Implantable probes with built-in light emitters have a promising potential for a range of applications, in particular optogenetic neural stimulation. However, where soft encapsulation methods are used, lifetime will be a function of the quality of encapsulation and the driving mechanism. We have found that a balanced driving mechanism – whereby the integral voltage on encapsulated contacts, can significantly prolong lifetimes. As such, in this work we have designed a driving circuit which drives current, but ensures balanced electric fields with an error less than 1%. The circuit has been designed in 0.18um TSMC CMOS technology and will be integrated with custom implantable optical brain probes for characterization.