An output stage for use in a therapeutic defibrillator enables practical use of specialized output waveforms optimized for cardiac defibrillation. A pulse-width modulated (PWM) switching amplifier, connected to a high voltage source capacitor and to one or more output bridges corresponding to different electrode placements, is adapted to operate with high efficiency, demonstrated at about 80%. The amplifier is capable of delivering a defibrillating electric shock to a heart in the form of a time-varying output voltage waveform of arbitrary shape. Efficiency improvement is accomplished through the use of a high voltage reservoir capacitor network configured to minimize a voltage differential between the high voltage reservoir and the output voltage. The switching amplifier features both step-up and step-down amplifier capability. A PWM control unit is positioned within the circuit so as to reduce complexity by eliminating a need for additional isolation circuitry.