关键词: 低场核磁共振, 信噪比, 低噪声前置放大器, 负反馈, 噪声系数

Abstract:

Nuclear magnetic resonance (NMR) technology enables the acquisition of multi-scale molecular dynamics by characterizing the variation of relaxation times under different magnetic field conditions, thereby offering a critical detection methodology for petroleum logging research. To minimize magnetic field gradients induced by susceptibility differences between solid and liquid phases, low-field NMR systems are predominantly employed in petroleum logging applications. However, the inherent limitations of reduced sensitivity in low-field environments present significant challenges for applied studies, necessitating stringent noise performance specifications for receiving circuitry. As the primary stage of the signal reception chain, the preamplifier’s noise characteristics fundamentally determine the signal-to-noise ratio (SNR) of NMR measurements. To address the requirements of low-field NMR, this paper adopts a cascade topology combining a common-emitter and common-collector two-stage amplification circuit, and integrates negative feedback to optimize input broadband matching and noise figure. Consequently, a high-gain, low-noise broadband preamplifier has been developed. The implemented preamplifier demonstrates excellent performance parameters: NF≤0.73 dB, gain≥31 dB, gain flatness≤0.35 dB, and equivalent input voltage noise density≤0.45 nV/$\sqrt{\text{Hz}}$ across the 10~30 MHz frequency band. Integrated into an NMR spectrometer system, the amplifier demonstrated favorable ¹H NMR signal SNR under 0.5 T, 0.35 T, and 0.25 T magnetic field conditions. These results indicate that the developed low-noise amplifier provides critical technical support for advancing low-field NMR applications.

Key words: low-field NMR, signal-to-noise ratio, low-noise preamplifier, negative feedback, noise figure