A signal-processing system has an amplifier that generates an amplified (RF) output signal based on an (RF) input signal. The amplifier receives a supply voltage that can be selectively set to an appropriate level between a lower power supply level and a higher power supply level. With one power supply permanently connected to the supply voltage node, a control unit executes software to toggle a supply switch to periodically connect and disconnect the other power supply thereby generating a weighted average value for the supply voltage between the two power supply levels. When a sudden and large increase occurs in the (input) power level, hardware-interrupt circuitry interrupts and supersedes the software-based control of the supply-voltage switch to quickly switch the supply voltage towards the higher power supply level. The hardware-interrupt circuitry handles such situations faster than the software-based control in order to prevent a limit violation of spectrum emission requirements.