Low-noise amplifier
The low-noise amplifier (LNA) is an electronic amplifier used to amplify very weak signals (for example, captured by an antenna). It is usually located very close to the detection device to reduce losses in the feedline. This active antenna arrangement is frequently used in microwave systems like GPS, because coaxial cable feedline is very lossy at microwave frequencies.
An LNA is a key component which is placed at the front-end of a radio receiver circuit. Per Friis' formula, the overall noise figure of the receiver front-end is dominated by the first few stages.
Using an LNA, the effect of noise from subsequent stages of the receive chain is reduced by the gain of the LNA, while the noise of the LNA itself is injected directly into the received signal. Thus, it is necessary for an LNA to boost the desired signal power while adding as little noise and distortion as possible so that the retrieval of this signal is possible in the later stages in the system.
For low noise the amplifier needs to have a high amplification in its first stage. Therefore JFETs and HEMTs are used and distributed amplifiers could be used. They are driven in a high current regime, which is not energy efficient, but reduces the relative amount of shot noise. Input and output matching circuits for narrow band circuits enhance the gain (see gain-bandwidth product) and do not use resistors, as these would add noise. Biasing is done by large resistors, because energy efficiency is not needed, and a large resistor prevents leakage of the weak signal out of the signal path or noise into the signal path.
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