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

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/ \sqrt\mathrmH\mathrmz @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.