A 21-Gb/s Dual-Channel Voltage-Mode Transmitter With Stacked NRZ and PAM4 Drivers

This paper describes a voltage-mode transmitter that stacks independent non-return-tozero (NRZ) and four-level pulse amplitude modulation (PAM4) drivers for dual channels. The stacked structure can improve the transmitter's power efficiency owing to its current-recycling capabilities. The PAM4 voltage-mode driver with voltage regulators, implemented to increase data rates, generates multilevel signals. The proposed dynamic load control inside the PAM4 driver minimizes the voltage variation in the output of the regulators, which is caused by the variations of the data patterns. A prototype chip was fabricated using a 28-nm CMOS process. The stacked 7-Gb/s NRZ and 14-Gb/s PAM4 transmitters have achieved a full aggregate transmission rate of 21 Gb/s while consuming 12.6 mW from a 1-V power supply. When transmitting 500-mVpp signals with the proposed transmitter through 2-cm FR4 channels, the eye heights obtained in the NRZ and PAM4 channels were 312 and 92 mV, respectively.