A bootstrap transimpedance amplifier for high speed optical transcutaneous wireless links

We report on a prototype of an experimental bootstrap transimpedance preamplifier (BTA) for high speed transcutaneous data transfer. Due to the strong scattering effect of the human skin and the corresponding widely spread optical power distribution, a large size photodiode is required at the external receiver in order to increase the transmission efficiency as well as the misalignment tolerance in the transcutaneous optical telemetric link (TOTL). However, the increased junction capacitance associated with a larger size photodiodes makes the design of high speed transimpedance amplifiers (TIA) challenging. To mitigate this tradeoff, in this paper, we use bootstrapping technique to achieve a large TIA bandwidth even in the presence of large-size photodiodes. Thereby, an emitter follower is employed as the bootstrapping buffer to minimize the junction capacitance seen by the transimpedance stage. The proposed circuit is tested on circuit board level in combination with a 10 mm² photodiode with a parasitic capacitance as large as 350 pF and achieves a 61.5 dBΩ transimpedance gain over a bandwidth of 61 MHz with an input referred current noise density of 158 pA/√Hz. Therewith, a transcutaneous optical link with 75 MBps is achieved.