Design and Experimental Verification of a Transimpedance Amplifier for 64-Gb/s PAM-4 Optical Links

The use of four-level pulse-amplitude modulation (PAM-4) has emerged as a solution to increase the serial rate in short-range optical links, offering twice the data throughput but requiring similar bandwidth as on–off keying. However, the receiver design should take into account the increased susceptibility of PAM-4 to noise, intersymbol interference, and nonlinearity. This papers explores these challenges, and details the design of a transimpedance amplifier (TIA) for 64-Gb/s PAM-4 optical links. The TIA was implemented in 0.13-µm SiGe BiCMOS, and has a power consumption of 180  mW. It contains a digital gain controller, which allows switching between four gain modes, to tradeoff sensitivity against linearity. Bit error rate (BER) measurements show that the dynamic range is significantly extended: Optical modulation amplitudes between $-$ 7 dBm and at least $-$ 0.2 dBm yield a BER lower than $10^{-3}$ .