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Inversion Bubbles and Active-LOW Logic

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The little circle has a meaning

The small circle used on logic symbols has several closely related names: inversion bubble, negation bubble, or simply bubble.

Bubble on the output

An output bubble means: perform the function, then invert the result.

A triangle followed by a bubble is therefore NOT:

Y = ~A

Bubble on an input

An input bubble means that particular input is logically inverted before the gate performs its normal function.

For example, an AND gate with a bubble on A means:

Y = (~A) • B

That is a compact shortcut for drawing a separate inverter ahead of input A.

Active-LOW signals

In practical electronics, a bubble often tells us that a pin or signal is active LOW: the function is asserted when that signal is at logic 0 rather than logic 1.

Datasheets may also mark active-LOW signal names with an overbar, a trailing slash, a leading slash, or a suffix such as _N, depending on the manufacturer.

Bubble rule:
bubble at an input → invert that input.
bubble at an output → invert the result.

Try these yourself

What is the small circle on a logic input or output commonly called? A fuse An inversion bubble A junction dot A capacitor

It is commonly called an inversion bubble, negation bubble, or simply a bubble.

An AND gate has a bubble on input A but not on B. Which expression matches it? A • B ~(A • B) (~A) • B A + ~B

The input bubble means the gate receives NOT A, so the function is (~A) • B.

Does the dashed or small-circle convention mean an extra electrical wire is present? Yes No Only for XOR Only at 120 volts

No. A bubble is part of the logic symbol. It indicates inversion or an active-LOW condition.

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