a low-noise amplifier (LNA)
/ ell-en-AY /
An antenna catches an unimaginably faint signal, a millionth of a millionth of a watt or less, riding on a sea of noise. The very first amplifier it meets has a special job: boost that whisper while adding almost no hiss of its own. That is the low-noise amplifier, the LNA. Picture a sound engineer recording a soft singer in a noisy room; the LNA is the careful first microphone preamp, chosen above all to not add its own hum, because any noise it adds here will be amplified by everything after it and can never be removed.
Why the first stage is decisive comes from a rule called Friis's formula: in a chain of amplifiers, the noise of the first stage dominates the noise of the whole receiver, because later stages amplify both the signal and the first stage's noise together. So the front-end LNA is designed to have the lowest possible noise figure, often under 1 dB, even at the cost of other things. It also needs enough gain (say 15 to 20 dB) to lift the signal well above the noise of the stages behind it, and a clean input match to the antenna so the faint signal is not partly reflected away. RF engineers deliberately accept a compromise here: the input is matched for best noise, which is not quite the same as the match for best power transfer, a famous and honest tension in LNA design.
LNAs sit at the front of every receiver: in your phone, WiFi card, GPS unit, satellite dish, and radio telescope. The best ones use special low-noise transistors and sometimes cooling. Honest caveat: an LNA cannot improve the signal-to-noise ratio of what arrives, the noise picked up by the antenna is already there, all the LNA can do is amplify the signal before later stages add more noise, so it preserves the ratio rather than improving it. And too much LNA gain can overload later stages on strong signals, so even a low-noise front end is a balance, not a free win.
A GPS receiver's antenna delivers a signal weaker than the background noise. An LNA with a 0.8 dB noise figure and 18 dB of gain sits right at the antenna, lifting the signal cleanly so the mixer and later stages, which are noisier, no longer dominate the receiver's overall noise.
The first amplifier sets the receiver's noise, so it is chosen for lowest noise figure.
An LNA preserves signal-to-noise rather than improving it; the antenna's incoming noise is already fixed. Its input is matched for minimum noise, which is generally not the same as the match for maximum power transfer.