A Low-noise amplifier (LNA) is an electronic amplifier used to amplify very weak signals (for example, signals captured by an antenna). Essentially, an LNA amplifies signals that are barely recognizable without adding a lot of noise, as the name implies.
An LNA is usually located close to the signal source in order to reduce losses in the feed-line or minimize interference. This arrangement is frequently used in microwave systems such as GPS, because coaxial cable feed-line has high loss at microwave frequencies. For example, 10 feet (3.0 m) of RG-174 coaxial cable has a loss of 3.2 dB (50 percent) at 1 GHz; the feedline would degrade the signal-to-noise ratio (SNR) by 3.2 dB (50 percent).
An LNA is a key component 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 only the first stage).
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. This enables retrieval of the signal in the later stages of the system. A good LNA has a low NF (e.g. 1 dB), enough gain (e.g. 10 dB) and should have large enough inter-modulation and compression point (IP3 and P1dB). Further criteria are operating bandwidth, gain flatness, stability, 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. 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).
Low noise amplifiers are the building blocks of any communication system. The four most important parameters in LNA design are: gain, noise figure, non-linearity and impedance matching. LNA design is based mainly on the S-parameters of a transistor. The steps required in designing an LNA are the following:
Broadly speaking, there are two large categories of transistor models to enable the design of low-noise amplifiers with standard circuit-design simulators:
- Small-signal models use a set of S-parameter measurements at different frequencies, but typically at a fixed bias point for the modeled device, which does not require any type of external biasing for preliminary simulations.
- Large-signal models are a more physical representation of the physical transistors and to those an external bias must be applied. The majority of discrete surface-mount transistors provide small-signal S-parameter models, as they can be sufficient for LNA design.
One of the crucial stages in designing an LNA is proper selection of a transducer. The transducer that is selected should have a maximum gain and minimum noise figure (NF).
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.
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.
LNAs are used in various applications like ISM Radios, Cellular/PCS Handsets, GPS Receivers, Cordless Phones, Wireless LANs, Wireless Data, Automotive RKE, and satellite communications.
In a satellite communications system, the ground station receiving antenna will connect to an LNA. The LNA is needed because the received signal is weak. The received signal is usually a little above background noise. Satellites have limited power so they use low power transmitters. The satellites are also distant and suffer path loss; low earth orbit satellites might be 200 km away; a geosynchronous satellite is 35 786 km away. A larger ground antenna would give a stronger signal, but a larger antenna can be more expensive than adding an LNA. The LNA boosts the antenna signal to compensate for the feed-line losses between the (outdoor) antenna and the (indoor) receiver. In many satellite reception systems, the LNA includes a frequency block down-converter that shifts the satellite down-link frequency (e.g., 11 GHz) that would have large feed-line losses to a lower frequency (e.g., 1 GHz) with lower feed-line losses. The LNA with down converter is called a low-noise block down-converter (LNB). Satellite communications are usually done in the frequency range of 100 MHz (e.g. TIROS weather satellites) to tens of GHz (e.g., satellite television).
Operating supply voltage
Usually LNAs require operating voltages in the range of 2 .. 10 V.
Operating supply current
LNAs require supply current in the mA range, the supply current required for an LNA is dependent on its design and the application for which it has to be used.
The Frequency Range of LNA operation is very wide. They can operate from 500 kHz to 50 GHz.
Operating temperature range
An LNA, like other semiconductor devices, is specified for operation in a specific temperature range. The temperature range where an LNA operates best is usually -30 to +50 °C or -22 to +122 °F.
The 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. A low noise figure results in better reception of signal.
With a low noise figure, an LNA must have high gain in order to process signal into post-circuit. Depending on requirements, high-gain LNAs are designed for application by manufacturers. If an LNA doesn't have high-gain, then the signal will be affected by noise in the LNA circuit itself. It may become attenuated, so the LNA's high gain is an important parameter. Like NF, the gain of an LNA also varies with the operating frequency.
- A 900MHz Low Noise Amplifier with Temperature Compensated Biasing. ProQuest. 2008-01-01. ISBN 9780549667391.