We propose a novel dual-mode neural stimulator circuit. For stimulation requiring small or medium amount of charge, the stimulator supplies a constant current to the stimulation load in the same way as any standard current-mode stimulator. For high-charge stimulation applications, the stimulator supplies a variable stimulus current, depending on the voltage available across the current generator circuit. By adjusting the bias current accordingly, the stimulator maintains a high output impedance, independent of the stimulus current and the load voltage at the time. The total stimulus charge from the proposed stimulator is theoretically as high as voltage-mode stimulators. Simulated results using a 0.6-μm CMOS process are presented. At low stimulus current, the output impedance of the proposed stimulator circuit is at least six times higher than a conventional current-mode generator with constant bias current.