基于同轴型高纯锗探测器的CMOS电荷灵敏前置放大器设计

Design of CMOS Charge-sensitive Amplifier for Coaxial High Purity Germanium Detector

  • 摘要: 本文设计了一款多通道低噪声的电荷灵敏前置放大器,专用于输入电容较大的同轴型高纯锗探测器。大电容探测器会带来更大的输入噪声,为保证高能量分辨率,对前端电子学的噪声性能提出了较高的标准。在多级放大器中,总噪声受第一级噪声的影响最显著。因此,输入管必须具有优异的噪声性能,通过采用优化后的噪声模型、多次仿真迭代和特殊的版图结构得到低噪声输入管。当输入管尺寸较大时,会产生栅极漏电流,漏电流会改变前置放大器输出的基准位置。本文采用了一种带漏电流补偿的反馈电阻模块,该模块消除了反馈电阻对电源、温度和工艺变化的敏感性,同时可补偿几μA的泄漏电流,且电路为自偏置,无需外部偏压设定反馈电阻阻值。此前置放大器在10 pF的探测器电容下,上升时间不超过50 ns,且无任何震荡现象。在低温下前置放大器展示出仅5.6个电子的低噪声性能,具有5 mV/fC的输出转换增益和0.15%的线性度及12.5 mW的较低静态功耗。在特定的低能量辐射检测应用中,电路性能良好。

     

    Abstract: High purity germanium detectors play an increasingly significant role in particle physics and astrophysics, particularly in low-background radiation measurement experiments, due to their exceptional energy resolution, high detection efficiency. These detectors are especially critical in the search for rare events, such as neutrinoless double-beta decay (0vββ) and direct detection of dark matter, as they operate effectively under extremely low-background conditions. To fully leverage the advantages of HPGe detectors, a specifically tailored front-end readout system was required to minimize the contribution of electronic noise from the system itself. This noise minimization is critical to ensure that weak event signals from inside the detector are not obscured by the system’s inherent noise. In this paper, the design of a multi-channel, low-noise charge-sensitive amplifier (CSA) optimized for use with coaxial HPGe detectors was proposed, particularly those with large input capacitance. Large-capacitance detectors tend to introduce significant input noise, which degrades the overall energy resolution of the system. Therefore, higher standards are necessary for the noise performance of front-end electronics in such systems to preserve the excellent resolution that HPGe detectors can achieve. In multi-stage amplification systems, the noise performance is primarily influenced by the first amplification stage, where the noise characteristics of the input transistor play a crucial role. To address this, the input transistor was designed using an optimized noise model, iterative simulations, and a specially engineered layout structure to ensure low noise. However, larger transistor sizes can lead to gate leakage currents, which can alter the baseline of the amplifier output. To address this issue, a low-noise CSA circuit with a feedback resistor module for leakage current compensation was developed. This resistor feedback module mitigates sensitivity to power supply variations, temperature changes, and process deviations, and can compensate for leakage currents up to several micro amperes. Importantly, the circuit is self-biased, eliminating the need for external bias to adjust the feedback resistance value. The proposed amplifier demonstrated a rise time of less than 50 ns when used with a detector capacitance of 10 pF, and no oscillations were observed under these conditions. At low temperatures, the amplifier exhibits outstanding noise performance, with a noise level as low as 5.6 electrons. Additionally, it provides an output conversion gain of 5 mV/fC, a linearity deviation of only 0.15%, and a low static power consumption of 12.5 milliwatts. The performance achieved is sufficient for gamma-ray spectroscopy and pulse shape analysis using coaxial high purity germanium detectors.

     

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