A 50-Gbaud Low-Noise CMOS Stacked Differential Difference TIA for Coherent Optical Receiver

This paper presents a new type of differential difference (DD) transimpedance amplifier (TIA) for coherent optical receivers. On top of the intrinsic differential signaling of the coherent receiver, the proposed TIA further reuses the signal current and receives differential signals from each individual photodiode (PD), forming a DD-TIA and achieving an input-referred noise (IRN) of √2 times better than that of a conventional coherent TIA. Through voltage domain stacking of the two differential TIAs, distinct PD biasing of cathode/anode and differential signaling are achieved. Fabricated in 28-nm CMOS process, the proposed TIA achieves a transimpedance gain of 71 dBΩ, a 3-dB bandwidth of 26 GHz, and an IRN density of 6.9 pA/√Hz. High-quality PAM-4 eye diagrams in 50 GBaud are also demonstrated. To the authors' best knowledge, this is the first differential TIA that reuses the cathode photocurrent of PDs for coherent detection.