On Fri, Apr 15, 2011 at 10:18 AM, Sean Breheny wrote: > Olin is assuming that you connect the battery to the collector of the > BJT, control voltage to the base, and the emitter to ground through a > sense resistor. The error comes from the fact that whatever current > the base terminal draws will ALSO flow through the emitter. In other > words, Ie=3DIc+Ib. > > The only way you could really avoid this (and still use a BJT) is to > either use a BJT with a sufficiently-high current gain that the base > current is negligible OR put the sense resistor in the collector lead. > However, the latter option will require a differential amplifier to > measure the sense resistor since it is no-longer ground referenced. > > Sean > > Oh! The battery will only be providing the collector current. The rest of the testing device will be powered by other means. My device will be something more like this: http://postimage.org/image/1n89aa144/full/ --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .