A dual-mode NRZ/PAM-4 differential low-swing voltage-mode transmitter employs a quarter-rate output multiplexing architecture for low-power operation. In NRZ mode, 2-tap feedforward equalization is realized with analog replica-bias tap control that is configurable in high-performance controlled-impedance or energy-efficient impedance-modulated settings. This analog control also allows for efficient generation of the middle levels in PAM-4 operation. Fabricated in GP 65nm CMOS, the transmitter supports an output swing range of 100–400mVPpd with up to 12dB of equalization in NRZ mode and achieves energy efficiencies of 1.48 and 0.91pJ/b at 20-Gb/s NRZ and 28-Gb/s PAM-4 data rates. Operation in the NRZ energy-efficient impedance-modulation setting allows for power savings of up to 32% relative to the controlled-impedance setting.