Typicaly to get good preformance from a stepper motor you drive the windings with a voltage about 6 times the rated winding voltage , you then sense the winging current and switch it off and on to maintain the current through the winding at the rated level. The chopping frequency is about 20 times the highest step frequency . This is done to force the winding to saturate as fast as possible by using the steepest part of the curve. regenerative feedback (recirculating diodes ) allow the winding to discharge fast at turn off contributing to the improvement in the maximum speed attainable. this whole circuit ,properly configured is available from many sources as has been mentioned, For a general over view see "Jones on Stepping Motors on the web, for a lucid operation description For Bipolar stepper drives see National Semiconductors Lm182XX data sheet and suggested app notes AN 828, and also see New Japan Radio's (NJR) 3770 and related app notes. Hope his helps Terry