Pad between LNA output and filter required?

[Ghost Tweaker]

Hi everyone,

I have an LNA whose output is matched to 50Ohm over a small frequency range (LC matching). The stage following the LNA is a filter who is designed to work with 50Ohm load at input and output. Now outside of the band of interest, the LNA is not matched to 50Ohms and therefore the filter doesn't see 50Ohm anymore! Is it a concern? Should I use a resistive pad to smooth out the mismatch? I have recently seen many designs (discrete designs) using this technique. I was wondering if it is really necessary...

Thanx for your help

 

[biff44]

Depends on what you are trying to accomplish. If you are looking for only modest out of band rejection, you might be ok (like with an image reject noise figure situation).

If you are looking for 60 dB of out of band rejection, then yes, the non 50 ohm source will be a problem. A pad, even just 3 dB, would help. Or you could do some sort of diplexer arrangement, where out of band you see a 50 ohm load, but in band you see the LNA output.