What Are Low-noise Amplifiers?

Low-noise amplifiers (LNAs) are electronic amplifiers designed to boost weak signals while adding as little noise as possible to the signal of interest. They occupy the first active stage of a receiver chain, positioned immediately after the antenna or sensor, where any noise introduced is amplified along with every subsequent stage in the system and therefore has the greatest impact on the overall sensitivity. LNAs are fundamental building blocks in radio-frequency (RF) and microwave systems, where signal levels from distant transmitters or small sensors may be far below the thermal noise floor.

The discipline draws on RF circuit theory, solid-state device physics, and electromagnetic compatibility. Key performance metrics include noise figure (the degradation in signal-to-noise ratio expressed in decibels), gain, input and output impedance matching, linearity (characterized by the third-order intercept point, IIP3), and stability. These parameters interact: achieving low noise figure often requires biasing and impedance conditions that differ from those maximizing gain or linearity, so LNA design is inherently a multi-objective trade-off.

Noise Figure and Matching

Every active device contributes noise, and the noise figure of an amplifier quantifies how much. The Friis formula shows that the noise figure of the first stage dominates the receiver noise floor: if the LNA has 15 dB of gain, the noise contributions of all subsequent stages are divided by the LNA's power gain, making that first-stage noise almost the only noise that matters. For any transistor, there exists an optimum source impedance that minimizes noise figure; designing the input matching network to present this impedance, rather than the standard 50-ohm termination, is the central task of LNA input design. Inductive source degeneration, a technique that adds a small inductance in series with the transistor's source terminal, is widely used in narrowband LNAs to achieve simultaneous noise and impedance matching, as described in the NXP practical considerations for LNA design whitepaper.

Circuit Topologies

The common-source and cascode topologies are the two most widely deployed LNA architectures. A common-source stage offers high transconductance and good noise properties at moderate frequencies but suffers from gain roll-off and limited reverse isolation at millimeter-wave frequencies due to the Miller effect across the drain-gate capacitance. The cascode topology cascades a common-source input transistor with a common-gate output transistor, which reduces the Miller capacitance, increases reverse isolation (improving stability), and extends the usable frequency range. Differential LNA topologies are used where common-mode noise rejection is important, such as in receiver front-ends integrated on a noisy digital chip. A third option, the common-gate topology, presents a well-defined input impedance directly from device physics rather than from a matching network, which simplifies broadband designs at the cost of a somewhat higher noise figure. Microwave and millimeter-wave CMOS LNA research from UC Berkeley surveys these topologies with respect to the scaling behavior of CMOS devices.

Technology Platforms

LNA performance depends strongly on the transistor technology. Gallium arsenide pseudomorphic high-electron-mobility transistors (GaAs pHEMTs) have historically offered the lowest noise figures in the microwave and millimeter-wave range and remain dominant in satellite receivers and electronic warfare systems. Silicon-germanium (SiGe) bipolar-complementary metal-oxide-semiconductor (BiCMOS) technology achieves noise figures within a fraction of a decibel of GaAs at frequencies up to W-band (75-110 GHz) while offering integration with digital logic on the same chip. Standard CMOS, driven by scaling to 65 nm and below, has become viable for LNAs at 5G millimeter-wave bands such as 26-28 GHz, as demonstrated in recent 5G LNA implementations that achieve sub-3 dB noise figure with adequate gain for the first receiver stage.

Applications

Low-noise amplifiers have applications across a wide range of fields, including:

• Cellular and 5G wireless receivers operating from sub-6 GHz through millimeter-wave bands
• Satellite ground station receivers and direct-broadcast satellite consumer electronics
• Radar and electronic warfare front-ends requiring high sensitivity and wide dynamic range
• Scientific radio telescopes and radio astronomy receivers
• Medical imaging systems, including magnetic resonance imaging (MRI) coil preamplifiers
• GPS and GNSS receivers integrating antennas with sub-1 dB noise figure LNAs