Logic gate notation — what does the small circle (bubble) on a gate output indicate? Choose the operation represented when a bubble is drawn on the output of a logic symbol.

Introduction / Context:
Schematic symbols in digital logic often include a small circle, called a bubble, on an input or output. Understanding bubble notation is crucial when reading data sheets and designing with NAND/NOR logic, because bubbles denote inversion points and change the logic function when De Morgan’s transformations are applied.

Given Data / Assumptions:

• The small circle is placed on a gate output (or sometimes input) in the logic symbol.
• Standard ANSI/IEEE logic symbols are assumed.

Concept / Approach:
A bubble explicitly represents a logical NOT (inversion). When a bubble appears on the output of an AND symbol, the device is a NAND. When it appears on the output of an OR symbol, the device is a NOR. Similarly, bubbles on inputs swap active polarity (active-LOW inputs).

Step-by-Step Solution:

Verification / Alternative check:
Compare the truth table of an AND gate to a NAND gate. A NAND is simply the complement of AND for every input combination, matching the meaning of an output inversion bubble.

Why Other Options Are Wrong:

• Comparator operation: comparators are analog/digital interface components; the bubble does not indicate comparison.
• OR operation / AND operation: the bubble is not a basic binary operator; it is an inversion modifier.
• Exclusive-OR operation: XOR has its own distinctive symbol; a bubble does not indicate XOR.

Common Pitfalls:

• Assuming a bubble indicates an active-LOW output without realizing it also changes the gate’s logic family (e.g., NAND instead of AND).

Final Answer:
NOT operation (inversion)