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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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5.4 Boolean algebra 187

Table5.12

Truthtablefor (A +B)· (A +C).

A B C C A+B A+C (A+B)·(A+C)

1 1 1 0 1 1 1

1 1 0 1 1 1 1

1 0 1 0 1 1 1

1 0 0 1 1 1 1

0 1 1 0 1 0 0

0 1 0 1 1 1 1

0 0 1 0 0 0 0

0 0 0 1 0 1 0

5.4.2 Logicalequivalence

We know from the distributive laws ofBooleanalgebrathat

A+(B·C)=(A+B)·(A+C)

Let us construct the truth table for the r.h.s. of this expression (Table 5.12). If we now

observethefinalcolumnofTable5.12weseeitisthesameasthatofTable5.11.Wesay

thatA+(B·C)islogicallyequivalentto (A+B)· (A+C).Figures5.12and5.13show

thetwowaysinwhichtheselogicallyequivalentcircuitscouldbeconstructedusingOR

gates, AND gates and inverters. Clearly different electronic circuits can be constructed

toperform the same logical task. We shall shortlyexplore a wayof simplifyingcircuits

toreduce the number ofcomponents required.

A

B

C

C

B . C

A + (B . C)

Figure5.12

Circuit to implementA + (B ·C).

A

A + B

B

A

C

C

A + C

(A + B) . (A + C)

Figure5.13

Circuit to implement (A +B)· (A +C).

Engineeringapplication5.1

Booleanexpressionandtruthtableforanelectroniccircuit

Find the Boolean expression and truth table for the electronic circuit shown in Figure

5.14.

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