Equivalent Circuit Model for Large-Area Photodiodes for VLC Systems

An equivalent circuit model for large-area PIN photodiodes used in visible light communication (VLC) applications is described. The modelling aims to gain insight into the effect of the photodiode intrinsic elements on the bandwidth of VLC receivers. The model parameters are extracted based on impedance measurements of two commercially available large-area PIN photodiodes. The extracted model parameters identify the photodiode series resistance as a major contributor to the bandwidth limitation of VLC receivers, especially when employing low input impedance transimpedance amplifiers (TIAs). However, such resistance is commonly ignored by most circuit designers, since it is assumed to have a negligible effect on the TIA performance. To demonstrate the accuracy of the photodiode equivalent circuit model, a design example of a low input impedance regulated cascode (RGC) TIA is described. The TIA is constructed using discrete components and a printed circuit board (PCB). The designed RGC TIA is measured using the proposed photodiode equivalent model versus a simplified model. In addition, a VLC link is constructed to measure the optoelectrical response of the large-area photodiode with the RGC TIA. The RGC frequency response measurements obtained from the full and simplified photodiode equivalent model are contrasted to the optoelectrical response, which verified that, unlike the simplified photodiode model, the proposed photodiode equivalent model can accurately predict the bandwidth performance of the VLC receiver.