Low-noise amplifier
Encyclopedia
Low-noise amplifier 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 amplifier
s 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.
, 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.
s, Cellular/PCS Handsets, GPS Receivers, Cordless Phones, Wireless LAN
s, Wireless Data, Automotive RKE
, satellite communications, etc.
It 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).
MAX 2640 operate at +2.7 V to +5.5 V.
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.
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.
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.
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.
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.
2.http://ece.iisc.ernet.in/~kjvinoy/study%20phase%20report%20on%20LNA.pdf
3.http://pdfserv.maxim-ic.com/en/ds/MAX2640-MAX2641.pdf
Electronic amplifier
An electronic amplifier is a device for increasing the power of a signal.It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude...
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
Active antenna
An active antenna is an antenna that contains active electronic components. This allows the construction of antennas of limited size and / or wide frequency range. Most active antennas use as antenna parts of minimal conducting area, e.g. a small whip, connected to the active component...
arrangement is frequently used in microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
systems like GPS
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
, because coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
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
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
(SNR).
An LNA is a key component which is placed at the front-end of a radio receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
circuit. Per Friis' formula, the overall noise figure
Noise figure
Noise figure is a measure of degradation of the signal-to-noise ratio , caused by components in a radio frequency signal chain. The noise figure is defined as the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard...
(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
Electronic noise
Electronic noise is a random fluctuation in an electrical signal, a characteristic of all electronic circuits. Noise generated by electronic devices varies greatly, as it can be produced by several different effects...
from subsequent stages of the receive chain is reduced by the gain
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...
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
Standing wave ratio
In telecommunications, standing wave ratio is the ratio of the amplitude of a partial standing wave at an antinode to the amplitude at an adjacent node , in an electrical transmission line....
(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 amplifier
Distributed Amplifier
Distributed amplifiers are circuit designs that incorporate transmission line theory into traditional amplifier design to obtain a larger gain-bandwidth product than is realizable by conventional circuits.-History:...
s could be used. They are driven in a high-current regime, which is not energy-efficient, but reduces the relative amount of shot noise
Shot noise
Shot noise is a type of electronic noise that may be dominant when the finite number of particles that carry energy is sufficiently small so that uncertainties due to the Poisson distribution, which describes the occurrence of independent random events, are of significance...
. Input and output matching
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
circuits for narrow-band circuits enhance the gain (see Gain-bandwidth product
Gain-bandwidth product
The gain–bandwidth product for an amplifier is the product of the amplifier's bandwidth, and the gain at which the bandwidth is measured....
) 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 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: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.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.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.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 noiseJohnson–Nyquist noise
Johnson–Nyquist noise is the electronic noise generated by the thermal agitation of the charge carriers inside an electrical conductor at equilibrium, which happens regardless of any applied voltage...
, 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.
LNA Application:
LNA is used in various applications like ISM RadioISM band
The industrial, scientific and medical radio bands are radio bands reserved internationally for the use of radio frequency energy for industrial, scientific and medical purposes other than communications....
s, Cellular/PCS Handsets, GPS Receivers, Cordless Phones, Wireless LAN
Wireless LAN
A wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...
s, Wireless Data, Automotive RKE
RKE
RKE is an abbreviation that may refer to the following:*A remote keyless entry system used to gain entry to automobiles*B Routsin Korpus Eestimal Swedish Corps In Estonia*In optics, RKE may refer to a type of Eyepiece...
, satellite communications, etc.
In Satellite:
During reception of radio signal sent by satellite in a communication system, in the receiver section, second element after antenna is LNA.It 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).
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 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
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
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.
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
1.http://www.ek.isy.liu.se/courses/tsek03/T1_LNA_2011.pdf2.http://ece.iisc.ernet.in/~kjvinoy/study%20phase%20report%20on%20LNA.pdf
3.http://pdfserv.maxim-ic.com/en/ds/MAX2640-MAX2641.pdf