Design of an auto-gain control transimpedance amplifier for optical sensing applications

An auto-gain control transimpedence amplifier (TIA) system has been developed in this work for optical sensing applications. In the design, a digitally controlled variable resistor is implemented in the TIA as the feedback resistance (gain) which is controlled by a micro-controller (μC) based board. The μC is programmed to monitor the TIA output and automatically adjust the gain in the TIA to keep the TIA output stable. This avoids signal saturation due to a large input photocurrent or a high ambient environmental condition and stabilize the TIA performance. In addition, the μC allows for a serial interface to a PC that enable the real time process analysis using a graphic user interface (GUI) developed in LabVIEW. With the GUI, the real time TIA gain and the voltage signal at the TIA output can be recorded which makes it easier for the calculation of the incident light's intensity and enhances the system's usefulness in the applications where the intensity measurement is required. Experimental results show that for the gain range up to 255 kΩ, the system developed is capable of automatically adjusting the TIA gain for the changes of incident light to stabilize the TIA performance. In addition, by recording the gain and output signal of the TIA using the PC, the real time photocurrent can be accurately calculated.