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The noise of a fast charge sensitive amplifier (CSA) with an input MOS transistor operating in the moderate inversion region is discussed. The MOS transistor operation in the moderate inversion region becomes especially important in multichannel readout systems where limited power dissipation is required. The ENC of a CSA followed by a fast shaper is usually dominated by the voltage noise of the input MOS transistor. We carried out noise minimization for such a CSA, searching for an optimum input transistor width. The analyses were made using a simplified EKV model and were compared to HSPICE simulations using a BSIM3v3 model. We considered several CMOS technology generations with minimum transistor gate length ranging from 0.13 mum to 0.8 mum. We studied the sensitivity of ENC to the input transistor width, and propose a simple formula to estimate the optimum transistor width, which is valid in a wide current density range.