A 256-Gbps PAM-4 Signal Generator IC in $0.25-\mu \text{m}$ InP DHBT Technology

This paper presents a 256-Gbps (128-GBaud) four-level pulse amplitude modulation (PAM-4) signal generator (SG) IC fabricated using developed 0.25- μm -emitter-width InP double heterojunction bipolar transistors (DHBTs), which have a peak f
_T
and f
_max
of 460 and 480 GHz, respectively. The IC is based on a 128-GS/s 2-bit R2R-ladder current-steering digital-to-analog converter (DAC) with integrated two 2:1 multiplexer (MUX) functions. Ultrahigh-speed clear PAM-4 signals of up to 256 Gbps (128 GBaud) were successfully generated using this PAM-4 SG IC. To the best of our knowledge, this is the first demonstration of 256-Gbps (128-GBaud) PAM-4 signal generation without any equalization.