Hi all,

a typical band-select filter is only matched over the desired band. However, it is normally designed by considering S-parameters. Furthermore, the LNA matching bandwidth depends on the Q of the input matching network. Likewise, and antenna is only 50 ohm over a certain BW. Therefore the filter transmission, T21, should be different from S21 and can be calculated using the S11 of the antenna and LNA.

I was wondering, for a commercial LNA is it customary to have a larger than necessary matching bandwidth in order to "idealize" the band-select filter's T21 over a wider band? In other words, if I have a band-select filter with a BW from 2.4-2.5 GHz and an LNA with a BW from 2.4-2.4 GHz, then the filter's T21 will not equal to its S21 for frequencies outside of 2.4-2.5 GHz. But if i make the LNA match from 2-3 GHz, then the filter's T21 will equal to its S21 from 2-3 GHz.

As a last point, a narrowband LNA has a matching network gain at the input which is frequency dependent. Is it normally taken into consideration when concerning the band-select filter's attenuation profile?

thanks,
Aaron