Power amplifier (PA) for a next generation X band T/R-modules in active array antennas is realized using low cost, high yield and high integration 0.18μm SiGe-HBT Technology. A single stage class AB cascode PA using only highspeed HBTs and double tuned transformers at the input and output matching networks with excellent performances has been designed. The PA achieve peak output power of 27dBm and maximum 36 % power added efficiency (PAE). The core RF size is 0.85mm × 0.56mm without pads and low frequency decoupling capacitors exhibiting an output power density of 1.0 W/mm2. To our knowledge, those values are the highest in SiGe-HBT power amplifiers.