Low-noise amplifier
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A low-noise amplifier (LNA) is an electronic component that amplifies a very low-power
LNAs are found in
Communications
An example is a feed line made from 10 feet (3.0 m) of RG-174
An LNA is a key component at the front-end of a
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. The LNA boosts the desired signals' power while adding as little noise and distortion as possible. The work done by the LNA enables optimum retrieval of the desired signal in the later stages of the system.
Design considerations
Low noise amplifiers are the building blocks of communication systems and instruments. The most important LNA specifications or attributes are:[2]
- Gain
- Noise figure
- Linearity
- Maximum RF input
A good LNA has a low NF (e.g. 1 dB), enough gain to boost the signal (e.g. 10 dB) and a large enough inter-modulation and compression point (IP3 and P1dB) to do the work required of it. Further specifications are the LNA's operating bandwidth, gain flatness, stability, input and output voltage standing wave ratio (VSWR).
For low noise, a high amplification is required for the amplifier in the first stage. Therefore, junction field-effect transistors
Gain
Amplifiers need a device to provide gain. In the 1940s, that device was a
Broadly speaking, two categories of transistor models are used in LNA design: Small-signal models use quasi-linear models of noise and large-signal models consider non-linear mixing.
The amount of gain applied is often a compromise. On one hand, high gain makes weak signals strong. On the other hand, high gain means higher level signals, and such high level signals with high gain may exceed the amplifier's dynamic range or cause other types of noise such as harmonic distortion or nonlinear mixing.
Noise figure
The noise figure helps determine the efficiency of a particular LNA. LNA suitability for a particular application is typically based on its noise figure. In general, a low noise figure results in better signal reception.
Impedance
The circuit topology affects input and output impedance. In general, the source impedance is matched to the input impedance because that will maximize the power transfer from the source to the device. If the source impedance is low, then a
Biasing
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Another design issue is the noise introduced by biasing networks.
Applications
LNAs are used in communications receivers such as in
In a satellite communications system, the ground station receiving antenna uses an LNA because the received signal is weak since satellites have limited power and therefore use low-power transmitters. The satellites are also distant and suffer path loss: low Earth orbit satellites might be 120 miles (190 km) away; a geosynchronous satellite is 22,236 miles (35,785 km) away.
The LNA boosts the antenna signal to overcome
LNAs can enhance the performance of software-defined radio (SDR) receiver systems. SDRs are typically designed to be general purpose and therefore the noise figure is not optimized for any one particular application. With an LNA and appropriate filter, performance is improved over a range of frequencies.
See also
References
- ISBN 9780549667391.
- ^ "An Introduction to Low Noise Amplifier Specifications". Software-Defined Radio Simplified. January 11, 2020. Retrieved January 11, 2020.
Further reading
- Motchenbacher, C. D.; Connelly, J. A. (1993), Low-Noise Electronic System Design, John Wiley, ISBN 978-0471577423