AC actuation contribution to the induced electromotive force in the miniaturized inductive planar microphone

In this paper, we aim to analyze the gain ratio in the output induced voltage of a CMOS-compatible miniaturized electrodynamic microphone which is based on the use of two coplanar concentric inductors. These latter are comparable to a concentric transformer with a secondary situated on the microphone diaphragm top. The study has been applied when the outer inductor is actuated using an AC bias instead of DC leading to a time-variable B-field. The induced output voltage expression has been evaluated using two distinct analytic methods: the magnetic field derivation and the electric field approach. When an AC current with frequency above that of the acoustic band is used to bias the primary, results show that the induced voltage increases and becomes independent from the incident pressure wave frequency. The paper demonstrates also that with an AC bias, the microphone induced voltage values can reach the order of hundreds of A mu Vs, as opposed to only few A mu Vs when applying a DC bias. The gain in the induced voltage has been investigated and it is shown that it can be correlated to the AC current frequency, omega(c), and the inner inductor fluctuation frequency, omega(p). Moreover, this has been quantified analytically and it is shown that it equals omega(c)/8 omega(p). This result will not only enhance the overall performance of the microphone, but will also make the preamplification design less complicated by increasing the SNR ratio.