New Perspectives in Low Noise Preamplifier Design for Room Temperature Detector Applications
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NEW PERSPECTIVES IN LOW NOISE PREAMPLIFIER DESIGN FOR ROOM TEMPERATURE DETECTOR APPLICATIONS P.F. MANFREDI Dipartimento di Elettronica, Via Abbiategrasso 209, 27100 Pavia, Italy, and INFN - Sezione di Milano, Via Celoria 16, 20133 Milano, Italy ABSTRACT After reviewing the noise limits in room temperature preamplifiers based on discrete elements, the paper discusses some results obtained with monolithic circuits and the perspectives opened-up by active devices directly integrated on the chip of a silicon detector. I - INTRODUCTION
In applications with room temperature detectors, the idea of the cooling front-end system is usually discarded, for it would cancel the advantage of absence of cryogenics. However, the noise requirements that arise in some applications are such that it is really hard to meet them with a preamplifier operating at room temperature. An example is provided by X-ray spectrometry with HgI2 detectors, where extremely low noise-induced dispersions, down to a very few tens of electrons, have to be attained [1]. In a preamplifier with a cooled input field-effect transistor, the equivalent noise charge (ENC) contribution brough about by the channel thermal noise can be reduced below 1.-C__D electrons rms, CD being the detector capacitance, by using processing times in the millisecond region[2]. With the input field-effect transistor operating at room temperature such a possibility is ruled out by the presence of the shot noise associated with the gate leakage current, which may become the dominant ENC contributions at such long processing times. Besides the ultra-low-noise spectrometry, several other applications of room temperature detectors have kept the activities related to front-end innovation well alive over the past few years. Position sensing and imaging with highy segmented detectors, in particular tracking applications of microstrip detectors in particle physics, have stimulated the development of front-end systems in monolithic form. These systems, consisting of several signal processing channels integrated on the same chip with the input of each channel wire-bonded to the relevant detector electrode, have introduced a remarkable innovation in the domain of preamplifiers for detector applications [3]. The innovation went farther on when detectors with very small electrode capacitances, like the solid-state drift chambers and the pixel detectors, were introduced [4, 5]. For these detectors a front-end system residing on a separate chip is often inadequate, for the capacitance added by the bonding wire may spoil the intrinsically high signal-to-noise ratio. It was, accordingly, necessary to think of preamplifiers or, in the first instance, of front-end active devices integrated on the detector chip. Such a step involves remarkable technological difficulties, related to the fact that ordinary electron devices are realised on substrate materials that have much lower resistivity than the detector-grade silicon. However,successful examples of integration of front-end devices on the detector ch
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