Abstract
The purpose of the work is to design an integrated amplifier system to use with silicon photodiodes for soft X-ray counting. The amount of charge generated in silicon diodes from soft X-rays is very low and therefore, a low noise design is a major concern in this study. Moreover, for portable and miniature dosimeter applications, the circuit is to be powered by a single supply voltage and requires low power consumption.The amplifier circuit consists of a charge sensitive amplifier (CSA) and a pulse shaping circuit. Both stages are based on a folded cascode operational trans-conductance amplifier (OTA) structure for noise reduction and a better frequency response. In the CSA section, the channel length of the frond end transistor M0 is optimized for noise based on a nominal detector capacitance 30pF. The feedback capacitor 0.3pF is used to reduce detector capacitance variations by the Miller effect. That the ENC of the front end section is less than 600 electrons. The pulse shaping circuit is a CR-RC filter. The operating frequency is set about 0.5 MHz and the CR and RC time constants are set about 1μs for high counting applications. To facilitate batch production, all passive components are integrated in the same chip using the 0.35 SPQM process. The overall chip size is about 480 μm x 420 μm and is dominated by the coupling capacitor C1.The width and length parameters of each MOS component used in the amplifier were iterated to optimize amplification factor and power consumption. The overall gain of the amplifier stage is about 255 mV/fC in the interested frequency range around 0.5 MHz. The IC manufacturing process has a threshold voltage about 0.6 volt for PMOS and NMOS components. The DC level of the output signal is set about 0.6 volt. Using a power supply of 3 volts, the maximum signal swing is about 0.6 volt and the total power consumption is less than 520μW. The design has passed the “four corner” analyses; sensitivity analyses were also performed for operating temperature variations from 0 to 70 ℃ and a 10% supply voltage fluctuation. An IC chip has been manufactured and the preliminary test showed a satisfactory result.