核磁共振（NMR）技术通过测量弛豫时间随磁场的变化特性，可获取多尺度下的分子动力学特征，从而为石油测井等领域的研究提供了关键的检测手段．为减小固液两相磁化率差异引起的磁场梯度效应，石油测井领域的应用研究多在低场下开展，但较低的灵敏度为应用研究带来了严峻的挑战，且对接收链路的噪声性能提出了高要求．作为接收链路的第一级，前置放大器的噪声特性直接影响NMR信号的信噪比．针对低场NMR的需求，本文采用共射-共集两级放大电路级联拓扑结构，并结合负反馈优化输入宽带匹配和噪声系数，由此研制了一种高增益、低噪声的宽带前置放大器．在10~30 MHz频段内，研制的前置放大器的噪声系数（NF）≤0.73 dB，增益（G）≥31 dB，不平坦度≤0.35 dB，等效输入电压噪声密度≤0.45 nV/$\sqrt{\text{Hz}}$．将其集成于NMR谱仪系统，在0.5 T、0.35 T、0.25 T磁场下均获得了较好的¹H NMR谱信噪比．结果表明，研制的低噪声放大器可为低场应用研究提供关键技术支持．

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.