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Concept
Low-noise amplifier
Summary
A low-noise amplifier (LNA) is an electronic component that amplifies a very low-power signal without significantly degrading its signal-to-noise ratio (SNR). Any electronic amplifier will increase the power of both the signal and the noise present at its input, but the amplifier will also introduce some additional noise. LNAs are designed to minimize that additional noise, by choosing special components, operating points, and circuit topologies. Minimizing additional noise must balance with other design goals such as power gain and impedance matching.
LNAs are found in radio communications systems, medical instruments and electronic test equipment. A typical LNA may supply a power gain of 100 (20 decibels (dB)) while decreasing the SNR 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 3dB degrades the receiver signal-to-noise ratio (SNR) by 3dB.
An example is a feed line made from of RG-174 coaxial cable and used with a global positioning system (GPS) receiver. The loss in that feed line is 3.2dB at 1GHz; approximately 5dB at the GPS frequency (1.57542GHz). This feed line loss can be avoided by placing an LNA at the antenna, which supplies enough gain to offset the loss.
An LNA is a key component at the front-end of a radio receiver circuit to help reduce unwanted noise in particular. Friis' formulas for noise models the noise in a multi-stage signal collection circuit. In most receivers, the overall NF is dominated by the first few stages of the RF front end.
By using an LNA close to the signal source, the effect of noise from subsequent stages of the receive chain in the circuit is reduced by the signal gain created by the LNA, while the noise created by the LNA itself is injected directly into the received signal.
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