A Mixed-Mode Variable Gain Amplifier for Hearing Aid Devices

A mixed mode variable gain amplifier for hearing aid application is presented. It consists of main amplifier stage and a gain control circuit. Based on the output of microphone, voltage levels are categorized into two gain regions and designed circuit automatically sets the close loop gain of main amplifier. Main amplifier consists of opamp with feedback resistors and gain control circuit consists of peak detector, high speed comparator and XNOR gate. Due to high speed digital control circuitry, attack and release time are as small as 60µSec which is 33 times faster than temporal resolution of human hearing. Along with preamplifier, proposed circuit achieves a gain range of 45dB to 65dB and offers an input referred noise of 0.13µVrms, with peak SNR of 77dB and consumes a power of 172µW from 1.8V supply. Circuit is designed in 0.18µm CMOS process and occupies an area of 493µm × 184µm.