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This article describes the design of a 32 Gb/s four-level pulse amplitude modulation (PAM- 4) optical transceiver in a 40 nm CMOS technology. At the transmitter side, the laser driver is composed of an asymmetric waveform equalizer, a 3-tap feed-forward equalizer (FFE), and a novel active-back termination (ABT) circuit. The ABT circuit provides a self-tracking, tunable source impedance to match the characteristic impedance of different laser diodes. At the receiver side, the fully integrated optical receiver consists of a transimpedance amplifier (TIA), a variable gain amplifier (VGA), an automatic threshold tracking circuit (ATC), and a quarter-rate decision feedback equalizer (DFE). By using the adaptive ATC, it reduces the BER induced by the harmonic distortion along the signal path by more than 27X under a THD of -20dB. Both the ATC and DFE are automatically adapted by an on-chip sign-sign LMS (SSLMS) engine. Fabricated in TSMC 40 nm CMOS process, the chip area for the transmitter and receiver are about 0.029 mm<sup>2</sup> and 0.23 mm<sup>2</sup>. The power consumptions are about 146.8 mW and 128.8 mW respectively for the PAM-4 transmitter and receiver.

A 32 Gb/s PAM-4 Optical Transceiver With Active Back Termination in 40 nm CMOS Technology