The proposed λ/4-transformer based Class-F^-1 load network effectively shapes the voltage and current waveform of the active device and increases the efficiency of the PA. The simple load structure makes it effective at millimeter-wave regime despite low quality (low-Q) of passive components. The effectiveness of the load network is verified with implementation of 2-stage Class-F^-1 PA at 40GHz achieving record measured power added efficiency (PAE) of 43% with saturated output power (PSAT) of 18dBm at 0.13 µm SiGe BiCMOS technology.