|Low noise amplifier (LNA)|
|Thursday, 14 December 2006|
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.