Large bandwidth tunable analog equalizer based on a differential cascode amplifier cell for 100-GBaud communication systems

We report a continuous-time analog equalizer based on a differential cascode amplifier cell. Realized with indium phosphide double heterojunction bipolar transistors reaching a f(T)/f(max) couple of 370/340 GHz, this circuit exhibits a maximal peaking of 10.5 dB at 85 GHz. The tunability is made thanks to a varactor, which can tune the peaking frequency from 30 to 85 GHz. This analog equalizer also provide a bandwidth of more than 110 GHz whatever the varactor settings, which makes it suitable for very high speed applications.