Development of Low-noise Preamplifier for Low-field NMR

doi:10.11938/cjmr20253162

Abstract

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

CLC Number:

O482.53

LI Keyan, CHENG Xin, CHEN Junfei, CAO Li, HUANG Zhen, LIU Chaoyang. Development of Low-noise Preamplifier for Low-field NMR[J]. Chinese Journal of Magnetic Resonance, 2025, 42(3): 321-333.

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结构	噪声系数	线性度	电路尺寸	阻抗匹配	晶体管数量
平衡式	小	好	较大	很好	中
负反馈式	中	比较好	较小	好	少
有源匹配式	小	比较好	中等	好	中
前馈结构	高	好	大	好	多
有损匹配结构	高	一般	中等	差	少
行波电路式	中	比较好	中等	中	多

	10 MHz	12 MHz	14 MHz	16 MHz	18 MHz	20 MHz	22 MHz
噪声系数/dB	0.664	0.628	0.632	0.628	0.616	0.636	0.656
噪声电压密度/(nV/$\sqrt{\text{Hz}})$	0.432	0.432	0.434	0.435	0.433	0.434	0.431

放大器	类型	典型增益	典型噪声
ATF-33143	射频窄带放大器^[8]	25 dB@128 MHz	0.42 dB@128 MHz
ATF-54143	射频窄带放大器^[17]	40 dB@20 MHz	0.6 dB@20 MHz
MAR-6SM+	射频宽带放大器^[19]	20.11 dB@62 MHz	1.7 dB@62 MHz
PHA-13HLN+	射频宽带放大器	24.3 dB@20 MHz	1.2 dB@20 MHz
本文LNA	射频宽带放大器	31.3 dB@20 MHz	0.63 dB@20 MHz