A 18 GHz Broadband Stacked FET Power Amplifier Using 130 nm Metamorphic HEMTs

A broadband power amplifier (PA) with a 3 dB power bandwidth of 72% is presented using metamorphic high electron mobility transistors (mHEMTs). A stacked FET structure, where transistors are series connected to combine voltage swings, is employed to overcome relatively low breakdown voltages of mHEMTs. Series-connected PA's show much higher load impedance compared to the parallel combined transistors, which allows output matching to be realized in the low quality (Q)-factor region, providing the broadband performance. The fabricated PA using quadruple-stacked 130 nm mHEMTs has a gain of 21.2 dB and saturated output power of 26.4 dBm with power added efficiency (PAE) of 33% at the design frequency of 18 GHz. The 3 dB output power bandwidth is from 10 to 23 GHz.