A system is provided for identifying reference locations within the environment of a device wearer. A driver stage of a power amplifier has a multi-stage configuration with multi-finger HBTs connected in shunt with each other, each multi-finger HBT having a single emitter. Sensors\u2014for example, inductive proximity sensors\u2014are positioned near the periphery of the first and second disks that detect the encoded pseudo-random sequences and are periodically sampled to generate first and second detected sequences.