Lawrence Berkeley National Laboratory

A high-speed, low-noise 16-channel amplifier IC has been fabricated in the HP 0.5 mm CMOS process. It is a prototype for use with a PET detector which uses a 4x4 avalanche photodiode (APD) array having 3 pF of capacitance and 75 nA of leakage current per pixel. The preamplifier must have a fast rise time (a few ns) in order to generate an accurate timing signal, low noise in order to accurately measure the energy of the incident gamma radiation, and high density in order to read out 2-D arrays of small (2 mm) pixels. A single channel consists of a charge-sensitive preamplifier followed by a pad-driving buffer. The preamplifier is reset by an NMOS transistor in the triode region which is controlled by an externally supplied current. The IC has 16 different gain settings which range from 2.085 mV/fC to 10.695 mV/fC. The gain is determined by four switched capacitors in the feedback loop. The switch state is set by two digital input lines which control a 64-bit shift register on the IC. A preamplifier 10-90 percent rise time as low as 2.7 ns with no external input load and 3.6 ns with a load of 5.8 pF was achieved. For the maximum gain setting and 5.8 pF of input load, the amplifier had 400 electrons of RMS noise at a peaking time of 0.7 us. The IC is powered by a +3.3 V supply drawing 60 mA.