When Noise Figure Formulas Fail: The Overlooked Condition in Many Textbooks

Noise figure is a widely used metric for comparing the noise performance of amplifiers, receivers, and passive components. Yet, it remains one of the most commonly misunderstood concepts in RF and communication engineering. Many textbook definitions neglect a critical condition, resulting in an incorrect value for the noise figure under certain conditions. This letter proposes a revision to the conventional definition of noise figure by explicitly stating the required condition. A generalized noise figure expression is derived, which remains valid under all conditions. The use of the noise temperature approach is advocated, as it leads to a more concise formulation. A few problems and solutions are provided to facilitate a deeper understanding of the noise figure concept for practicing engineers and students.