Current Mode Feed-Forward Gain Control for 0.8V CMOS hearing aid

In an ultra low voltage hearing aid front-end system on chip, a novel Current Mode Feed-Forward Gain Control (CMFGC) technique is presented. Compared with the conventional automatic gain control (AGC), CMFGC significantly reduces the total harmonic distortion (THD) without worsening other performances by digitally controlling the passive resistor array. To attain the digital control codes according to extremely weak signal from the microphone, a novel high precision current mode rectifier is proposed. A prototype chip has been designed based on a 0.13μm standard CMOS process and tested with 0.8V supply voltage. The measurement results show that the THD of the system is between 0.02% and 0.06% at the output level of 500mV_p-p. In addition, the typical input referred noise within the system´s bandwidth is 4μV_rms and the typical power consumption of the complete system is 40μW.