The function of the LNA is to take the extremely weak and uncertain signal from the antenna, usually on the order of microvolts or under -100 dBm, and amplify it to a more useful level, usually about one-half to one volt.
While providing this gain itself is not a major challenge with modern electronics, it is severely compromised by any noise that the LNA may add to the weak input signal. This noise can overwhelm any benefits of the amplification that the LNA adds.
Note that the LNA functions in a world of unknowns. As the front end of the receiver channel, it must capture and amplify a very low-power, low-voltage signal plus associated random noise that the antenna presents to it within the bandwidth of interest. In signal theory, this is called the unknown signal/unknown noise challenge, the most difficult of all signal-processing challenges.
For LNAs, the primary parameters are noise figure (NF), gain, and linearity.
- Noise is due to thermal and other sources, with typical noise figures in the 0.5 to 1.5 dB range.
- Typical gain is between 10 and 20 dB for a single stage. Some designs use cascaded amplifiers with a low-gain, low-NF stage, followed by a higher-gain stage that may have higher NF, but this is less critical once the initial signal has been “gained up.”
- Nonlinearity is another issue for the LNA, as the resultant harmonics and intermodulation distortion corrupt the received signal and make demodulating and decoding it with sufficiently low bit error rate (BER) more difficult. Linearity is usually characterized by the third-order intercept point (IP3), which relates nonlinear products caused by the third-order nonlinear term to the linearly amplified signal; the higher the IP3 value, the more linear the amplifier performance.
https://www.digikey.com/en/articles/low-noise-amplifiers-maximize-receiver-sensitivity
https://www.nxp.com/docs/en/white-paper/RFLNAWP.pdf