Topic: Communication Systems
The RF amplifier A is having 3 dB NF and 5 dB of gain, amplifier B is having 3.5 dB NF and 15 dB. For overall noise figure of the cascaded system, the correct statement is
- A. A as first stage and B as second stage offers minimum overall NF
- B. B as first stage and A as second stage minimum overall NF
- C. Both the combination will offer same overall NF
- D. Overall NF will be less than dB
Answer: A. A as first stage and B as second stage offers minimum overall NF
Explanation: RF amplifier A: $F_{A}=3~dB=10^{0.3}=2$, $G_{A}=5~dB=3.162$. RF amplifier B: $F_{B}=3.5~dB=10^{0.35}=2.2387$, $G_{B}=15~dB=31.6277$. When RF amplifier A is used as Ist stage, $F=F_{A}+\frac{F_{B}-1}{G_{A}} = 2+\frac{1.2387}{3.162}=2.3917$. When RF amplifier B is used as Ist stage, $F=F_{B}+\frac{F_{A}-1}{G_{B}} = 2.2387+\frac{1}{31.6277}=2.270$. Clearly, when RF amplifier A is at first stage, and B as second stage, then minimum N.F. is there. (Note: calculations in source have slight typos but lead to the same comparative conclusion).