This letter describes an ultra-broadband power amplifier (PA) in a 130 nm SiGe:C BiCMOS technology. To achieve this broadband while keeping a compact chip size, an on-chip transformer-based dual-LC-tank power matching method is proposed and implemented for achieving this 54.8 % fractional large-signal 3 dB bandwidth (28.5-50 GHz). Additionally, a high quality factor (Q) by-pass MOM capacitor is proposed and implemented in the transistor layout to decrease the parasitic inductance influence for increasing the gain and keep unconditional stability of the PA. The complete PA achieves a measured saturated output power of 19.2 dBm with 21.5 GHz (28.5-50 GHz) large-signal -3 dB bandwidth at 3.3 V power supply. This broadband PA is only 0.63 mm, and the measured S21 is higher than 22.5 dB from 18.5 to 50 GHz with recorded 70.3 % fractional small-signal 3 dB bandwidth for silicon-based cascode PA. The measured peak OP1dB is 16.8 dBm at 44 GHz, and the maximum power-added efficiency (PAE) is 23.9 % at 32 GHz.