Low-noise amplifier

A low-noise amplifier (LNA) is an electronic amplifier that amplifies a very low-power signal without significantly degrading its signal-to-noise ratio. An amplifier increases the power of both the signal and the noise present at its input. LNAs are designed to minimize additional noise. Designers minimize noise by considering trade-offs that include impedance matching, choosing the amplifier technology (such as low-noise components) and selecting low-noise biasing conditions.

LNAs are found in radio communications systems, medical instruments and electronic equipment. A typical LNA may supply a power gain of 100 (20 decibels (dB)) while decreasing the signal-to-noise ratio by less than a factor of two (a 3 dB noise figure (NF)). Although LNAs are primarily concerned with weak signals that are just above the noise floor, they must also consider the presence of larger signals that cause intermodulation distortion.

Antennas are a common source of weak signals. An outdoor antenna is often connected to its receiver by a transmission line called a "feed line". Losses in the feed line lower the received signal-to-noise ratio: a feed line loss of 3 dB degrades the signal-to-noise ratio (SNR) by 3 dB.

An example is a feed line made from 10 feet (3.0 m) of RG-174 coaxial cable and used with a global positioning system (GPS) receiver. The loss in that feed line is 3.2 dB at 1 gigahertz (GHz); approximately 5 dB at the GPS frequency (1.57542 GHz). This feed line loss can be avoided by placing an LNA at the antenna, which supplies enough gain to offset the loss.

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