Logic Gates

In simple terms

A logic gate is a tiny circuit, usually made from a handful of transistors, that takes one or two electrical signals as input and produces an electrical signal as output — following a rule like AND, OR, or NOT.

More detail

The common gates correspond directly to boolean operations:

Gate	Symbol idea	Output is 1 when…
NOT	¬A	input is 0
AND	A · B	both inputs are 1
OR	A + B	either input is 1
NAND	¬(A · B)	NOT of AND
NOR	¬(A + B)	NOT of OR
XOR	A ⊕ B	inputs differ

Gates can be wired together to build:

• Adders that compute binary sums
• Flip-flops that remember a bit (the basis of memory and registers)
• Multiplexers that pick one of several inputs
• Eventually, the CPU itself

A modern processor packs billions of gates onto a fingernail-sized chip.

Why it matters

Logic gates are the bridge between physics and information. They are where electricity stops being voltage and starts being computation.

Real-world examples

• A single 7400-series chip from the 1970s contains four NAND gates.
• An FPGA is a chip whose gates can be rewired in software.
• The CPU in your phone is, structurally, a very large and very clever pile of logic gates.

Common misconceptions

• “Each gate uses one transistor.” A CMOS NAND gate uses four transistors. Even a basic NOT (inverter) uses two.
• “Logic gates are obsolete.” They are everywhere — they just live inside larger building blocks.

Learn next

Gates combine into a CPU.