Low-noise amplifier

A Low-noise amplifier (LNA) is an electronic amplifier that amplifies a very low-power signal without significantly degrading its signal-to-noise ratio. An amplifier will increase the power of both the signal and the noise present at the amplifier's input; amplifiers are not perfect, so they also effectively add noise to the input signal. Low-noise amplifiers are designed to minimize the additional noise. Designers minimize the additional noise by considering tradeoffs that include impedance matching. choosing the amplifier technology, and selecting low-noise biasing conditions. Low-noise amplifiers are found in radio communications systems, medical instruments, and electronic equipment. A typical low-noise amplifier may supply a power gain of 100 (20 dB) while decreasing the signal-to-noise ratio by a factor of two (a 3 dB noise figure). Although low-noise amplifiers are primarily concerned with weak signals that are just above the noise floor, they must also consider the presence of larger signals causing intermodulation distortion. Consequently, low-noise amplifiers often do not have high gains.

Communications

Antennas are a common source of weak electronic signals. An outdoor antenna is often connected to its radio receiver by a transmission line called the feed line. Any losses in the feedline adversely affect the received signal-to-noise ratio: a feed line loss of 3 dB degrades the SNR by 3 dB. For example, consider a feed line made from 10 feet (3.0 m) of RG-174 coaxial cable and used with a Global Positioning System (GPS) receiver. The loss in that feed line is 3.2 dB (50 percent) at 1 GHz; the GPS signal is at a higher frequency (1.57542 GHz), so the feed line would be higher and would degrade the signal-to-noise ratio (SNR) by at least 3.2 dB (50 percent). This feed line loss may be avoided by placing a low-noise amplifier at the antenna; the LNA should supply at least enough gain to offset the feedline loss.

An LNA is a key component at the front-end of a radio receiver circuit to help reduce unwanted noise in particular. There is a formula for calculating noise in a multi stage signal collection circuit. When using Friis' formulas for noise, the overall noise figure (NF) of the receiver's front-end is dominated by the first few stages of the signal system even though in some cases there may be only one first stage. By using an LNA close to the signal source, the effect of noise from subsequent stages of the receive chain in the circuit is reduced by the signal gain created by the LNA, while the noise created by the LNA itself is injected directly into the received signal. This is important for effective noise reduction in the receive part of the circuit. The LNA boosts the desired signal power while adding as little noise and distortion as possible. The work done by the LNA enables optimum retrieval of the desired signal in the later stages of the system.

A good LNA has a low NF (e.g. 1 dB), enough gain to boost the signal (e.g. 10 dB) and should have a large enough inter-modulation and compression point (IP3 and P1dB) to do the work required of it. Further criteria to consider are the LNA's 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, Junction Field-Effect Transistors (JFETs) and High Electron Mobility Transistors (HEMTs) are often used. They are driven in a high-current regime, which is not energy-efficient, but it reduces the relative amount of shot noise. It also needs input and output matching circuits for narrow-band circuits enhance the gain (see Gain-bandwidth product).

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, 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:

Transistor

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.

Transducer

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).

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, (represented by ∆) 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 the 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.

Applications

LNAs are used in applications such as ISM radios, cellular telephones, GPS receivers, cordless phones, wireless LANs (WiFi), automotive remote keyless system, and satellite communications.

Satellite

In a satellite communications system, the ground station receiving antenna will connect to an LNA because the received signal is weak. The received signal is usually a little above background noise since satellites have limited power and 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).

Utilization requirements

Operating supply voltage

Usually LNAs require operating voltages in the range of 2V to 10V.

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.

Operating frequency

The Frequency Range of LNA operation is very wide. They can operate between 500 kHz and 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°C to 50 °C (-22°F to 122 °F).

Important factors

Noise figure

The Noise figure is 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.

High gain

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 Noise Figures (NF), the gain of an LNA also varies with the operating frequency.

See also

• Low-noise block converter
• Transducer
• Directional antenna

External links

• Conversion: distortion factor to distortion attenuation and THD