Inversion bubble (negation circle)

How to read it

Kuphaldt’s Lessons in Electric Circuits calls the small circle, or bubble, on an input or output terminal the standard mark for the inversion function: take it off the inverter and the triangle that is left is a buffer, which only amplifies. On the NAND symbol the bubble on the output line marks the output’s inversion. Texas Instruments’ guide to IEEE Std 91 defines logic negation at an input as an external 0 producing an internal 1, and at an output as an internal 1 producing an external 0.

Also searched as: active low, bubble on a logic gate, circle on a logic gate, inverting bubble, negation indicator.

Bubbles on the inputs

Bubbles can sit on the inputs too. Kuphaldt explains that inverting all of a gate’s inputs swaps AND for OR, with an inverted output: with all inputs inverted, a NAND behaves as an OR, a NOR as an AND, an AND as a NOR and an OR as a NAND. In Boolean algebra the change is De Morgan’s theorem. So an OR with bubbled inputs is another drawing of a NAND gate, and an AND with bubbled inputs another drawing of a NOR.

The triangle in place of the circle

On IEC-style drawings the circle may be replaced by a small triangle, the polarity indicator. Texas Instruments’ guide explains that it marks an input where the low (L) level is the active state, or an output whose active state produces the low level.

Look-alikes

A filled dot where two wires cross is a connection, as a University at Buffalo course note puts it, not an inversion. The bubble is an open circle, drawn against the gate’s input or output.