The present invention is a dual mode quadrature RF power amplifier system that is efficient and meets linearity requirements in both modes of operation. In a high power mode of operation, the power amplifier functions in a traditional manner, wherein both quadrature amplifier legs feed a quadrature power combiner to provide a combined RF output signal at an RF output. Any imbalanced signals are routed to an isolation output. In a low power mode of operation, one quadrature amplifier leg is disabled, which would normally cause the signal being fed from the enabled amplifier leg to be split between the two quadrature outputs; however, the impedance at the isolation output is shifted to reflect the isolation output signal back into the quadrature combiner to appear at the RF output. The impedance shift at the isolation output causes an impedance increase at the active input, thereby lowering output power.