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 Ω, reducing the impedance transformation ratio and allowing the broadband matching networks. Two power-amplifier channels are power-combined using on-chip baluns which performs the impedance transformation from 25 to 50 Ω. 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.