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2011
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Noise from a chain of amplifiers
Amplifier chain: the first amplifier’s noise dominates
G
1
NF
1
G
2
NF
2
G
n
NF
n
NF = NF
1
+
NF
2
− 1
G
1
+
NF
3
− 1
G
1
G
2
+ . . . (Friis formula)
(Not to confuse with transmission chain, which has strong
attenuation between amplifiers)
Attenuation: quantum noise not affected
⇒ NF (attenuator) = attenuation
Insertion loss: attenuation at amplifier input
⇒ Strong influence on NF
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