A 50-GBaud Low-Noise CMOS Stacked Differential-Difference TIA with PD Cathode-Current Reuse

This brief presents a stacked differential-difference (SDD) transimpedance amplifier (TIA) for coherent optical receivers. The proposed TIA reuses the cathode current of the photodiodes (PDs) and senses signals from both the anode and cathode nodes. Compared with conventional balanced detection, the proposed sensing scheme ideally improves the front-end sensing signal-to-noise ratio (SNR) by √2. Two differential transimpedance stages are stacked in the voltage domain to reuse current while maintaining about 2 V reverse bias across the PDs. The prototype is fabricated in a 28-nm CMOS process. The measured transimpedance gain is 71 dBΩ with a 25-GHz bandwidth and the input-referred noise density is 7 pA/√Hz. The proposed TIA also demonstrates 50-GBaud PAM4 operation.