A design methodology to achieve low input impedance and non-constant gain-bandwidth product in TIAs for optical communication

This paper demonstrates a method to achieve low input impedance and a non-constant gain-bandwidth product using SiGe NPN and PNP devices in order to attain a large bandwidth and gain in transimpedance amplifiers (TIA) for optical communication. The major obstacle to achieving wide bandwidth and high gain in a TIA design is the large parasitic capacitance associated with the input photo-diode. This challenge can be overcome by using a common-base input stage to decrease the input impedance and employing a technique to introduce a non-constant gain-bandwidth product into the system. The nonconstant gain-bandwidth product can be realized by utilizing global feedback (as opposed to local) with a common-base input stage. The circuit was designed and simulated using a complementary SiGe HBT on SOI process technology.