An 80-Gbaud PAM-4 $g_{\mathrm{m}}$ -Boosted Variable-Gain TIA in 22-Nm FDSOI

A variable-gain transimpedance amplifier (VG-TIA) with over $80-\text{dB}\Omega$ differential $Z_{21}$ and over 45 dB of gain control, 36-GHz bandwidth, operating at record symbol and data rates of up to 80 GBaud PAM-4 and 160 Gb/s, respectively, is presented. The differential multi-stage TIA features a 6-mA, 53- $\text{dB}\Omega\ g_{\mathrm{m}}$ -boosted cascode input stage with 70-GHz bandwidth which minimizes the differential input equivalent noise and input impedance, making its bandwidth less sensitive to the PIN photodiode capacitance, while simultaneously providing $59-\Omega$ input impedance per side for ease of on-die testing without a photodiode. A double-cascode Gilbert-Cell VGA and two fixed-gain Cherry-Hooper stages provide the extra 25 dB of gain and help maintain the THD below 3 % for differential input currents up to 4.8 $\text{mA}_{\text{pp}}$ . The equivalent input noise current remains below 21.4 $\text{pA}/\sqrt{}\text{Hz}$ up to 50 GHz.