OK, I will work out the first scenario as an example, but it is up to you to figure out which order of amplifiers yields the lowest overall noise figure. The rule of thumb for cascading noise figure is that the NF of the first stage adds directly to the system NF, and you want as much gain as possible at the front end of the chain because the contribution of subsequent stages is reduced by gain of previous stages.
The first step is to convert the decibel values to ratios:
amplifier A , gain = 6dB (3.981), noise figure = 2dB (1.586)
amplifier B , gain = 10dB (10.00), noise figure = 4dB (2.512)
amplifier C , gain = 8dB (6.310), noise figure = 3.5dB (2.239)
Then, plug all the values in to the recursive formula provided in my previous post:
nf = 1.586 + (2.512-1)/(3.981) + (2.239-1)/(3.981*10.00)
= 1.586 + 0.380 + 0.0311
NF = 10 * log (1.997)
= 3.004 dB
There you have it. Get to work. We oldtimers are well aware of students trying to get other people to do all their schoolwork for them.
- Kirt Blattenberger
RF Cafe Progenitor & Webmaster