NOT Gate (Inverter)

The simplest logic gate - inverts the input signal, flipping 0 to 1 and 1 to 0.

The NOT gate, also called an inverter, is the simplest logic gate. It has one input and one output. Whatever logic level you put in, the opposite comes out. The NOT gate performs logical negation — in everyday language, "NOT true" equals "false".

Objectives

  • Understand the NOT gate truth table
  • Recognize NOT gate schematic symbols
  • Write Boolean expressions using NOT notation
  • Build a NOT gate from transistors conceptually
  • Apply NOT gates in practical circuits

Key Takeaways

  • NOT gate has one input and one output
  • Output is always the opposite of the input
  • Notation: Ā, A', !A
  • The bubble (circle) on symbols indicates inversion
  • Double inversion returns original: NOT(NOT A) = A
  • Common IC: 74HC04 (six inverters)

Applications

  • Signal Inversion: Flip a signal's logic level
  • Active-Low Conversion: Convert active-high to active-low signals
  • Ring Oscillators: Odd number of inverters creates oscillation
  • Buffer (2 inverters): Two inverters in series strengthen signals without changing logic

Practice Problems

Problem 1: If A = 1, what is NOT A?

Problem 2: Simplify NOT(NOT(NOT A))

Problem 3: If a NOT gate outputs HIGH, what is the input?