An output circuit (116) for use in an implantable cardiac defibrillator (10) provides an output pulse (170) having a waveform of virtually any desired shape. The device includes a sensing circuit (82,84) that senses cardiac activity. An arrhythmia detector detects fibrillation responsive to the cardiac activity signal. The device further includes an output circuit (120) that provides a stimulation output pulse (170) when the arrhythmia detector detects a cardiac arrhythmia. The output circuit includes an H-bridge (140) having a pair of switching devices which control the output pulse waveform with pulse-width modulation (128) and a second pair of switching devices (130) that control the output pulse polarity.
