A 4×56 -Gbaud PAM-4 Linear and Low-Noise TIA for Linear-Drive Pluggable Optics

This letter presents a $4\times 56$ -Gbaud linear transimpedance amplifier (TIA) with low noise and high linearity, designed for linear-drive pluggable optics (LPO) and implemented in 0.13- $\mu $ m SiGe-BiCMOS technology (fT/f ${_{\text {MAX}}} {=} 260$ /350 GHz). The TIA features an inductive shunt-feedback (ISFB) transimpedance stage (TIS) with a $\pi $ -topology L-C network as the input stage, achieving wide bandwidth (BW) and low noise despite large photodiode (PD) and packaging parasitic capacitances. Two current-splitting variable gain amplifiers (VGAs) with continuous-time linear equalizer (CTLE) functionality are cascaded after the TIS, providing a gain control range of –12 to +18 dB and accommodating input currents up to 2mApp. A 50-ohm output buffer with T-coil further extends the BW. Measurement results demonstrate a maximum optical-to-electrical transimpedance gain (O/E.ZT) of 73.8 dB $\Omega $ , an O/E O/E.BW exceeding 40 GHz, an input-referred noise (IRN) current of $1.9~\mu $ Arms, and total harmonic distortion (THD) ¡ 4.5% for a 700-mVpp output swing. The TIA supports 56-Gbaud PAM-4 eye diagrams with PRBS31Q and achieves 9.7-dBm optical modulation amplitude (OMA) sensitivity at the pre-FEC BER limit of $2.4\times 10^{-4}$ for 56-Gbaud PAM-4 SSPRQ. The design achieves a power efficiency of 1.67 pJ/bit and occupies an area of $3.14\times 1.04$ mm2.