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Low noise amplifier for transponder


Low-noise amplifier: Part Of Transponder

The low noise amplifier (LNA) is a special type of electronic amplifier or amplifier used in communication systems to amplify very weak signals captured by an antenna. It is often located very close to the antenna. If the LNA is located close to the antenna, then losses in the feedline become less critical. This "active antenna" arrangement is frequently used in microwave systems like GPS, because coaxial cable feedline is very lossy at microwave frequencies.  It is a key component, which is placed at the front-end of a radio receiver circuit. Per Friis' formula, the overall noise figure of the receiver front-end is dominated by the first few stages.

Using an LNA, the noise of all the subsequent stages is reduced by the gain of the LNA and the noise of the LNA 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.

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

A low noise amplifier (LNA) is an electronic device used to filter out the noise of input signals received at the front ends of communication systems. Low noise amplifiers are used in a wide variety of applications such as RF communication systems including wireless computer networks and mobile phones. Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. Generally, communications systems transfer information from a source to a destination using a combination of a transmitter and a receiver. The transmitter typically includes a transducer and a transmission element which together convert an electrical signal into an electromagnetic signal. The electromagnetic signal then propagates through a transmission medium to the receiver, which converts the signal with the help of another transducer into a desired form for a use by an end user. In the direct wireless communication mode, the participating wireless communication devices tune their receivers and transmitters to the same channel and communicate over that channel. For indirect wireless communications, each wireless communication device communicates directly with an associated base station and/or an associated access point via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network. A transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna. Data signals are being transmitted through the air continuously. Those data signals are received at a variety of amplitudes. Consequently, receivers must be able to process data signals of varying received signal strength. Therefore, many modern receivers contain amplifiers that produce variable gain amplification outputs. Such amplifiers are aptly called variable gain amplification amplifiers. With the explosive growth of wireless communications, the airwaves are rapidly being filled with signals of varying strengths and frequencies. Immunity to jamming has subsequently become a significant concern to any communication system. Noise is produced by all electrical and electronic components, as well as by ambient conditions surrounding circuitry. Noise can be passed into an electrical system by external components, and may also produced by the electrical system itself. Consequently, the intrinsic noise level of a system determines the lower limit or minimum signal strength of a signal that can be detected in the presence of noise. In a radio frequency (RF) signal receiving apparatus such as a cellular phone and a base station of a wireless communication system, a received signal has very weak intensity and includes considerable noise mixed therein. Accordingly, such a signal receiving apparatus requires a circuit for amplifying the received signal while reducing a noise figure of the signal.

