Low-noise amplifier
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, 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.
LNA Design
Low noise amplifiers are the building blocks of any communication system.The four most important parametres in LNA design are:Gain, NF, non-linearity and impedance matching.The design for LNA is mainly based upon the S-parameters of transistor.Different steps required in designing a LNA are as follows:-
Selecting type of Design:
There are mainly two types of devices - 1.S-parameter and 2.Normal device. S-parameter is an in-built device does not require any type of external biasing because it has fixed S-parameters.Normal devices are like other transistors to which external bias can be applied.For designing a LNA S-parameter devices are generally used.
Selecting Transducer
One of the crucial stage in designing a Low Noise Amplifier is proper selection of a transducer.The transducer selected should have a maximum Gain and minimum Noise Figure(NF).Some of the examples of transistors that can be selected are- ATF-34143 and ATF-35143.
LNA Application:
LNA is used in various applications like ISM Radios,Cellular/PCS Handsets,GPS Receivers,Cordless Phones,Wireless LANs,Wireless Data,Automotive RKE, satellite communications etc.
In Satellite:
During reception of radio signal send by satellite in a communication system the receiver have second element after antenna is LNA. It required because the signal is travelling very large distance and carrying important information. While travelling through medium it get suffered by various types on noise so this signal has very small signal to noise ratio. Hence due to this direct reception of signal is not possible hence LNA is used to boost up the signal of desire energy from the weak information signal of required frequency. Usually satellite communication is done in the frequency range 100 MHz to several GHz (30 GHz).
PARAMETERS
Here some electrical parameters of LNA: parameters of MAX 2640.
Operating supply voltage
Usually LNA require less operating voltage in the range of 2 V to 10 V. MAX 2640 operate at +2.7 V to +5.5 V.
Operating supply current
LNA require supply current in the range of mA, the supply current require for LNA is depend on the its design and the application for which it has to be used. MAX 2640 which is used for satellite application is require supply current nearly 6mA.
Operating frequency
Range of LNA operation is very wide, it operate from 500 KHz to 50 GHz. frequency is the basic thing for any electronic device specially which are used in communication system. MAX 2640 operate in the frequency range 300 MHz - 1500 MHz.
Operating temperature range
Every electronic device have some limit on operating temperature, like that LNA also has some limit on its operating temperature. general range for faithful operation of LNA is -30°C to +50°C. MAX 2640 operates on very wide temperature range i.e. -40°C - +125°C.
Noise figure
noise figure is also one of the important factor which decide the efficiency of a particular LNA. hence we can decide which LNA is suitable for a particular application. for example: In satellite communication LNA having very low noise figure is required. MAX 2640 which is used for communication purpose have noise figure in the range 0.9 dB - 1.5 dB. noise figure varies according to the operating frequency for a same LNA. e.g. MAX 2640, NF 0.5 dB for 900 MHz, NF 1.2 dB for 1575 MHz, NF 1.3 dB for 1900 MHz, NF 1.5 dB for 2450 MHz. low noise figure results in better reception of signal.
High gain
with the low noise figure LNA must have high gain for the processing of signal into post circuit. According to requirement high gain LNA are designed for application by manufacturer. if LNA will not have high gain then the signal will be affected in by noise in LNA circuit itself and maybe attenuated so high gain of LNA is the important parameter of LNA. like NF gain of LNA also varies with the operating frequency. e.g. for MAX 2640 15.1 dB at 900 MHz, 15.7 dB at 1575 MHz, 14.4 dB at 1900 MHz, 13.5 dB at 2450 MHz.
Reference
See also
External links
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