In this thesis, a readout circuit with a simple preamplifier for an uncooled pyroelectric infrared detector is discussed. The preamplifier is composed of a CMOS inverter and is biased by a high-value feedback resistor implemented by an n-type MOSFET operating in sub-threshold region. The feedback resistance can be adjusted by gate voltage and body voltage of the n-type MOSFET. Because the output voltage of preamplifier is proportional to the feedback resistance, we can obtain high-value output voltage, as the feedback resistance is high. The HSPICE electrical model shows how temperature change of pyroelectric material is transformed into voltage signal. Using impurity diffusion p-well CMOS process, the preamplifier is fabricated and the operation of the preamplifier is evaluated by measurement.