I was asked: Why LNA is low noise?

Hello All,

I was on phone interview for RF designer, and was asked, "Tell me how does the LNA make it all happen in low noise?" In fact, I never thought that before (though, I did a class project on LNA design in Cadence). I replied from my gut feeling:
1. It does not have resistors that add thermal noise.
2. There are inductors in the circuit, so it helps reduce high frequency noises.

These sounded stupid

Anyway, I got called for onsite interview. Now, I need to answer it properly. Google search brought up lots of LNAs but did not specifically answer, how it works keeping the noise low.

Thanks is advance!

Read about optimization for noise and for power. I do not learned this topic well, but i remember there was formulas for lowest noise, highest power, something like that

The first important thing in LNA design is to carefully select a LOW NOISE active device which can meet simultaneously all the design parameters as: Noise Figure, Gain, Linearity, and Stability.
There are many things to follow designing a good LNA but most important are: designing the input matching network for good compromise between Noise Figure and Gain, and second, to do the proper LNA bias design spending time on the envelope termination of the bias circuit filtering. This would improve the LNA linearity (IP3 and P1dB).

Maybe what he wanted to hear was that the LNA on the input of a receiver block is very important and will set the overall noise figure of the system.

Thank you. Actually, I looked at the formulas that talked about F_min and F_min,P, but I have those formulas to make adjustments within limitations and keep the noise low. I was about to answer that, "Because, I used equations that dictates what should be the values for low noise". But, I wasn't sure how dumb I might sound.




Thank you all for your taking the time to answer me!

I sincerely appreciate your contributions!