Systems and techniques for reading a radio frequency identification transponder are described. A transmit coil produces a transmit field to power and interrogate the transponder. A reference coil and a receive coil furnish inputs to a differential amplifier, balanced so that the differential amplifier produces a predetermined signal. The receive coil senses changes produced by the transponder, while the reference coil is relatively isolated from such changes. The differential amplifier produces an amplified signal based on the difference between the inputs from the receive coil and the reference coil, so that the signal produced by the differential amplifier is based on the field effect changes produced by the transponder.