An amplifier is a linear electronic circuit that may be used amplify an input signal and provide an output signal that is a magnified replica of the input signal. Radio frequency power amplifiers are widely used to transmit signals in communications systems. Typically a signal to be transmitted is concentrated around a particular carrier frequency occupying a defined channel. Information is sent in the form of modulation of amplitude, phase, and/or frequency such that the information is represented by energy spread over a band of frequencies around the carrier frequency. Various amplifier applications are known in which low noise and high linearity performance are required. Examples of such applications include low noise amplifiers (LNAs), variable gain amplifiers (VGAs), transmit/receive (T/R) modules, power amplifiers, transimpedance amplifiers, and optical receivers used in various products. Such products can include wireless communications, cellular telephony, fiber optic data links, portable electronics. Variable gain amplifiers (VGAs) are frequently used in modern radio receivers to amplify or attenuate incoming signals to properly drive an associated analog-to-digital converter (A/D). In a radio frequency (RF) transceiver, the received signal has a high dynamic range. In order to supply a signal of constant amplitude to a baseband section of the transceiver, a variable gain amplifier (VGA) with equivalent or better dynamic range is required. Typically, the variable gain is distributed among radio frequency (RF), intermediate frequency (IF), and low-frequency or baseband circuits. As appliances and circuit designs continue to decrease in size and increase in speed, the need for low power, low noise, current efficient amplifier circuitry increases. The noise contribution of a single amplifier is a function of many fundamental parameters such as the device type, and its operating bias-point. Other considerations such as the source and load impedance presented to the device, have a particularly strong influence on both the added noise contribution from the amplifier to any input signal, as well as the subsequent gain of the input signal. The magnitude of the input signal is increased by the gain of the amplifier by the same magnitude, as is the incident receiver noise. Radio receivers typically receive a radio frequency (RF) signal, for example, via an antenna. The received RF signal is typically amplified and then sent to a mixer where the frequency is downconverted via a mixer to a lower frequency that is easier to process by the receiver. The amplifier should raise the level of the RF input signal above the equivalent input noise of the mixer so that an adequate signal to noise ratio is maintained. A low-noise amplifier (LNA) is an electronic circuit of the receiver that amplifies the low-voltage pulse while minimizing the amount of electronic noise added to the low-voltage pulse. Both base stations and subscriber units include radio frequency transmitters and RF receivers. These devices service the wireless links between the base stations and subscriber units. Each RF receiver typically includes a low noise amplifier (LNA) that receives an RF signal from a coupled antenna, a mixer that receives the output of the LNA, a band-pass filter coupled to the output of the mixer, and a variable gain amplifier coupled to the output of the mixer. These RF receiver components produce an intermediate frequency (IF) signal that carries modulated data. The low noise amplifier receives inbound RF signals via the antenna and amplifies then. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard. A low noise amplifier is often utilized in an input stage of a signal receiving apparatus or receiver. For example, the LNA amplifies a received RF input signal so that an amplified RF signal has a desired gain and noise figure.

Low noise amplifiers (LNAs) are used in many systems where low-level signals must be sensed and amplified. Low noise amplifiers are typically used in communication transceivers for the amplification of weak electrical signals. Generally, the LNA is capable of decreasing most of the incoming noise and amplifying a desired signal within a certain frequency range to increase the signal to noise ratio (SNR) of the communication system and improve the quality of received signal as well. A low noise amplifier (LNA) is utilized in various aspects of wireless communications, including wireless LANs, cellular communications, and satellite communications. A critical building block in a radio receiver is the low noise amplifier (LNA). Low noise amplifiers are used in conjunction with numerous radio frequency functions, for example, mixers, voltage controlled oscillators, limiters, filters, etc. A typical receiver for a radio frequency signal (RF signal) comprises a combination of an amplifier and a mixer for signal amplification and frequency conversion. The amplifier, usually a low-noise amplifier (LNA), receives the RF signal, amplifies the RF signal and feeds the amplified RF signal to the mixer which in addition receives a local signal from a local oscillator (LO). The local oscillator signal has a frequency which is different from the frequency of the RF signal. The mixer generates an output signal that includes more frequencies than the frequencies of the RF signal and the local signal. The output signal is usually filtered to block undesired frequencies which include the original frequencies, their harmonics and their sum frequencies. The LNA amplifies the received signal and boosts its power above the noise level produced by subsequent circuits. As such, the performance of the LNA greatly affects the sensitivity of the radio receiver. In a typical RF receiver the RF signal is received by an antenna, then amplified using a fixed gain amplifier and transmitted over a transmission line having a characteristic impedance to the receiver circuitry. Since the received RF signal strength can vary significantly depending on the distance between the receiver and the transmitter, RF receivers typically include a circuit for automatic gain control (AGC). The signal at the other end of the transmission line is then typically amplified in a variable gain low noise amplifier (LNA) whose gain is controlled by the AGC circuit before being converted from RF to baseband. In typical wireless applications, low noise amplifiers are generally fabricated in bipolar semiconductor or GaAs MOSFET technologies. For low noise amplifiers, the gain linearity applied to a signal is an important operating characteristic, especially when the incoming signal is large. The gain linearity is typically related to the transconductance of a MOSFET in an input stage of the amplifier. Depending on signal frequency, an LNA can be implemented as an open loop or closed-loop amplifier and may also have a requirement to match a specific source impedance.

 

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