NF LNA design

[Leiser]

Hi,

I would like to know how can I know what is the optimum impedance for the BFG520 transistor at any frequency to achieve the best noise figure value. The datasheet gives two noise circle plots at 900 and 2GHz but I need to design it for FM band.

Thanks a lot.

 

[zappedagain]

I've only done that with a Noise Figure option on a spectrum analyzer. Good luck.

 

[Leiser]

Ok, thanks. I also have this option on my spectrum analyzer. How can I then find in practice the optimum input impedance? Measurements are taken inside a small faraday chamber. Maybe using a tuner at the input of the amplifier?

Thanks,

 

[logbook]

What usually works is to pick a part optimised for the band in question and it will then be characterised for operation in that band. This applies to all specs of course.

Other than that I would think you have to perform your own characterisation. I doubt there is a magic formula that allows you to extrapolate the optimum noise figure impedances.

 

[logbook]

Looking at the NXP (Philips) data sheet for this part they do give quite a lot of data. The optimum noise figure is pretty flat from 900MHz down to 200MHz. Also the 900MHz circle going from optimum at 1.1dB to 1.5dB is pretty big.

The point is you don't have to hit it spot-on to get a good 1.5dB noise figure. Of course if you are desperate for those last few tenths of a dB you will have to do more hand tuning.