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Description
Optical data transmission is essential for on-detector readout electronics in large particle physics experiments. Multi-level pulse amplitude modulation (PAM4) can increase the single-channel data rate without altering the analog bandwidth, thereby reducing the number of data transmission channels and the material budget. In 55-nm CMOS, a 20-Gbps transmitter has been designed for front-end detector data transmission, necessitating a custom optical receiver at the corresponding speed for the backend electronics. This work presents a 20-Gbps PAM4 pre-amplifier for such an optical receiver, fabricated in the same process. The initial prototype design mainly consists of a photodiode (PD) bias circuit, a transimpedance amplifier (TIA), a variable gain amplifier (VGA), a continuous-time linear equalizer (EQ), a DC offset cancellation (DCOC) circuit and a test output driver. The bias circuit uses active inductors to provide an appropriate reverse bias voltage for the PD, achieving high optical efficiency while also outputting differential current to the TIA. The TIA uses a pseudo-differential modified regulated cascode structure to break the gain-bandwidth product limitation. The VGA employs a tunable source degeneration structure to maintain output linearity. The EQ compensates for high-frequency loss while reducing low-frequency gain to balance noise and bandwidth. The output driver and DCOC circuit improve driving capability and eliminate DC offset. Post-simulation results show that the initial design achieves 20-Gbps PAM-4 eye diagrams with a transimpedance gain of 62 dBΩ and consumes 45 mW from a 1.2V supply. More detailed designs and test results will be reported.