6 Vpp - 66 GHz-ultrabroadband amplifier for fiber-optical transmission systems

State of the art two-stage drivers for optical modulators (Mach-Zehhder configuration) perform a voltage swing exceeding 3 V with a gain of 18 dB. Most of them have a bandwidth better than 40GHz, which is required to achieve a satisfactory performance of a 40Gbit/s fibre optical system. We have succeeded in extending the bandwidth to 45 GHz making use of a resonant peaking technique already applied to non-distributed broadband amplifiers. So far, our distributed amplifiers contained a small number of cells. The possibility of high voltage swing - or high saturation output power - combined with a large bandwidth is limited by the design of the distributed amplifiers. Due to the large impedance of the transmission lines or the inductance of the leads connecting the various cell ports of the circuit, the input and output networks act as lowpass filters. The bandwidth of these filters is influenced by the parasitic capacitances of the high electron mobility transistors (HEMTs). According to the theory of distributed amplification, circuits with many cells and small transistors have a small parasitic capacitance per cell. We prefer this for large bandwidth designs. Unfortunately, losses due to parasitic resistance of the gate fingers and to the transmission lines themselves limit the number of cells that can be driven.