Abstract: In order to solve the problems in weak optical signal detection with wide dynamic range and high background noise, a circuit for detecting weak light based on the principle of logarithmic integral amplification was designed and fabricated. And the Si photodiodes with two different properties (PN and PIN) were used as the photoelectric conversion element to verify the circuit characteristics. Measurement results show that for the incident light power characteristics, the two photodiodes- output are similar and have signal compression. For the incident light frequency characteristics, when f20 Hz, the output amplitude of PN type is 1.2 times more than PIN type, the PN type has higher sensitivity; when f40 Hz, the output amplitude of PN type varied about 90%, the PIN type output is more stable with about 50% maximum variation. Additional, the spectra measured by two photodiodes have the same trend and peak wavelength as the actual spectra.