Bubble

Also called: inversion bubble, inversion circle, negation bubble, inverting bubble

A small circle drawn on a gate's input or output meaning the signal is inverted at that point. It turns AND into NAND, OR into NOR and XOR into XNOR.

A bubble is the small circle you see at the tip of a NOT gate and on the output of NAND, NOR and XNOR. Wherever it sits, it means invert this signal here.

On an output, it inverts the gate's result. AND plus an output bubble is NAND, . OR becomes NOR, and XOR becomes XNOR.
On an input, it inverts that one input before the gate sees it. An AND with a bubble on input B computes .

So a bubble is really a tiny NOT gate drawn without its triangle. That makes diagrams shorter and shows at a glance where inversions happen.

Two bubbles on the same wire, one at each end, cancel out, because inverting twice gets you back where you started (double inversion). This fact, together with de morgans laws, is the basis of bubble pushing, the technique for redrawing gates and converting AND-OR circuits into all-NAND ones.

To identify a gate symbol, look at two things in order: the shape of the body (AND, OR or XOR), then whether there is a bubble on the output.

Worked example

Example

Where the bubble sits matters

Compare an OR with a bubble on its output against an OR with a bubble on each input, for A = 1, B = 0.


0	0	1	1
0	1	0	1
1	0	0	1
1	1	0	0

1.
Output bubble: the OR gives 1 + 0 = 1, then the bubble inverts it. Y = 0. This is NOR, .
2.
Input bubbles: A becomes 0 and B becomes 1 before the OR, so Y = 0 + 1 = 1. This computes , which is NAND.
3.
Same body, same inputs, different answers. Bubbles on the inputs are not the same as one bubble on the output.

Common mistakes

Treating input bubbles and an output bubble as interchangeable. An OR with bubbled inputs is a NAND, not a NOR.
Thinking a bubble inverts every wire on the gate. It inverts only the one wire it sits on.
Forgetting that two bubbles on the same wire cancel.

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Bubble is taught in Logic Gates.

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