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III. Gm-C Filtering - Epublications - Université de Limoges

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A.5 Friis’ Formula<br />

In case of multistage systems, as it may be seen in Figure 195, the total noise factor<br />

can be computed as a function of the noise factors Fi and the gains Gi of each bloc, provi<strong>de</strong>d<br />

that the chain of quadripoles is conjugately matched at all interfaces. This equation is known<br />

as the Friis’ formula [A.1].<br />

Figure 195. Multistage System<br />

F2<br />

− 1 F3<br />

−1<br />

F4<br />

−1<br />

Ftotal = F1<br />

+ + + + ...<br />

(A.14)<br />

G G G G G G<br />

1<br />

1<br />

2<br />

The Friis’ formula un<strong>de</strong>rlines how critical for noise the first stage of a receiver chain<br />

is. In<strong>de</strong>ed, to lower the total noise factor of the chain, the most efficient solution is to increase<br />

the gain and to lower the noise factor of the first stage.<br />

- 185 -<br />

1<br />

2<br />

3

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