[SOLVED] trade off between NF and return loss of lna

[justtry114]

Hi all , i know that trade-off between noise figure and return loss must be make when design a LNA. But i am confuse how to balance these two things. If my LNA's NF is 0.8dB if RL is -7dB, while NF is 1dB then -12dB RL can be achived? Then I want to know which one I should choose? Which is better for my system? Suppose the lna is the first stage of active gps antnena.
Thanks for your kindly help in advance

 

[Joel-Tang]

The use of source inductance will improve the return loss performance and low frequency stability without affecting the noise figure very much at all. The trade off is gain and stability at high frequencies. If you are using a device with large Gate widths you will have the ability to add more inductance and maintain stability. 400 microns of width should give you plenty of room to play.
Another Alternative is to balance the amplifier with 90 degree hybrids.
You might find something of interest on this EDAboard forum post: https://www.edaboard.com/threads/282989/

Joel Richard

 

[justtry114]

Thanks, Joel,
Acutally i am using BGA915N7 to design a gps antenna. I haven't verify the stablity of the circuit yet. But I have got the pcb, that's to say I can't add more inductance to soure now. I can only tune the matching component of the input to change the S11 and NF slightly. And suppose that's all i can do, then i have to make a decision which will be adopted at the end. Then still comes the question, which i should choose?

Regards,
Derek

 

[Joel-Tang]

The difference between **broken link removed** at the input of the amplifier. If you do have a "good" match looking into the antenna you are better off going for the 12dB return loss. If your antenna is not well matched it is less predictable so I would just make sure the antenna is matched.

 

[justtry114]

Thank you, Joel. I totally agree with you. it's the same as how i understand it.