How it works
Thermal noise power N = kTB where k = 1.38×10⁻²³ J/K (Boltzmann's constant), T is temperature in Kelvin, and B is bandwidth in Hz. At T = 290K (standard temperature) and B = 1 Hz, N = −174 dBm/Hz — the thermal noise floor. Noise factor F = SNR_in/SNR_out > 1 for any physical amplifier; noise figure NF = 10 log(F) in dB. An amplifier with NF = 3 dB (F = 2) adds noise equal to the thermal noise floor — it doubles the noise power at its output compared to an ideal noiseless amplifier.
Key points to remember
Friis's formula for cascaded stages: F_total = F₁ + (F₂−1)/G₁ + (F₃−1)/(G₁G₂) + ... — first stage dominates when G₁ is large. Noise temperature T_e = (F−1)·T₀ where T₀ = 290K, giving an alternative measure. System noise temperature T_sys = T_antenna + T_receiver is used in satellite link budgets. White noise has flat power spectral density; 1/f (flicker) noise dominates at low frequencies in transistors and increases at −10 dB/decade below a corner frequency. Shot noise arises from discrete charge carriers crossing a junction: i²_n = 2qI_DC·B, where q = 1.6×10⁻¹⁹ C.
Exam tip
The examiner always asks you to calculate overall noise figure for a two or three-stage cascaded amplifier using Friis's formula — compute each term numerically and confirm that later stages contribute negligibly when first-stage gain exceeds 20 dB.