A programmable output current source for use within an implantable tissue or nerve stimulator, e.g., an implantable cochlear stimulator or spinal cord stimulator, includes parallel-connected P-FET current source sets connected between a positive voltage rail and an electrode node, and parallel-connected N-FET current source sets connected between the electrode node and a negative voltage rail. The N-FET current source sets include n N-FET current sources, where n is an integer 0, 1, 2, 3, 4, . . . , and wherein each N-FET current source, when enabled, respectively sinks a current 2
I from the electrode node to the negative rail, where I is a selectable fixed current. Similarly, the P-FET current source sets include n P-FET current sources, where n=0, 1, 2, 3, . . . n, and wherein each P-FET current source, when enabled, respectively sources a current 2
I from the positive voltage rail to the electrode node. An individual current pulse is formed by selecting an electrode pair, and enabling a desired combination of P-FET current sources so that a desired total current is sourced to one of the electrodes of the selected electrode pair, and at the same time enabling a corresponding combination of N-FET current sources so that the same total current is sunk from the other electrode of the selected electrode pair. Sequences of current pulses, e.g., biphasic or multiphasic stimulation pulse pairs, are formed by combining individual current pulses having the desired polarity and timing relationship.