What Is a Low Noise Amplifier?

Low noise amplifier, amplifier with very low noise figure. Generally used as a high-frequency or intermediate-frequency preamplifier for various types of radio receivers, and amplifying circuits for high-sensitivity electronic detection equipment. In the case of amplifying a weak signal, the interference of the amplifier's own noise on the signal may be serious, so it is desirable to reduce this noise to improve the signal-to-noise ratio of the output.

Low noise amplifier, amplifier with very low noise figure. Generally used as a high-frequency or intermediate-frequency preamplifier for various types of radio receivers, and amplifying circuits for high-sensitivity electronic detection equipment. In the case of amplifying a weak signal, the interference of the amplifier's own noise on the signal may be serious, so it is desirable to reduce this noise to improve the signal-to-noise ratio of the output.

Chinese name

Low noise amplifier

Foreign name

low noise amplifier

Function

Input signal to noise ratio is equal to output signal to noise ratio

Use

Radio receiver

LNA body

Amplifier with very low noise figure. Generally used as a high-frequency or intermediate-frequency preamplifier for various types of radio receivers, and amplifying circuits for high-sensitivity electronic detection equipment. In the case of amplifying a weak signal, the interference of the amplifier's own noise on the signal may be serious, so it is desirable to reduce this noise to improve the signal-to-noise ratio of the output. The degree of deterioration of the signal-to-noise ratio caused by the amplifier is usually expressed by the noise figure F. The noise figure of an ideal amplifier is F = 1 (0 dB), and its physical meaning is that the input signal-to-noise ratio is equal to the output signal-to-noise ratio. Most modern low-noise amplifiers use transistors and field-effect transistors; microwave low-noise amplifiers use variable-capacitance parametric amplifiers. The noise temperature Te at room temperature can be lower than tens of degrees (absolute temperature). In the following, the application of GaAs field effect transistor low noise microwave amplifiers has become increasingly widespread, and its noise figure can be lower than 2 dB. The noise figure of the amplifier is also related to the operating state of the transistor and the internal resistance of the source. In order to take into account the requirements of low noise and high gain, a low-noise amplifier circuit with a common emitter and a common base is often used.

Where weak signals are amplified, the amplifier

Low noise amplifier

The interference of the own noise on the signal may be serious, so it is desirable to reduce this noise to improve the signal-to-noise ratio of the output. The deterioration of the signal-to-noise ratio caused by the amplifier is usually

Low noise amplifier

Number F (see enlargement)

Low noise amplifier

Or logarithmic noise figure F N means F N = 10lg F (dB)

Ideal amplifier

Low noise amplifier

The noise figure F = 1 (0 dB), its physical meaning is that the output signal-to-noise ratio is equal to the input signal-to-noise ratio. The F N of a well-designed low-noise amplifier can be below 3 dB. When the noise figure is very low, the noise temperature T e is usually used as a measure of the noise performance of the amplifier: T e = T 0 ( F -1). Where T 0 is room temperature. Here, it and the unit of noise temperature T e are both Kelvin (K).

The noise figure F of a multi-stage amplifier depends mainly on its pre-stage. If F 1, F 2, ..., F n are the noise figures of the amplifiers in each order, then

Low noise amplifier

Where A 1, ..., A n-1 are the power gains of the amplifiers in each order. The larger the gain A 1 of the pre-stage, the smaller the impact of the subsequent stages of the amplifier on the total noise figure F.

The noise figure of a single-stage amplifier depends mainly on

Low noise amplifier

Depending on the active device used and its operating status. Most modern low-noise amplifiers use transistors and field-effect transistors; microwave low-noise amplifiers use variable-capacitance parametric amplifiers. The noise temperature T at room temperature can be lower than tens of degrees (absolute temperature). Below 20K. GaAs field effect transistor low-noise microwave amplifiers have been increasingly used, and their noise figure can be lower than 2 decibels.

The transistor's own noise consists of the following four parts. Flicker noise, whose power spectral density increases as frequency f decreases, so it is also called 1 / f noise or low frequency noise. This noise is greater at very low frequencies and can be ignored at higher frequencies (above a few hundred hertz). The thermal noise sum of the base resistance r b'b. scattered grain

Low noise amplifier

Noise, the power spectral density of these two kinds of noise is basically independent of frequency. Distribution noise, whose intensity is proportional to the square of f . When f is higher than the cut-off frequency of the transistor, this noise increases sharply. Figure 1 is a graph of the transistor noise figure F as a function of frequency. For low-frequency, especially ultra-low-frequency and low-noise amplifiers, transistors with low 1 / f noise should be selected; for mid- and high-frequency amplifiers, high-transistors should be used as much as possible so that their operating frequency range lies on the flat part of the noise figure-frequency curve .

The field effect transistor has no shot noise. At low frequencies, it is mainly flicker noise, and at higher frequencies, it is mainly thermal noise generated by channel resistance. It usually has less noise than a transistor and can be used in much lower-noise amplifiers.

The noise figure of the amplifier is also related to the operating state of the transistor and the internal resistance of the source. Figure 2 is an amplifier model that considers its own noise. u s and R s are the source voltage and internal resistance, respectively. The thermal noise voltage mean square value of R s is equal to 4 k T R s f , where T is the absolute temperature, k is the Boltzmann constant, and f is Amplifier passband. Noise voltage mean square and noise current mean square for amplifier own noise

Low noise amplifier

The values indicate that they are a function of the operating state of the transistor and can be measured by an appropriate method. In this way, the noise figure F of the amplifier can be written as

Low noise amplifier

Once the DC operating point of the amplifying tube is determined, and it is determined accordingly, the noise figure F will be mainly a function of the internal resistance R s of the source. R s has an optimal value that minimizes F (Figure 3).

