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A Low-noise amplifier (LNA) is an electronic amplifier used to amplify very weak signals (for example, signals captured by an antenna). Essentially, an LNA amplifies signals that are barely recognizable without adding a lot of noise, as the name implies. An LNA is usually located close to the signal source in order to reduce losses in the feed line or minimize interference. This arrangement is frequently used in microwave frequency systems such as Global Positioning Systems (GPS), because coaxial cable feed-line has a high loss at microwave frequencies. For example, of RG-174 coaxial cable has a loss of 3.2 dB (50 percent) at 1 GHz; the feedline would degrade the signal-to-noise ratio (SNR) by 3.2 dB (50 percent). An LNA is a key component at the front-end of a radio receiver circuit. Per Friis' formula, the overall noise figure (NF) of the receiver's front-end is dominated by the first few stages (or only the first stage). 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. This enables retrieval of the signal in the later stages of the system. A good LNA has a low NF (e.g. ), enough gain (e.g. ) and should have large enough inter-modulation and compression point (IP3 and P1dB). Further criteria are operating bandwidth, gain flatness, stability, input, and output voltage standing wave ratio (VSWR). For low noise, the amplifier needs to have a high amplification in its first stage. Therefore, Junction Field-Effect Transistors (JFETs) and High Electron Mobility Transistors (HEMTs) are often 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''). == LNA design == Low noise amplifiers are the building blocks of any communication system. The four most important parameters in LNA design are: gain, noise figure, non-linearity and impedance matching. LNA design is based mainly on the S-parameters of a transistor. The steps required in designing an LNA are the following: 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Low-noise amplifier」の詳細全文を読む スポンサード リンク
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