Dual-mode gain control for a 1 V CMOS hearing aid device with enhanced accuracy and energy-efficiency

A dual-mode gain control (DMGC) technique is presented for accurate and energy-efficient pre-amplification in the front-end system of a hearing aid chip. Compared with the conventional automatic gain control (AGC) approach, the DMGC approach is characterized by an amplification switching mechanism between the pro-linearity discrete gain setting mode and the energy-efficient AGC mode. Thus, the total harmonic distortion (THD) should be significantly improved without incurring any degradation concerning other performances parameters (e. g. gain, noise and power consumption). In order to further enhance the system power efficiency, a self current-adapting (SCA) circuit design technique is proposed. Such SCA circuits are capable of automatically adjusting the bias current in accordance with the sound level. A prototype chip was designed with a 0.13 mu m standard CMOS process and tested with 1 V supply voltage. The measurement results show that, for a typical output level of 500 mV(p-p), the THD is somewhere below -64 dB, achieving approximately ten times reduction compared to the previously reported works. The power consumption of less than 45 mu W has also been obtained. In addition, the typical input referred noise is only 2 mu V-rms and the maximum gain attainable is up to 39 dB.