boolean Algebra
1. Developed by George Boole in 1847
2. Applied to the Design of Switching
Circuit by Claude Shannon in 1939
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
Boolean algebra: f : {0, 1} {0, 1}
Basic operations
Inverse (complement)
AND
OR
Inverse is denoted by (’)
0' = 1
1' = 0
Inverter
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
11 1
01 0
00 1
00 0
C=A·BA B
Input Output
All possible
combinations
of inputs
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
11 1
11 0
10 1
00 0
C=A+BA B
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
Basic operations of switching circuits
A switch
A · B Two switches in a series
A + B Two switches in parallel
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
Boolean Expressions are formed by applications of basic
operations to one or more variables or constants, e.g.
AB '+C
[A(C+D)] '+BE
Priority of operators: NOT > AND > OR
Each expression corresponds directly to a circuit of logic gates
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
A truth table specifies the values of a Boolean expression
for every possible combinations of variables in the
expression
E.g. AB '+C
If an expression has n-variables, the number of different
combinations of variables is 22…=2n
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
Basic Theorems
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
Commutative, associative and distributed laws
Commutative laws :
XY = YX X+Y = Y+X
Associative laws :
(XY)Z = X(YZ) = XYZ
(X+Y)+Z = X+(Y+Z) = X+Y+Z
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
Distributed law
AND operation distributes over OR:
X(Y+Z) = XY+XZ
OR operation also distributes over AND
X+YZ = (X+Y)(X+Z)
= XX + XY + XZ + YZ
= X ( 1+ Y + Z) + YZ
= X + YZ
This distributive law does not hold for ordinary algebra
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
Simplifications of Boolean expressions
Each expression corresponds to a circuit of logic gates.
Simplifying an expression leads to a simpler circuit
Some useful theorems
E.g. F = A(A’+B) By the second distributive law
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
Example 1
Example 2
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
An expression is said to be in sum-of-products form when
all products are the products of only single variables
E.g. : AB’+ CD’E + AC’E
ABC’+ DEFG + H
When multiplying out an expression, the second
distributive law should be applied first when possible
E.g. : (A + BC)(A + D + E) = A + BC(D + E) = A + BCD + BCE
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
An expression is in product-of-sums when all sums are
the sums of single variables
E.g. : (A+B’)(C+D’+E)(A+C’+E’)
The second distributive law can be applied for
factorization
E.g. :
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
Example 1
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
Two-level circuits
Sum-of-products
Product-of-sums
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
DeMorgan’s Laws
The complement of the sum is the product of the complements
(X+Y)’= X’Y’
The complement of the product is the sum of the complements
(XY)’= X’+ Y’
Can be verified by using a truth table
DeMorgan’s Laws are easily generated to n variables
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Logic Design Unit 2 Boolean Algebra Sau-Hsuan Wu
Example 1
Example 2