This work develops automatic gain control (AGC) system based on a low power and low area mix mode for the front-end circuitry of a hearing aid. By incorporating a direct feedback method for the sigma-delta modulator output without a decimation filter, the proposed AGC system can reduce the chip area, latency and system power consumption. