Energy efficient electrical pulse generating device

An implantable neurostimulator for delivering an electrical pulse to a target region within a patient's body, the neurostimulator comprising:

• a housing;
• a controller located in the housing;
• an energy source located in the housing;
• a stimulation circuit located in the housing, wherein the stimulation circuit generates a stimulation current that provides the electrical pulse, wherein the stimulation circuit includes:
• a converter for generating the stimulation current;
• a current source in electrical communication with the converter;
• and
• wherein the current source includes a pair of cascode arranged transistors that are controlled to maintain an amplitude of the stimulation current substantially constant, wherein the controller is configured to control the pair of cascode transistors to minimize a voltage across one of the pair of cascode arranged transistors in order to reduce an amount of energy withdrawn from the energy storage device while the current source operates to provide the substantially constant stimulation current.