Gate symbol

Also called: logic symbol, logic gate symbol, gate symbols, distinctive-shape symbol, schematic symbol

The standard drawing used for each logic gate in a circuit diagram. The body shape gives AND, OR or XOR, and a bubble on the output adds an inversion.

Every gate has a symbol used in logic diagrams. Inputs attach on the left (the back) and the output leaves on the right (the front). To name any symbol, check two things in order.

1. The body shape.

AND: a flat back and a round front, like a D.
OR: a curved back and a pointed front, like a shield on its side.
XOR: the OR shape with an extra curved line behind the back.
NOT / buffer: a triangle pointing the way the signal flows.

2. A bubble on the output. It inverts the result:

AND + bubble = NAND
OR + bubble = NOR
XOR + bubble = XNOR
triangle + bubble = NOT

Bubbles can also appear on inputs, which inverts just those inputs. Such drawings are the alternative forms produced by bubble pushing; an OR with bubbled inputs is a NAND, for example.

These curved shapes are the distinctive-shape symbols from the US standard, which most textbooks use. You may also meet the rectangular IEC style, where every gate is a box with a label inside: & for AND, ≥1 for OR, =1 for XOR, and 1 for a buffer or NOT, with a small circle or triangle marking inversion.

Worked example

Example

Naming the symbol above

Identify the gate drawn above, then give its output for A = 1, B = 1.

1.
The back is curved and there is a second curved line behind it: an XOR body.
2.
There is a bubble on the output, so the result is inverted: XNOR.
3.
XNOR is 1 when the inputs are equal. A = B = 1, so Y = 1.

Common mistakes

Missing the extra back line and calling an XOR an OR.
Calling a triangle without a bubble a NOT gate. It's a buffer, which doesn't invert.
Looking only at the body and ignoring the output bubble, which turns AND into NAND and so on.

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