A current-mode front-end circuit with low voltage and low power for analog hearing aids is presented.The circuit consists of a current-mode AGC (automatic gain control) and a current-mode adaptive filter.Compared with its conventional voltage-mode counterparts,the proposed front-end circuit has the identified features of frequency compensation based on the state space theory and continuous gain with an exponential characteristic.The frequency compensation which appears only in the DSP unit of the digital hearing aid can upgrade the performance of the analog hearing aid in the field of low-frequency hearing loss.The continuous gain should meet the requirement of any input amplitude level,while its exponential characteristic leads to a large input dynamic range in accordance with the dB SPL (sound pressure level).Furthermore,the front-end circuit also provides a discrete knee point and discrete compression ratio to allow for high calibration flexibility.These features can accommodate users whose ears have different pain thresholds.Taking advantage of the current-mode technique,the MOS transistors work in the subthreshold region so that the quiescent current is small.Moreover,the input current can be compressed to a low voltage signal for processing according to the compression principle from the current-domain to the voltage-domain.Therefore,the objective of low voltage and low power (48 μW at 1.4 V) can be easily achieved in a high threshold-voltage CMOS process of 0.35 μm (VToN + |Vrop｜≈ 1.35 V).The THD is below -45 dB.The fabricated chip only occupies the area of 1 × 0.5 mm2 and 1 × 1 mm2.
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