A Linear 65-GHz Bandwidth and 71-dBΩ Gain TIA With 7.2 pA/√Hz in 130-nm SiGe BiCMOS

This letter presents a differential transimpedance (TI) amplifier with a maximum gain of 71 dB Ω and a bandwidth (BW) of 65 GHz, including the effect of a photodetector with 65-fF capacitance. The implemented circuit in a 130-nm SiGe BiCMOS process with f
_T
= 300 GHz has an averaged input-referred noise current of 7.2 pA /√{Hz}, while the total harmonic distortion is better than 1.5% for peak-to-peak differential input currents lower than 0.8 mA at a differential output voltage of 0.8 V. Time-domain measurements show an open eye at 112 and 100 Gb/s for a non-return-to-zero and pulse-amplitude-modulation 4-level signals, respectively. The presented design, to the best of our knowledge, shows the lowest noise and one of the highest gain-bandwidth (GBW) products compared to the state-of-the-art. The improvement of the TI limit is achieved by optimizing the open-loop GBW product of the core amplifier in a shunt-feedback structure. This includes tradeoffs associated with the selection of a high value for the feedback resistor to keep the input-referred noise low. The BW is further increased by using a series inductor at the input and a T-coil at the output.