A Compact Multilayer-Stacked Guanella Transformer Based 50–680-MHz GaAs Power Amplifier

This letter presents a sub-GHz ultra-wideband power amplifier (PA) in a 0.25-
${\mu }\text{m}$
GaAs pHEMT process. A compact Guanella-type transformer is designed with a space-saving multilayer-stacked topology to achieve single-ended to differential conversion and impedance transformation over a multioctave frequency range. The proposed PA with differential cascode topology andOFF-chip transformers can achieve a high saturated output power larger than 31 dBm across 50680 MHz (173% fractional bandwidth) in a small dimension. When with a 5-V supply, the fabricated PA microsystem delivers a 34.5-dBm peak output power and achieves a 24.0-dB maximum gain, a 52.7% peak power-added efficiency (PAE), as well as high linearity.