Noise Sources in RF Low Noise Amplifiers

The electronic components that make up an RF low-noise amplifier, such as transistors and resistors, generate thermal noise during normal operation due to the random thermal motion of internal charge carriers. This noise is an inherent physical phenomenon of the component and is not directly controlled by the external circuit design. It is solely dependent on the component's material, temperature, and operating bandwidth. Higher temperatures increase the intensity of carrier motion, which in turn increases the intensity of thermal noise. This in turn interferes with weak signals received by the amplifier, affecting signal purity.

Transistors, the core of amplifiers, generate two typical types of noise. Shot noise originates from the random fluctuations of carriers as they travel through the PN junction within the transistor. This irregular movement of carriers causes tiny fluctuations in the current, creating a noise signal. Flicker noise, on the other hand, is related to the transistor's manufacturing process and material properties. It is more pronounced at low frequencies, increasing in intensity as frequency decreases, significantly impacting the RF low-noise amplifier's performance in processing weak signals at low frequencies.

Parasitic parameters in RF low-noise amplifier circuit design, such as parasitic capacitance and inductance, can contribute to noise. These parasitic parameters are not inherent to the design but rather arise from factors such as PCB layout, component packaging, and wire connections. They interact with active components in the circuit, altering the signal's transmission path and phase characteristics, resulting in additional signal fluctuations and interference. The noise impact of parasitic parameters is particularly prominent in high-frequency operating bands.

In actual operation, RF low-noise amplifiers are subject to various interference signals from the external environment. Surrounding electromagnetic devices, such as communication base stations and industrial motors, radiate electromagnetic energy, which can enter the amplifier through spatial coupling or power supply lines, generating interference noise. Furthermore, slight fluctuations in the power supply voltage and unstable ground loops can also introduce external noise into the amplifier, disrupting the amplification of weak signals and reducing the output signal quality.