A Tunable Passband Logarithmic Photodetector for IoT Smart Dusts

This paper presents a tunable passband low-power logarithmic photodetector for IoT smart dusts using optical wireless communications. The photodetector can selectively receive optical signals and filter out unwanted optical noise. All the transistors work in the subthreshold region. Each transistor and circuit pole is designed based on theoretical analysis. The output of the photodetector is independent of the absolute illumination intensity and proportional to the modulation depth of the intensity. The photodetector requires a giga-ohm resistor, which is realized by a Gm circuit with an extreme low bias current. The Gm bias voltage can be used to tune the lower cut-off frequency of the passband. The upper cut-off frequency is determined by the photodetector bias and illumination intensity. The photodetector was fabricated using a standard $0.25~\mu \text{m}$ CMOS process and tested under different illumination intensities of 830 nm laser. Measurement results showed that the lower cut-off frequency increases as Gm bias and the minimum is approximately 100 Hz when Gm bias is 0 V. The photodetector output and Gm DC offset matches with theoretical analysis.