A 4 × 56-Gbaud PAM-4 Optical Receiver Integrated With SiGe-BiCMOS TIA

A chip-on-board (COB) packaged four-channel optical receiver integrated with a transimpedance amplifier (TIA) in 0.13-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>m SiGe-BiCMOS technology with fT/fMAX =260/350 GHz demonstrating 56-Gbaud four-level pulse-amplitude modulation (PAM-4) with low noise and high linearity is presented in support of optical receivers required in linear-drive pluggable optics (LPO) and beyond-400G optical links. The proposed four-stage optical receiver comprises an inductive shunt-feedback TIA followed by two variable gain amplifiers (VGAs) with a continuous-time linear equalizer (CTLE) function and a 50-ohm output buffer. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula>-topology L-C network and inductive shunt feedback techniques are used in TIA to meet the specifications of wide bandwidth (BW) and low noise. Two current-splitting VGA stages with CTLE function cascaded after TIA featuring high reliability can compensate for the loss of BW which caused by photo diode (PD) parasitic capacitance and provide a gain control range of −12dB ~ +18dB that accommodates input current up to 2mApp with low group delay variation. The 50-ohm output buffer with a T-coil and degeneration resistor achieves better output matching and high linearity. Measurement results show that TIA provides a maximum DC transimpedance gain (ZT) of 74 dB<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula> while O/E.BW is beyond 40 GHz, the 50-ohm output buffer offers 580-mV output swing and PAM-4 electrical eye diagrams up to 56-Gbaud of optical receiver with PRBS31Q. The optical receiver achieves a sensitivity of −9.7-dBm optical modulation amplitude (OMA) at pre-FEC BER limit of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.4\times 10^{-4}$ </tex-math></inline-formula> operating 56-Gbaud PAM-4 SSPRQ and consumes 720mW with a power supply of 3.3 V.