Frequency equivalent circuits for the on and off state of an optically triggered switch based on a PIN photodiode

In this work we have developed equivalent circuit models for the frequency response of an optically triggered switch based on a positive-intrinsic-negative photodiode (PIN-PD) for the On state (light applied to the PIN-PD) and the Off state (without applying light to the PIN-PD). From measurements it was found the frequency response in Off state behaves as a high-pass filter whereas in the On state as a low-pass filter; this means that depending on the input signal frequency there could be an approach or even an overlap between the output amplitude in the On and in the Off state. The developed models allow the calculation of the highest permissible frequency in the input signal to avoid distortion due to overlap of the On and Off state response, and the models take into account circuit parameters such as bias voltage, load resistance, resistance, and capacitance of PIN-PD, which in turn depend on the operating point and the desired linear range for the input’s amplitude. Experimental measurements of the frequency response were made in both conditions (On and Off state) using three commercial photodiodes of bandwidth higher than 1 GHz. The measured frequency response as well as amplitude separation between the On and Off states to avoid the overlap is very well matched by the calculated results. Although the tests were performed for photodiodes with 1 GHz bandwidth, the technique can be applied for photodiodes with higher bandwidth.