Work out noise figure instantly with clear inputs, formula shown and shareable results.
Friis' formula shows why the first stage dominates: Ftotal = F1 + (F2 - 1)/G1. With 15 dB of first-stage gain, a 6 dB second stage adds only about 0.5 dB to the system figure. The noise floor is -174 dBm/Hz plus 10log10(bandwidth) plus the noise figure.
Friis cascade
Ftotal = F1 + (F2 - 1)/G1 (linear ratios); noise floor = -174 dBm/Hz + 10log10(BW) + NF
Its noise is amplified by every following stage, while later stages' noise is divided by the gain ahead of them. That is why the LNA goes at the antenna.
It is kTB expressed in dBm for a 1 Hz bandwidth at 290 K, the thermal noise floor of any resistive source at room temperature.