What Is Amplifier Noise?

Amplifier noise is the unwanted random electrical signal that an amplifier introduces to its output in addition to the amplified version of the input signal. Every electronic amplifier adds noise derived from physical processes within its active devices and passive components, and this noise sets a fundamental lower bound on the smallest signal the amplifier can usefully process. The ratio of output signal-to-noise ratio to input signal-to-noise ratio is the noise factor, or equivalently the noise figure when expressed in decibels. A noise figure of 0 dB would represent a theoretically perfect, noiseless amplifier; practical electronic amplifiers range from below 1 dB in optimized microwave designs to 10 dB or more in general-purpose circuits.

The field of amplifier noise analysis draws from statistical physics, semiconductor device physics, and communication systems theory. It is a central concern in any application where weak signals must be detected or processed reliably, including radio astronomy, cellular handsets, satellite receivers, and biomedical instruments. The noise behavior of an amplifier is determined by the physical mechanisms within its transistors and resistors, by the circuit topology, and by the source impedance presented to the amplifier input.

Noise Sources in Amplifiers

The dominant noise mechanisms in electronic amplifiers are thermal noise, shot noise, and flicker noise. Thermal noise, also called Johnson-Nyquist noise, arises from the random thermal motion of charge carriers in any resistive element and produces a noise power spectral density of 4kTR watts per hertz, where k is Boltzmann's constant, T is temperature in kelvin, and R is resistance. Shot noise originates from the granular nature of current flow across a potential barrier, such as a p-n junction, and has a power spectral density proportional to the DC bias current. Flicker noise, also called 1/f noise, is a low-frequency phenomenon with power spectral density increasing as frequency decreases, caused by carrier trapping and release at surface states or crystal defects, and it is particularly significant in MOSFET amplifiers at audio frequencies. These mechanisms and their modeling are described in detail in Tektronix white paper resources on noise figure measurement methods. The thermal noise floor at room temperature sets an absolute limit of approximately −174 dBm per hertz of bandwidth on the minimum detectable signal power.

Noise Figure and Cascaded Systems

Noise figure (NF) is the standard metric for characterizing how much noise an amplifier or component adds. It is measured by applying a calibrated noise source to the input and comparing the output noise power with the source connected and disconnected. For a cascade of amplifier stages, the Friis formula governs the total system noise factor: the total noise factor equals the first stage's noise factor plus the second stage's noise factor minus one, divided by the first stage's power gain, and so on for subsequent stages. The implication is that the first stage in a signal chain dominates the overall noise performance, which is why placing a low-noise amplifier as close as possible to the signal source, before any cable loss or mixer stage, is a foundational principle of receiver design. The Friis formula and its practical consequences are explained in RF design application notes from Marki Microwave on noise figure and receiver sensitivity.

Low-Noise Amplifier Design

Low-noise amplifiers (LNAs) are the first active stage in receivers where sensitivity is critical. GaAs pseudomorphic high-electron-mobility transistors (pHEMT) achieve noise figures below 0.5 dB at microwave frequencies due to their high electron mobility and low parasitic resistance. Silicon-germanium heterojunction bipolar transistors are preferred in integrated front-end circuits combining LNA, mixer, and frequency synthesizer functions on a single chip. In optical fiber systems, erbium-doped fiber amplifiers (EDFAs) add amplified spontaneous emission noise characterized by a noise figure typically between 4 and 6 dB. Design guidelines for RF LNAs, including transistor selection and input matching strategies, are covered in application notes from NXP Semiconductors on practical LNA design.

Applications

Amplifier noise is a central design constraint in a wide range of systems, including:

• Radio telescope and deep-space communication receivers requiring cryogenically cooled LNAs
• Cellular base station and handset receiver front ends
• Medical magnetic resonance imaging coil preamplifiers
• Satellite transponder input stages
• Test and measurement instruments such as spectrum analyzers and network analyzers