A surgical apparatus comprises a body, an ultrasonic transducer, a shaft, and an end effector. The ultrasonic transducer is operable to convert electrical power into ultrasonic vibrations. The shaft couples the end effector and the body together. The end effector comprises an ultrasonic blade in acoustic communication with the ultrasonic transducer. The ultrasonic blade includes a recess region having a plurality of recesses. The recess region is tapered such that the cross-sectional area of the recess region decreases along the length of the recess region. The ultrasonic blade is also curved such that a central longitudinal axis of the ultrasonic blade extends along a curved path. A reference circuit is used to account for voltage drops of unknown values during operation of the surgical apparatus.