Understanding the Low Noise Amplifier (LNA)

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. Amplifier noise critically impacts weak signals, degrading their signal-to-noise ratio. Therefore, it is hoped to reduce this noise to improve the signal-to-noise ratio of the output. The noise figure (F) quantifies how much an amplifier degrades the signal-to-noise ratio.

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. LNAs are critical when the input SNR is low, as they minimize added noise to preserve the signal quality 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. Superconducting parametric amplifiers now operate near 4KThe application of GaAs field-effect transistor low-noise microwave amplifiers has become increasingly widespread. Its noise figure can be lower than 2 decibels. Modern GaN (Gallium Nitride) amplifiers now achieve noise figures below 1 dB in some applications (e.g., 5G infrastructure). 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). 

For a 1 dB noise figure （$F = 1.23$F=1.23），$T_e = 290 \times (1.23 - 1) = 69K$，aligning with modern LNAs.

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 Diffusion 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

Applications of the low-noise amplifier

LNA has experienced the early development of liquid helium-cooled parametric amplifiers and room temperature parametric amplifiers. These are now largely replaced by solid-state amplifiers in most applications. 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.

Modern Advancements

5G Base Stations:
GaN-based LNAs eliminate the need for input limiters, reducing noise figures and enabling higher integration in 5G infrastructure. They support frequencies up to 3.6 GHz with gains of 13.5 dB, critical for millimeter-wave applications.

Satellite and Radar Systems:
Broadband LNAs (0.2–100 GHz) are used in military radar, Earth observation, and satellite links. Their lightweight design and waveguide compatibility make them ideal for aerospace applications.

Emerging Technologies:

Cryogenic LNAs (e.g., SiGe HBTs) achieve ultra-low power consumption (290 µW) and noise temperatures below 5 K for quantum computing and advanced radiometry.

Wireless Infrastructure: LNAs improve base station receiver sensitivity in crowded spectra, enabling better coverage in urban environments.

LNAs continue to evolve, driven by demands for higher frequencies, lower noise, and greater integration in telecommunications, astronomy, and healthcare technologies.

Transistor Comparison

Bipolar Junction Transistors (BJTs) vs. FETs in LNAs, highlighting noise performance trade-offs

Practical Case Studies

Satellite Receiver Design (Ka-Band)

A GaN HEMT-based monolithic microwave integrated circuit (MMIC) LNA demonstrated a measured noise figure (NF) of 2.5 dB and a small-signal gain of 18.3–20.2 dB across the 24.25–29.5 GHz frequency range. This design, implemented using 0.15 μm GaN-on-SiC technology, achieved enhanced signal clarity for millimeter-wave radar and 5G applications. The LNA’s hybrid matching network topology improved stability and bandwidth, making it suitable for Earth observation and high-frequency satellite communication systems.

X-Band Radar and Satellite Communications

A two-stage common-source GaN HEMT LNA designed for X-band (8–12 GHz) applications exhibited a noise figure below 1.4 dB and a flat gain of 14–16 dB with minimal ripple (±1.4 dB). This design eliminated the need for input protection circuits due to GaN’s inherent robustness, reducing system complexity and cost. Such LNAs are critical for military radar and satellite links, where high linearity and resilience to power surges are essential.

Quantum Computing and Cryogenic Systems

A cryogenic GaN HEMT LNA achieved a noise temperature of <5 K at 4 GHz, enabling ultra-sensitive signal detection in quantum computing readout circuits. Operating at 290 µW power consumption, this design highlights GaN’s potential for cryogenic applications where low thermal noise and high electron mobility are critical.

Reference

1. GaN HEMT Low Noise Amplifiers for Radio Base Station Receivers
   -1 GaN HEMT Low Noise Amplifiers for Radio Base Station Receivers, Chalmers University of Technology.
   

2. GaN based LNA MMICs for X-Band Applications
   -2 Salahuddin Zafar et al., GaN based LNA MMICs for X-Band Applications, Bilkent University.
   

3. A 26–30 GHz GaN HEMT Low-Noise Amplifier
   -3 A 26–30 GHz GaN HEMT Low-Noise Amplifier Employing a Series Inductor-Based Stability Enhancement Technique, Electronics, 2022.
   

4. Design of a Low Cost X-Band LNA with Sub-1-dB NF for SATCOM
   -4 Galip Orkun ARICAN et al., Design of a Low Cost X-Band LNA with Sub-1-dB NF for SATCOM, GU J Sci, 2023.
   

5. Ka-band High-linearity and Low-noise Gallium Nitride MMIC
   -5 Ka-band High-linearity and Low-noise Gallium Nitride MMIC, scispace.com.
   

6. A highly survivable X‐band low noise amplifier based on GaN
   -6 A highly survivable X‐band low noise amplifier based on GaN, Wiley Online Library.
   

7. A K-Band MMIC Low Noise Amplifier in GaN-on-Si 100-nm
   -7 A K-Band MMIC Low Noise Amplifier in GaN-on-Si 100-nm, J-Stage.
   

8. RF Front-End Design for X Band using 0.15µm GaN HEMT
   -8 RF Front-End Design for X Band using 0.15µm GaN HEMT, scispace.com