how a filter before an LNA degrades its NF ?

[mmarwan]

filter before lna

I read in some papper that for a receiver, if a band pass filter is inserted between an antenna and an LNA, it will degrade the noise performance of that LNA.
Why is that?! ........ is there any physical explanation for that?
Thanks

 

[khouly]

coz the filter has insertion loss or passband attenuation , and any passive network ots noise figure equal the attenuation of the network

so if u insert a filter before the LNA , the filter NF will dominate the total noise figure of the system . so it degrades the noise figure

so begin with amplifier and the filter

wish this help

khouly

 

[biff44]

Sometimes you do not have a choice. If you have a receiver right next to a transmitter, you might need the filter there just to keep the LNA from getting saturated. In those cases you need a really low loss filter, possibly with only a few poles, and then follow it up with another filter after the LNA.

Of course, you usually need some sort of bandpass or lowpass filter after the LNA to cancel out image noise in a system with an IF frequency.