In this paper, we demonstrate the result of combining a low noise amplifier (LNA) and a dielectric microcalorimeter (DMC) at 2.3 GHz. The DMC and the LNA system detected infrared rays that are used as a fundamental experiment of X-ray detection. DMCs require LNAs for detected weak signals of X-ray and infrared ray. The monolithic microwave integrated circuit (MMIC) LNA was designed for a DMC of a new type X-ray detector. The LNA was fabricated using a 0.15 µm gate length GaAs-based pseudomorphic high electron mobility transistor (pHEMT) process. The LNA module has a gain of 12.5 dB around 300 K and 21.6 dB at 77 K at 2.3 GHz.