Dr. Ahmed G. Abo-Khalil

Electrical Engineering Department

Low-noise amplifie

Low-noise amplifier (LNA) is an electronic amplifier used to amplify possibly 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, e.g. a loss of 10% coming from few meters of cable would cause a 10% degradation of the signal-to-noise ratio (SNR).

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 (NF) of the receiver's front-end is dominated by the first few stages (or even the first stage only).

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. A good LNA has a low NF (like 1dB), a large enough gain (like 20dB) and should have large enough intermodulation and compression point (IP3 and P1dB). Further criteria are operating bandwidth, gain flatness, stability and input and output voltage standing wave ratio (VSWR).

For low noise, the amplifier needs to have a high amplification in its first stage. Therefore JFETs and HEMTs are often 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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