these particular embodiments in other instances well known circuits control signals timing protocols and software operations have not been shown in detail in order to avoid unnecessarily obscuring the invention fig2 illustrates a power source 200 according to an embodiment of the invention the power source 200 includes a first power source 202 that provides a voltage v 1 and a second power source 204 that provides a voltage v 2 coupled to the power sources 202 and 204 are respective power source control circuits 210 and 230 in some embodiments the first power source 202 may represent a primary power source and the second power source 204 may represent a secondary e g backup power source in some embodiments the magnitude of the v 1 voltage provided by the first power source 202 may be different than the magnitude of the v 2 voltage provided by the second power source 204 in some embodiments the second power source represents a power storage circuit for example a charged energy storage device a capacitor and or battery in some embodiments the second power source represents an active energy device a solar panel and or an environmental energy harvester as will be explained in greater detail below the power source control circuits 210 and 230 control the application of the voltages provided by the first and second power sources to the output node vout a resistance 206 coupled to the vout node represents a load to be driven by the power source 200 in the embodiment illustrated in fig2 the power source control circuit 210 includes transistors 212 and 214 coupled in series between the first power source 202 and the vout node to provide a current path from the first power source diodes 226 and.