A new family of hybrid stacked power amplifiers (named as ‘Class-K’) are presented where each of the series stacked transistors can operate independently as different class of switching amplifiers. The voltage and current waveforms of the stacked transistors are shaped by independent harmonic load networks connected to the collector nodes of each of the stacked HBTs. A properly-designed Class-K amplifier can simultaneously achieve the high efficiency of Class-E/F amplifiers, high output power of Class-EF amplifiers, and high power gain of Class-E amplifiers. A proof-of-concept two-stage two-stacked balanced Class-K amplifier implemented in a 0.18 µm SiGe HBT BiCMOS process demonstrates 25.5 dBm output power and 26% peak PAE at 34 GHz.