What is noise figure?

Noise figure (NF) is a measure of how much an amplifier degrades the signal-to-noise ratio (SNR). It is expressed in dB, where 0 dB means a perfect, noiseless amplifier. A lower NF indicates better performance.

What is the Friis formula?

The Friis formula calculates the total noise factor of cascaded stages: F_total = F1 + (F2-1)/G1 + (F3-1)/(G1*G2) + ... where F is the linear noise factor and G is the linear gain of each stage.

Why does the first stage dominate the noise figure?

Each subsequent stage's noise contribution is divided by the cumulative gain of all preceding stages. High gain in the first stage reduces the impact of noise from later stages, making the first LNA the most critical component.

What is noise temperature?

Noise temperature (Te) is an alternative way to express noise performance: Te = T0 * (F - 1), where T0 is the standard reference temperature of 290 K and F is the linear noise factor. Lower noise temperature means less added noise.

What does a gain of 0 dB mean for a stage?

A gain of 0 dB (linear gain = 1) means the stage neither amplifies nor attenuates the signal. A passive attenuator with insertion loss L dB is entered as a gain of -L dB.

How do I calculate NF for a passive attenuator?

For a passive attenuator at room temperature, the noise figure in dB equals its insertion loss in dB. For example, a 3 dB attenuator has a noise figure of 3 dB. Enter a negative gain (e.g. -3 dB) and NF equal to the loss magnitude.

Noise Figure Calculator

Name: Noise Figure Calculator
Availability: InStock
Author: Nham Vu

Compute noise figure, noise temperature, and cascaded NF for multi-stage RF amplifier chains using the Friis formula.

Amplifier Stages

Friis cascade

Reference Temperature T₀

K (standard = 290 K)

Total NF

—

Noise Temp

—

Total Gain

—

Per-Stage Breakdown

Stage	NF (dB)	Gain (dB)	Te (K)	Contrib (dB)
Add at least one stage to see results.

Friis Formula Reference

F_total = F₁ + (F₂-1)/G₁ + (F₃-1)/(G₁·G₂) + …
NF = 10·log₁₀(F) | T_e = T₀·(F−1)

Summary

Compute noise figure, noise temperature, and cascaded NF for multi-stage RF amplifier chains using the Friis formula.

How it works

Enter the noise figure (dB) and gain (dB) for each amplifier stage.
Add as many stages as needed using the "Add Stage" button.
The calculator converts NF and gain from dB to linear factors.
The Friis formula is applied to cascade all stages into a single total NF.
Total noise figure, noise temperature, and per-stage contributions are displayed.
Adjust any value to see results update instantly.

Use cases

Design and optimize low-noise amplifier (LNA) chains for receivers.
Calculate system noise figure for satellite or radio telescope front-ends.
Verify that the first-stage LNA adequately suppresses downstream noise.
Compare different component orderings to minimize cascaded noise figure.
Estimate receiver sensitivity based on total system noise figure.
Educational demonstrations of the Friis noise formula.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu