A power amplifier develops a modulated high power signal by summing the outputs of multiple voltage controlled oscillators. A phase control circuit synchronizes the phase relationship of multiple voltage controlled oscillators. The voltage control oscillators provide amplitude modulation of an output signal of the voltage controlled oscillator by adjusting transconductance of amplification devices of the voltage controlled oscillator. Another embodiment of the power amplifier provides buffer circuits at the outputs of the voltage controlled oscillators. The gain or delay of the buffer circuits is varied to provide amplitude modulation of the power amplifier.