In the case where the operating frequency and the internal resistance of the source are given, the noise figure is also related to the DC operating point of the transistor. The emitter current I E has an optimal value that minimizes the noise figure. A typical F - I E curve is shown in Figure 4.

The noise figure of a transistor amplifier is basically related to the circuit configuration

Low noise amplifier

Nothing. However, the common-emitter amplifier has a moderate input resistance. The optimal source resistance R s when F is minimum is close to this input resistance. The input circuit is generally in a matched state with a large gain. The input resistance of the common base amplifier is small and the input impedance of the common collector amplifier is high. Both of them are not easy to meet the conditions of low noise figure and high amplifier gain at the same time, so they are not suitable for the pre-stage of the amplification key. In order to take into account the requirements of low noise and high gain, a low-noise amplifier circuit with a common emitter-base cascade is often used.

LNA Product Information

Low Noise Amplifier Applications

The noise amplifier (LNA) is mainly designed for mobile communication infrastructure base station applications, such as transceiver wireless communication cards, tower-mounted amplifiers (TMA), combiners, repeaters, and remote / digital wireless broadband head-end equipment. The low noise figure (NF, Noise Figure) sets a new benchmark. At present, the wireless communication infrastructure industry is facing the challenge of providing the best signal quality and coverage in the congested spectrum. Receiver sensitivity is one of the most critical requirements in the design of the receiving path of the base station. Suitable LNA selection,

Low noise amplifier

In particular, the first-level LNA can greatly improve the sensitivity performance of the base station receiver, and the low noise figure is also a key design goal. Avago provides the best noise figure at the same level of 0.48dB at 1900MHz. The other key design is linearity, which affects the receiver s ability to distinguish between close and false signals. The third-order intercept point OIP3 can be used to define linearity. Under typical operating conditions of 1900MHz and 5V / 51mA, Avago is unique. The GaAs enhanced mode pHEMT process technology can bring about a noise figure of 0.48dB and an OIP3 of 35dBm. Under typical operating conditions of 2500MHz and 5V / 56mA, the noise figure is 0.59dB and OIP3 is 35dBm. With low noise figure and high OIP3, these new low-noise amplifiers from Avago can provide more design space for base station receiver paths than existing amplifier products.

Adjustable capability and common pin arrangement bring design optimization and flexibility

Low Noise Amplifier Power

Built-in active bias circuit, the working current of Avago low-noise amplifier can be adjusted, so that the design engineer can choose between working power consumption and output linearity, through OIP3 measurement and maintain the best noise index

Low noise amplifier

However, base station design engineers can have the flexibility to use the same Avago low-noise amplifier to meet various design needs and requirements in different regions. As more communication channels must be added to the transmit and receive circuit cards, the printed circuit board space has also become another key design challenge for base station design engineers. Avago chose a small 4 mm2 QFN package to meet this market demand. These two new low-noise amplifiers use the same package size, pin arrangement and external matching circuit as Avago's existing 900MHz low-noise amplifier MGA-633P8, which can use a common printed circuit board for all base station RF front-end designs operating in different frequency bands. Design, reducing the number of printed circuit board designs required to provide base station solutions for different frequency bands and regional markets.

Key features of LNA

1500MHz to 2300MHz operation

Best noise figure (NF) at the same level: 0.48dB @ 1900MHz

35dBm OIP3

17.8dB gain

21dBm P1dB @ 1900MHz

2300MHz to 4000MHz operation

Low noise figure (NF): 0.59dB @ 2500MHz

35dBm OIP3

17.5dB gain

22dBm P1dB @ 2500MHz

· Single 5V power supply, low power consumption

51mA (1500MHz-2300MHz)

56mA (2300MHz-4000MHz)

· The device adopts common pin arrangement and matching circuit

Simplify printed circuit board design and production

· Special process: 0.25m GaAs enhanced mode pHEMT

Package and temperature range

The two low-noise amplifiers are available in 2.0 x 2.0 x 0.85 mm and RoHS-compliant 8-pin surface-mount QFN packages. All devices can operate over a wide temperature range from -40oC to + 85oC.

Low Noise Amplifier Principle

The figure of merit ( G / T ) of an earth station depends mainly on the performance of the antenna and the low noise amplifier (LNA). The noise temperature T _s of the receiving system refers to the equivalent noise temperature of the system converted to the LNA input. It is mainly composed of three parts: antenna noise temperature T _A , feeder loss L _A L _A, and low noise receiver noise, as shown in the figure. .

Noise composition of the receiving system

So the value of T _s

T _ss == T _e ++ T _a T _A / L _aA ++ (1 - 1 / L _a L _A ) T _o

In the formula:

T _s T _s is the noise temperature of the receiving system

T _o is the equivalent noise temperature converted from the receiving system to the input of the LNA

T _a T _A is the antenna noise temperature

L _a L _AA is the feeder loss (true value)

T _o is the ambient temperature ( T _o == 293K)

It can be calculated that when the feeder loss increases by 0.1dB, the system noise temperature will increase by about 6.7K. It can be seen that the feeder loss has a great impact on the system noise temperature, so the feeder should be as short as possible. In fact, the LNA of the earth station is often installed directly in the cabin at the end of the feed.

Low Noise Amplifier Applications

LNA has experienced the development of early liquid helium-cooled parametric amplifiers and room temperature parametric amplifiers. With the rapid development of modern science and technology, it has been replaced by microwave field effect transistor amplifiers in recent years. Such amplifiers have small size and light weight And low cost excellent characteristics. Especially in terms of radio frequency characteristics, it has the characteristics of low noise, wide frequency band and high gain. It has been widely used in C, Ku, Kv and other frequency bands. The noise temperature of low-noise amplifiers currently used can be lower than 45K.