Fundamental logic function — inverter (NOT gate) Identify the basic logic gate whose output is always the logical complement (opposite) of its single input in a positive-logic system.

Introduction / Context:
Combinational logic is built from a handful of primitive operations. The simplest unary operation is inversion, which produces the logical complement of its input. Recognizing the inverter is foundational for interpreting active-low signals, building NAND/NOR gates, and designing control logic.

Given Data / Assumptions:

• Positive logic (HIGH = 1, LOW = 0).
• Single-input gate mapping X → X̅ (the complement).

Concept / Approach:
An inverter (NOT gate) outputs 1 when its input is 0, and outputs 0 when its input is 1. It is represented by a triangle with a small bubble at the output in schematic symbols, with the bubble denoting inversion. This operation is used to create active-low control lines and to implement De Morgan transformations.

Step-by-Step Solution:

Verification / Alternative check:
Observe that both NAND and NOR include inversion internally, but they are multi-input and do not simply complement a single input without additional wiring. The comparator is an analog device comparing magnitudes, not a single-input logic complementer.

Why Other Options Are Wrong:

• OR/AND: Multi-input gates performing sum/product, not inversion.
• Comparator: Analog magnitude comparison, not a Boolean NOT operation.

Common Pitfalls:
Confusing an inverting buffer with a non-inverting buffer; overlooking inversion bubbles on schematics; assuming comparators (with TTL outputs) are logic gates—comparators are analog front-ends with digital-like outputs but not basic logic gates.

Final Answer:
inverter