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    2021-10-21 17:39:24
    Understanding the Low Noise Amplifier (LNA)

    Topics covered in this article:
    What is a Low Noise Amplifier?
    The noise figure of the low-noise amplifier
    The transistor's own noise composition
    Applications of the low-noise amplifier

    What is a Low Noise Amplifier?

    A low noise amplifier is generally used as a high-frequency or intermediate-frequency preamplifier for various types of radio receivers, as well as amplifying circuits for high-sensitivity electronic detection equipment. When amplifying weak signals, the noise generated by the amplifier might substantially interfere with the signal. Therefore, it is hoped to reduce this noise to improve the signal-to-noise ratio of the output. The degradation of the signal-to-noise ratio caused by the amplifier is usually expressed by the noise figure F.

    Low noise amplifier.jpg

    Low noise amplifier

    Low-noise amplifiers are an important component of the receiver circuit, which processes and converts the received signal into information. LNAs are meant to be close to the receiving device in order to reduce interference loss. They contribute only a small amount of noise (useless data) to the received signal since any more will severely degrade the already weakened signal. An LNA is employed when the signal-to-noise ratio (SNR) is high and needs to be reduced by roughly 50% while power is increased. The first component of a receiver to intercept a signal is the LNA, making it a critical component in the communications process.

    The noise figure of the low-noise amplifier

    Modern low-noise amplifiers mostly use transistors and field-effect transistors. Microwave low-noise amplifiers use variable capacitance diode parametric amplifiers. The standard temperature amplifier's noise temperature Te can be in the tens of degrees (absolute temperature). The refrigeration parametric amplifier can reach 20K. The application of GaAs field-effect transistor low-noise microwave amplifiers has become increasingly widespread. Its noise figure can be lower than 2 decibels. The noise figure of the amplifier is also related to the working state of the transistor and the internal resistance of the signal 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.

    The ideal amplifier has a noise figure F=1 (0 decibels). The output signal-to-noise ratio is equal to the input signal-to-noise ratio, which is its physical meaning. The FN of a well-designed low-noise amplifier can reach below 3 decibels. When the noise figure is very low, the noise temperature Te is usually used as a measure of the amplifier's noise performance: Te=T0(F-1). In the formula, T0 is room temperature. Here, the unit of it and the noise temperature Te are both Kelvin (K).

    The noise figure of a single-stage amplifier mainly depends on the active devices used and their working conditions. The noise figure F of a multi-stage amplifier mainly depends on its pre-stage. If F1, F2,..., Fn are the noise figure of each amplifier in turn, then A1,..., An-1 is the power gains of each amplifier in turn. The greater the gain A1 of the pre-stage, the smaller the influence of the subsequent amplifiers on the total noise figure F.

    The transistor's own noise composition

    The self-noise of the transistor is composed of the following four parts.

    ① Flicker noise. Its power spectral density increases with the decrease of frequency f. So it is also called 1/f noise or low-frequency noise. When the frequency is very low, this noise is relatively large. When the frequency is high (above several hundred Hz), this noise can be ignored.

    ②The thermal noise sum of the base resistance rb'b.

    ③ Shot noise. The power spectral density of these two kinds of noise is basically independent of frequency.

    ④ Distribution noise. Its intensity is proportional to the square of f. When f is higher than the cut-off frequency of the transistor, this noise increases sharply. The figure below is the curve of the transistor noise figure F with frequency. For low frequency, especially ultra-low frequency low noise amplifiers, transistors with low 1/f noise should be used. For medium and high-frequency amplifiers, high transistors should be used as much as possible, so that the operating frequency range is in the flat part of the noise figure-frequency curve.

    Noise figure-frequency characteristic curve of Low noise amplifier.jpg

    Noise figure-frequency characteristic curve of Low noise amplifier

    Applications of the low-noise amplifier

    LNA has experienced the early development of liquid helium-cooled parametric amplifiers and room temperature parametric amplifiers. With the rapid development of technology, it has been replaced by microwave field-effect transistor amplifiers in recent years. This type of amplifier has excellent characteristics of small size, low cost, and lightweight. 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. And the noise temperature of commonly used low-noise amplifiers can be lower than 45K.

    The low 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. Low noise figure (NF, Noise Figure) has set a new standard. At present, the wireless communication infrastructure industry is facing the challenge of providing the best signal quality and coverage in the crowded spectrum. Receiver sensitivity is one of the most critical requirements in the design of the base station receiving path. The appropriate LNA selection, especially the first Level LNA can greatly improve the sensitivity performance of base station receivers, and low noise index is also a key design goal.

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