Low-noise amplifier
 |
This article may be too technical for most readers to understand. Please help improve this article to make it understandable to non-experts, without removing the technical details. The talk page may contain suggestions. (November 2011) |
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.
Contents |
[edit] 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, and non-linearity and impedance matching.The design for LNA is based mainly upon the S-parameters of a transistor. The steps required in designing a LNA are as follows:
[edit] Design
There are two widely used types of devices the S-parameter and normal device. An S-parameter is a built-in device which 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. In designing a LNA, the S-parameter design is the most used.
[edit] Transducer
One of the crucial stages 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 examples of transistors that can be selected are- ATF-34143 and ATF-35143.
[edit] Stability check
While designing any amplifier, it is important to check the stability of the device chosen, or the amplifier may function as an oscillator.For determining stability, calculate Rollet's Stability factor, (represented as variable K) using S-parameters at a given frequency.For a transistor to be stable, parameters must satisfy K>1 and |∆|<1.
[edit] Stability enhancement
Some of the techniques for enhancing the stability are adding a series resistance and adding a Source Inductance. In the former, a small resistance may be added in series with gate of the transistor. This technique is not used in LNA design because the resistance generates thermal noise, increasing the noise figure of the amplifier.
Alternatively, an inductor may be added in series with the transistor gate. As an ideal inductor has zero resistance, it generates no thermal noise. It improves stability by reducing the gain of the amplifier by a small factor. Some of the inductors like 5.98nH and 3.1nH are used in 1st and 2nd stage respectively to improve the stability.
[edit] 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.
[edit] Satellite
During reception of radio signal sent by satellite in a communication system, in the receiver section, second element after antenna is the LNA. It is required because the signal is travelling through a very large distance and carrying important information. While travelling through a medium it suffers due to various types of noise. So this signal has very small signal to noise ratio. 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 of 100 MHz to several GHz (30 GHz).
[edit] Parameters
Here some electrical parameters of LNA: parameters of MAX 2640.
[edit] 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.
[edit] Operating supply current
LNA require supply current in the range of mA, the supply current require for LNA is dependent on the its design and the application for which it has to be used. MAX 2640 which is used for satellite application requires a supply current of nearly 6mA.
[edit] 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 among the ones used in communication systems. MAX 2640 operate in the frequency range 300 MHz - 1500 MHz.
[edit] 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.
[edit] Noise figure
Noise figure is also one of the important factors which determines the efficiency of a particular LNA. Hence, we can decide which LNA is suitable for a particular application. For example: In satellite communication, an LNA having a 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.
[edit] 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.
[edit] Reference
- http://www.ek.isy.liu.se/courses/tsek03/T1_LNA_2011.pdf
- http://ece.iisc.ernet.in/~kjvinoy/study%20phase%20report%20on%20LNA.pdf
- http://pdfserv.maxim-ic.com/en/ds/MAX2640-MAX2641.pdf
