Non-Foster impedance matching of an electrically small loop antenna for biomedical telemetry

Abstract

The demand for miniaturized antennas in biomedical telemetry has been steadily increasing. In biomedical telemetry, electrically small antennas are integrated into exterior portable devices owing to their size in respect of the wavelength to process the received sensing information and power battery-less implantable medical devices (IMDs). However, electrically small antennas particularly loop antennas have high intrinsic reactive part of their input impedance and low radiation resistance. In this paper, a non-Foster impedance matching network is adopted using a negative impedance converter (NIC) to compensate for the reactive part and extend the bandwidth of a loop antenna used for an exterior portable reader in biomedical telemetry. The negative inductance is realized by a transistor-based active circuit and lumped elements balun to properly feed a differential input loop antenna and improve radiation resistance and the bandwidth. Based on the non-Foster impedance matching of the exterior loop antenna, the operating bandwidth is increased to 15.7 MHz from 1.6 MHz using a conventional positive lumped elements matching network.