A new CMOS current source is described for biomedical implantable microstimulator applications, which utilizes MOS transistors in deep triode region as linearized voltage controlled resistors (VCR). The VCR current source achieves large voltage compliance, up to 97% of the supply voltage, while maintaining high output impedance in the 100 M range to keep the stimulus current constant within 1% of the desired value irrespective of the site and tissue impedances. This approach improves

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... efficiency, extends power supply lifetime, and saves chip area especially when the stimulation current level is high in the milliampere range. A prototype 4-channel microstimulator chip is fabricated in the AMI 1.5m, 2-metal, 2-poly, n-well standard CMOS process. With a 5-V supply, each stimulating site driver provides at least 4.25-V compliance and 10 M output impedance, while sinking up to 210 A, and occupies 0.05 mm 2 in chip area. A modular 32-site wireless neural stimulation microsystem, utilizing the VCR current source, is under development.