What is a logic gate?

A logic gate is a building block of digital circuits that takes one or more binary inputs (0 or 1) and produces a single binary output according to a Boolean function such as AND, OR, or NOT.

Why are NAND and NOR called universal gates?

Any Boolean function can be built using only NAND gates, or only NOR gates, so each is functionally complete. This makes NAND and NOR convenient primitives for IC manufacturing.

What is the difference between XOR and OR?

OR outputs 1 when at least one input is 1, including when both are 1. XOR (exclusive OR) outputs 1 only when the inputs differ, so 1 XOR 1 is 0.

What does the bubble on a gate symbol mean?

The small circle (bubble) on a gate's output denotes inversion. AND with a bubble becomes NAND, OR with a bubble becomes NOR, and XOR with a bubble becomes XNOR.

What is De Morgan's theorem?

De Morgan's theorem states that NOT (A AND B) equals (NOT A) OR (NOT B), and NOT (A OR B) equals (NOT A) AND (NOT B). It lets you convert between AND and OR forms by swapping the operator and inverting all terms.

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Reference

Logic Gates Truth Table

Truth tables, ANSI/MIL and IEC symbols, and Boolean expressions for the seven basic gates. Toggle inputs A and B to highlight every output live.

Inputs

Boolean algebra laws

Law	Identity