Low-noise, wide-band CMOS charge-sensitive preamplifier for use with APD/LSO PET detectors

The recent emergence of LSO as a potential scintillator for positron emission tomography (PET) and recent improvements in avalanche photodiode (APD) technology offer encouragement that an APD/LSO based PET detector may be commercially viable in the near future. An important component of any APD/LSO based PET detector will be the preamplifier used to read out the low-level detector signals. Due to the large number of detectors (>18,000) in a high-resolution PET scanner, the preamplifier must be implemented as a monolithic integrated circuit. Additionally, in order to achieve the timing resolution required for high resolution PET, the preamplifier must have a large band-width and a low equivalent input noise voltage. This paper presents a CMOS charge-sensitive preamplifier design which uses local feedback to improve the performance of the common gate transistor. The modified cascode circuit is analyzed and compared with a previously reported simple folded cascode circuit. A prototype circuit was fabricated in a 2 μm NWELL CMOS process. The prototype amplifier has a measured 10-90% rise-time of 7 ns with an external input capacitance of approx.6 pF and has an equivalent input noise voltage of approx.1.1 nV/rt-Hz above the flicker noise corner. A pulse height resolution of 14.3% FWHM and a timing resolution of 1.57 ns FWHM (vs. plastic) were obtained with the preamplifier, an Advanced Photonix 5 mm diameter beveled-edge APD and a 3.5×3.5×22 mm³ Teflon wrapped LSO crystal.