H-Band Broadband Balanced Power Amplifier in 250-nm InP HBT Technology Using Impedance-Transforming Balun

A broadband power amplifier in a 250-nm InP HBT technology is presented, working at full $H$ -band (220–320 GHz). A cascode transistor is employed as a power cell to achieve high gain and power at $H$ -band. Two-stage cascode power amplifier is designed with input and output impedance of not 50 but 25 $\Omega $ , reducing the impedance transformation ratio and allowing the broadband matching networks. Two power-amplifier channels are power-combined using on-chip baluns which perform the impedance transformation from 25 to $50~\Omega $ . The balun operation is based on the electromagnetic field conversion between coplanar waveguide and microstrip line. It does not rely on long transmission lines with frequency-dependent length, so that it occupies a very compact area and exhibits a broadband performance. It also enables power amplifiers to operate in the balanced mode, allowing the area reduction and stable operation. Substrate modes and in-band resonances in InP substrate, which seriously degrade circuit performance and limits bandwidth, are effectively suppressed by the reduced on-chip ground and balanced operation. The fabricated power amplifier shows a measured small-signal gain of 20.8 dB at 232 GHz with a 3-dB bandwidth larger than 51 GHz (20.8%). Output power also exhibits a broadband performance of 8.0± 1.5 dBm from 220 to 295 GHz with power gain larger than 10.5 dB, corresponding to a large-signal 3-dB bandwidth larger than 75 GHz. This belongs to an excellent bandwidth in both small- and large-signal performance among the reported $H$ -band power amplifiers.