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 increases the power of both the signal and the noise present at its input. LNAs are designed to minimize additional noise. Designers minimize noise by considering trade-offs that include impedance matching, choosing the amplifier technology (such as low-noise components) and selecting low-noise biasing conditions.

LNAs are found in radio communications systems, medical instruments and electronic equipment. A typical LNA may supply a power gain of 100 (20 decibels (dB)) while decreasing the signal-to-noise ratio by less than a factor of two (a 3 dB noise figure (NF)). Although LNAs are primarily concerned with weak signals that are just above the noise floor, they must also consider the presence of larger signals that cause intermodulation distortion.

Communications

Antennas are a common source of weak signals.[1] An outdoor antenna is often connected to its receiver by a transmission line called a "feed line". Losses in the feed line lower the received signal-to-noise ratio: a feed line loss of 3 dB degrades the signal-to-noise ratio (SNR) by 3 dB.

An example is 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 at 1 gigahertz (GHz); approximately 5 dB at the GPS frequency (1.57542 GHz). This feed line loss can be avoided by placing an LNA at the antenna, which supplies enough gain to offset the loss.

A good LNA has a low NF (e.g. 1 dB), enough gain to boost the signal (e.g. 10 dB) and a large enough inter-modulation and compression point (IP3 and P1dB) to do the work required of it. Further specifications are the LNA's operating bandwidth, gain flatness, stability, input and output voltage standing wave ratio (VSWR).

For low noise, a high amplification is required for the amplifier in the 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 reduces the relative amount of shot noise. It also requires input and output matching circuits for narrow-band circuits to enhance the gain (see Gain-bandwidth product).

Noise reduction

An LNA is a key component at the front-end of a radio receiver circuit to help reduce unwanted noise in particular. Friis' formulas for noise models the noise in a multi-stage signal collection circuit. In most receivers, the overall noise figure (NF) is dominated by the first few stages of the receiver's 'front-end'.

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. The LNA boosts the desired signals' 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.

LNA design

Low noise amplifiers are the building blocks of communication systems and instruments. The four important parameters in LNA design are: gain, noise figure, non-linearity and impedance matching. The required LNA design steps are:

Gain device

Amplifiers need a device to provide gain. In the 1940s, that device was a vacuum tube, but now it is usually a transistor or an integrated circuit. The transistor may be one of many varieties of bipolar transistors or field-effect transistors. Other gain devices, such as tunnel diodes, may be used.

Broadly speaking, two categories of transistor models are used in LNA design:

Circuit topology

Circuit topology covers issues such as gain and input impedance.

Gain is often a compromise. On one hand, having lots of gain is good because it takes weak signals above the noise floor. On the other hand, lots of gain means higher level signals and more problems with non-linear mixing.

The circuit topology also affects input and output impedance. In general, the source impedance is matched to the input impedance because that will maximize the power transfer from the source to the device. If the source impedance is low, then a common base or common gate circuit topology may be appropriate. For a medium source impedance, a common emitter or common source topology may be used. With a high source resistance, a common collector or common drain topology may be appropriate.

An input impedance match may not produce the lowest noise figure. There is another notion of a noise impedance match.

Another design issue is the noise introduced by biasing networks.

Applications

LNAs are used in applications such as industrial, scientific and medical band (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 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 (120 miles) away; a geosynchronous satellite is 35,786 kilometres (22,236 mi) 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 downlink frequency (e.g., 11 GHz) that would have large feed line losses to a lower frequency (e.g., 1 GHz) with lower 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).

Requirements

Important factors

Noise

The noise figure helps determine the efficiency of a particular LNA. LNA suitability for a particular application is typically based on its noise figure. In general, a low noise figure results in better signal reception.

High gain

With a low noise figure, an LNA must have high gain. An LNA without high-gain allows the signal to be affected by LNA circuit noise; the signal may become attenuated, so the LNA's high gain is an important parameter. Like noise figures, LNA gain also varies with operating frequency.

See also

References

  1. ↑ A 900MHz Low Noise Amplifier with Temperature Compensated Biasing. ProQuest. 2008-01-01. ISBN 9780549667391.

Further reading

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