The NOT Gate

Definition. A NOT gate, also called an INVERTER, is a logic circuit with a SINGLE input and a single output, whose output is always the exact OPPOSITE (complement) of its input: HIGH when the input is LOW, and LOW when the input is HIGH. Unlike the OR and AND gates of Sections 9.26-9.27, a NOT gate always has exactly one input -- 'inverting two things at once' is not a meaningful operation the way ORing or ANDing two inputs is.

Symbol. The NOT-gate symbol (shown in the accompanying figure) is a simple triangle, with the input entering its flat left edge and the output emerging from its pointed apex on the right -- but crucially, a small open circle (the 'bubble', the universal symbol for logical inversion in digital-circuit diagrams) is drawn right at the apex. This bubble alone is what makes the symbol a NOT gate rather than a plain (non-inverting) BUFFER, whose triangle would otherwise be identical but bear no bubble -- and, as Sections 9.29-9.30 show directly, the very same bubble, attached instead to the output of an AND or OR gate's own symbol, is exactly what turns that symbol into a NAND or NOR gate. …