An Ultra Low Noise Preamplifier for Room Temperature X-Ray Detectors
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AN ULTRA LOW NOISE PREAMPLIFIER FOR ROOM TEMPERATURE X-RAY DETECTORS
G. Bertuccio'), P. RehakV, D.M. Xi?2
1)Politecnico di Milano, Dipartimento di Elettronzca e Informazione, P.za Leonardo da Vinci 32, 20133 Milano, Italy 2J Brookhaven National Laboratory,Upton, NY 11973, USA
ABSTRACT A new circuital configuration for the charge amplifier is presented. By means of a double feedback loop, the input field-effect transistor can operate with its gate junction sligtly forward biased, collecting the detector current and discharging the feedback capacitor. The feedback resistor is so avoided and no resetting device or circuit is required for the preamplifier operation. The noise is limited by the input transistor, an equivalent noise charge of 19.5 r.m.s. electrons has been measured at room temperature by employing a commercial JFET. 1. INTRODUCTION X-ray detectors have always acquired a great interest in many field of fundamental and applied research [1]. The state of the art in X-ray spectroscopy is reached employing silicon or germanium detectors operating at liquid-nitrogen temperature and charge amplifiers with cooled MET, an electronic resolution of 100 eV FWHM (12 r.m.s. electrons) is a typical value [2]. In the last decade several detectors of new conception have been proposed and realized in order to reach high resolution at higher temperature. Silicon Drift Detectors [3] Charge Coupled Devices [5,6] and HgI2 detectors [1] has been proven to be excellent room temperature X-ray detectors. The main limitation to the performances at room temperature operation is imposed by the amplification of the detector signal. For CCD, source follower amplifier stages are used with excellent results [5,8]. Other detectors employs generally charge amplifiers, which offers an higher gain stability. A great effort has been done in order to lower the noise of these amplifiers as much as possible. One of the main noise source in the classic charge-amplifier configuration is the resistor Rf placed in the feedback loop for the DC stabilization of the amplifier. Several solutions, which avoid Rfi as optoelectronic feedback and transistor reset feedback are presently employed in several systems [2]. These amplifiers require a switching circuit to work, so that spurious transients are present during their operation, this causes dead-time and cross-talk noise in multi-channel systems. In this paper a charge preamplifier operating at room temperature is presented. No switching circuit is required for its operation, so that it is a good candidate also for multichannel detectors. In paragraph 2 the principle of operation and the preamplifier circuit is described. In paragraph 3 an analysis of the noise sources is presented with the expected performances with different input transistors. In paragraph 4 some experimental results are shown.
Mat. Res. Soc. Symp. Proc. Vol. 302. ©1993 Materials Research Society
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Figure 1 Charge amplifier without the feedback resistor. A is a transresi
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