A medical apparatus includes an extracorporeal power source that transmits electrical power via a radio frequency signal to a medical device implanted inside an animal. The extracorporeal power source has a Class-E amplifier with a choke and a semiconductor switch connected in series between a source of a supply voltage and circuit ground. An output node of the amplifier is formed between choke and the switch and connected to a transmitter antenna. A shunt capacitor couples the amplifier's output node to the circuit ground. Controlled operation of the switch produces bursts of the radio frequency signal that are pulse width modulated to control the amount of energy being sent to the implanted medical device.