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Digital Electronics

Boolean Algebra

Boolean algebra is the mathematics of writing logic expressions like equations and simplifying them — George Boole founded it in 1854, and today it underlies every processor. Variables take only 0 and 1. The basic rules are intuitive: A·1 = A, A·0 = 0, A+0 = A, A+1 = 1. Also A·A = A, A + A = A (idempotent) and A·Ā = 0, A + Ā = 1 (complement).

The most powerful tool is De Morgan’s laws: taking the inverse of a group "flips" the operations. (A·B)̄ = Ā + B̄ and (A+B)̄ = Ā · B̄. So a NAND is really an "OR with inverted inputs", and a NOR an "AND with inverted inputs". This is the key to rebuilding a circuit with only NAND or only NOR gates.

Distribution works just like in ordinary algebra: A·(B+C) = A·B + A·C. The absorption rule A + A·B = A shortens expressions. Example: A·B + A·B̄ = A·(B + B̄) = A·1 = A — two terms collapse to one variable. For larger expressions, Karnaugh maps make this simplification visual; fewer gates = a cheaper, faster, lower-power circuit.

Formulas

De Morgan: (A·B)̄ = Ā + B̄
De Morgan: (A+B)̄ = Ā · B̄
Distribution: A·(B+C) = A·B + A·C
Absorption: A + A·B = A

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Test Yourself

By De Morgan’s law, what does (A + B)̄ equal?
Answer: Ā · B̄ — The inverse of a sum is the product of the inverses: (A+B)̄ = Ā·B̄.
What does A·B + A·B̄ simplify to?
Answer: A — Factor out the common A: A·(B + B̄) = A·1 = A.