Development And Application Of Ultra-Low Noise And High Impedance Preamplifier For Capacitive Probe Electronics Acquisition

Beam diagnostics system,as the"eye"of the accelerator,is one of the most important systems.Measuring beam parameters,monitoring beam transmission efficiency and determining accelerator performance and parameters by beam diagnostic techniques is a necessary method to realize the stable operation of the machine and the matching in the different accelerator components.Beam Position Monitor（BPM）system plays a significant role in accelerator beam diagnostics,and is also the largest quantity in beam instrumentation.BPM system is usually composed of BPM,front-end electronics and subsequent BPM signal processing system.The front-end electronics is mainly used for amplifying the weak signals.In this paper,low noise and high impedance preamplifiers are developed for extremely weak signal amplification inducted by capacitive BPM during weak beam commissioning operation of synchrotron.The amplifiers will be installed in the in-operation Heavy Ion Research Facility in Lanzhou（HIRFL）and in the under-construction High Intensity Heavy-ion Accelerator Facility（HIAF）.Based on previous research at home and abroad,and according to the beam parameters of HIRFL and HIAF,the design requirements and performance of amplifiers are analyzed.Therefore,two kinds of amplifying circuits are proposed.The first type of circuit is based on JFET.The circuit structure includes cascode,source follower,constant current source,resistance-capacitance feedback and the output amplifier stage which is inserted between the output of input stage amplifier and feedback resistance.After theoretical analysis,the circuit is fabricated and tested.The test results show that the input impedance is about 3.1 MΩ,the gain is 36.2 d B,and the gain flatness is±0.3 d B.The input voltage noise density is 0.63 n V/Hz@1MHz,0.76 n V/Hz@10 MHz,the 1/f noise inflection-point is approximately 160.85k Hz,the output power 1 d B compression point(OP_{1d B})is about 26.2 d Bm,and the THD is-55.34 d B at 100 k Hz.The second type of circuit is based on high speed and low noise operational amplifier.The circuit structure mainly includes two non-inverting amplifier,resistance-capacitance coupling circuit and input compensation network.The input voltage noise density of the single-input stage circuit does not reach the ideal value.Therefore,according to the theorem that within a certain range,the voltage noise can be reduced by the trend of 1/Ntimes when multiple sub-circuits in parallel.After testing and exploration,it is found that when 3 input-sub-circuits are in parallel,The input voltage noise density reaches the ideal value of 0.68 n V/Hz@500 k Hz and0.665 n V/Hz@10 MHz,and this optimization does not significantly increase the circuit complexity and has little effect on other parameters.The measurement results of other parameters are as follows:The input impedance is 1 MΩ,the gain is 40.166d B,the bandwidth is 64.7 Hz to 90.8 MHz,and the OP_{1d B}is 19.32 d Bm,the phase margin is about 76.03°.The low noise and high impedance broadband preamplifiers of the two circuits both achieve the desired design objectives.By comparing the two circuits,the circuit structure of the amplifier based on JFET is complex,and the static operating point debugging is difficult.If the beam frequency bandwidth increases in the later stage,the bandwidth expansion is not very easy to implement.The simple circuit structure of amplifier based on operational amplifier lays a foundation for meeting the needs of higher bandwidth or different gain at the same time during beam acceleration and injection.