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.
['Fanyang Li', 'H. X. Yang', 'Yu Wang', 'Qisong Wu']
Current Mode Feed-Forward Gain Control for 0.8V CMOS hearing aid