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 its input, but the amplifier will also introduce some additional noise. LNAs are designed to minimize that additional noise. Designers can minimize additional noise by choosing low-noise components, operating points, and circuit topologies. Minimizing additional noise must balance with other design goals such as power gain and impedance matching.

What is purpose of low noise amplifier?

A low-noise amplifier (LNA) is commonly found in all receivers. Its role is to boost the received signal a sufficient level above the noise floor so that it can be used for additional processing. The noise figure of the LNA therefore directly limits the sensitivity of the receiver.

LNAs are found in radio communications systems, medical instruments and electronic test 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.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 receiver signal-to-noise ratio (SNR) by 3 dB.

Low Noise Amplifiers

Analog Devices low noise amplifiers cover the frequency range from DC (IF) to RF Microwave and W-Band (95 GHz). These MMIC-based designs cover various gains and bandwidths with noise figures as low as 0.7 dB. Our low noise amplifiers offer some of the lowest noise and highest linearity available in the industry. Many of the designs offer a self-biased topology, and are internally matched to 50 ohms. They are used in a wide range of applications including telecom, instrumentation, and military/aerospace. All Analog Devices low noise amplifiers are fully specified over frequency, temperature, and supply voltage.

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 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.

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 NF is dominated by the first few stages of the RF 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.

Design considerations

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 impedance matching circuits for narrow-band circuits to enhance the gain (see Gain-bandwidth product).

Gain

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

Broadly speaking, two categories of transistor models are used in LNA design: Small-signal models use quasi-linear models of noise and large-signal models consider non-linear mixing.

The amount of gain applied is often a compromise. On one hand, high gain makes weak signals strong. On the other hand, high gain means higher level signals, and such high level signals with high gain may exceed the amplifier's dynamic range or cause other types of noise such as harmonic distortion or nonlinear mixing.

Noise figure

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.

The gain is this amplifier is very precisely set to -1000, because I wanted to check the accuracy of the input attenuator with it.

the output amplifiers for the left and right channel.

The signal generator.

The signal generator I use for the measurements is the Neutrik "Minirator MR1".

That is a very handy battery operated device, it can for instance generate, sine waves, square waves, white noise and pink noise.

The frequency of the waves can in certain steps be set from 20 to 20000 Hz.

For instance, one frequency setting is 1000 Hz, and the next one is 1250 Hz.

The output level can in steps be set from 0.13 mV to 1.6 Volt RMS.

The output voltage is very accurate, on the 1.00 Volt setting, I measure 0.998 V RMS on a digital voltmeter.

the Neutrik "Minirator MR1" signal generator.

On the top side of the device is a cinch output connector, and on the bottom side as you see a XLR connector.

The output impedance of the generator is 200 Ω.

The spectrum analyser.

As spectrum analyser I use the sound card of my PC, with the program: Visual Analyser, (version: 10.0.5 NE) running on the PC (go to the Visual Analyser download page).

these are the main settings I use for the spectrum analyser.

The analyser displays the audio spectrum in certain small blocks of frequency, in analyser terms these blocks are called: bins.

With the settings from figure 10 , the bins are 5.86 Hz wide.

You can select a certain window function, which determines how a discrete tone is displayed on the spectrum analyser, for instance how wide the peak in the spectrum is, and how steep it falls off.

I use the Hanning window function.

With the Hanning function, the noise bandwidth is 1.5 times the bin width.

The bin width is in this case: 5.86 Hz, so the noise bandwidth becomes 8.79 Hz.

This means, each bin measures the noise within a bandwidth of 8.79 Hz

When your input signal is noise, the noise bandwidth determines the level at which the noise is displayed.

Doubling the noise bandwidth, means double the noise power per bin, and a 3 dB higher value for the noise measured..

But heights of discrete tones are not affected by the setting of the noise bandwidth.

frequency response of the spectrum analyser.

For this test, a white noise signal is connected directly to the spectrum analyser inputs.

"White noise" contains an equal amount of noise power for every Hz in the spectrum.

With such a signal , we can determine the frequency response of the spectrum analyser.

The level of the white noise signal must be set so high, that it is as least 15 dB higher then the noise floor of the analyser itself.

The displayed amplitude is the average of 200 measurements, this average function removes most of the noise you see.

And it becomes possible to read the amplitude with 0.5 dB accuracy.

As figure 11 shows, the response is very flat over the entire frequency range.

The window function can also be set to: no window function (select: none) , in that case the noise bandwidth is equal to the the bin width.

Switching the window function from "none" (noise bandwidth = 1 bin) to "Hanning" (noise bandwidth = 1.5 bin), should in theory increase the measured noise level by 10.log (1.5) = 1.8 dB, and that is indeed what I measure.

Another window function is the "Flattop function", which has a theoretical noise bandwidth of 3.83 x the bin width.

Switching the window function from "none" to "Flattop" should increase the measured noise level by 10.log (3.83) = 5.8 dB.

But strange enough, I only measure 4.8 dB increase when doing this.

For that reason I don't use the Flattop window function.

Impedance

The circuit topology 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.

Biasing

This section needs expansion. You can help by adding to it. (August 2019)

Another design issue is the noise introduced by biasing networks.

Applications

LNAs are used in communications receivers such as in cellular telephones, GPS receivers, wireless LANs (WiFi), and satellite communications.

In a satellite communications system, the ground station receiving antenna uses an LNA because the received signal is weak since satellites have limited power and therefore use low-power transmitters. The satellites are also distant and suffer path loss: low Earth orbit satellites might be 120 miles (190 km) away; a geosynchronous satellite is 22,236 miles (35,785 km) away.

The LNA boosts the antenna signal to overcome feed line losses between the antenna and the receiver.

LNAs can enhance the performance of software-defined radio (SDR) receiver systems. SDRs are typically designed to be general purpose and therefore the noise figure is not optimized for any one particular application. With an LNA and appropriate filter, performance is improved over a range of frequencies.