With a ladder network of several noisy two-ports the noise power is summed. The noise figure of the entire system can be derived from the noise figure (Section III.4.2) of the individual discrete two-ports in series and their power gain figure. (With passive two-ports like attenuation pads, feeder lines, etc. the attenuation figure can be viewed as a negative gain figure.)
Here, the use of the available power gain figure  is essential since especially noise-optimized stages lack general matching (Section III.3) or have no uniformly constant wave impedance. The available power gain figure is the difference between the available power level at the output of the respective two-port and that of the upstream element supplying the signal. The available power of a signal source is generally
= available power at the output, in W
= rms value of the source voltage (Section III.4.7), in V
= internal resistance, in Ω
The higher the amplification of the first element of the ladder network, the ...