用于重离子加速器束流位置测量的JFET低噪声高阻宽带前置放大器

A JFET Low Noise and High Impedance Wideband Preamplifier for Beam Position Measurement of Heavy Ion Accelerator

为解决重离子加速器束流位置探测中横跨低中高频段的微弱信号难以数字化的难题,设计了一款低噪声高阻宽带前置放大器。放大电路采用多级放大结构,输入级由两级低噪声N沟道结型场效应晶体管(NJFET)级联,并采用共源共栅与源极跟随低噪声结构,利用恒流源阻抗特性增大开环增益;输出级采用宽带电流反馈型运算放大器提供足够的输出功率和带宽,负反馈网络与两级放大电路并联提高灵敏度、减小失真。实测带宽34 kHz~38.47 MHz,带内增益36.2 dB,平坦度小于±0.3 dB,输入电压噪声密度为0.76 nV/(Hz)1/2@10 MHz,1/f噪声拐点160.85 kHz,增益压缩1 dB的输出功率(P^(1dB))为26.2 dBm,总谐波失真为-55.34 dB@100 kHz,满足了低噪声、宽带前置放大器的设计需求。

To solve the measurement of the weak coupled signals from beam position monitors in a heavy ion accelerator with low intensity beam,a broad band low noise preamplifier with a high input impedance is designed and developed,which has a broad band response over almost 2 decades of frequency.This design is based on a cascade multistage amplification.The input stage,consisting of 2 folded N-channel junction field effect transistors(N-JFET),adopts a cascade and source following structure to minimize the input voltage noise.A high voltage open-loop gain is obtained by using the characteristic impedance of a constant current source.The output stage is a current mode feedback op-amp which provides a broad band frequency response and enough output power.The whole circuit has a high sensitivity,low noise and distortion.From the measurement,the achieved gain is 36.2 dB with a flatness of less than±0.3 dB from 34 kHz to 38.47 MHz.The input voltage noise density is 0.76 nV/√Hz@10 MHz and the 1/f corner frequency is 160.85 kHz.The output power is 26.2 dBm at 1 dB compression point and the total harmonic distortion is-55.34 dB@100 kHz.The obtained results meets the design requirements of low noise and broadband preamplifiers.