A　56Gb/s　PAM-4　low-noise　linear　optical　receiver　is　presented.　To　reduce　the　input　referred　noise　(IRN)　and　improve　the　sensitivity,　the　optical　receiver　front-end　comprises　of　a　high-gain　narrow-bandwidth　transimpedance　amplifier　(TIA)　followed　by　a　two-stage　equalizer.　The　gain　tunable　TIA　and　the　variable　gain　amplifier　(VGA)　with　automatic　gain　control　(AGC)　loop　ensure　the　linearity　across　the　whole　input　range.　Designed　and　implemented　in　40nm　CMOS　technology,　the　receiver　accomplishes　1.7μArms　IRN,　78dB　transimpedance　gain　and　23GHz　bandwidth　while　consuming　108mW　from　1.6V　supply.　The　dynamic　range　of　transimpedance　is　40dB,　enabling　large　input　overload　current　of　2.5mApp　and　total　harmonica　distortion　(THD)　is　below　3%　with　800mVpp　swing　at　the　output.　©　2018　IEEE.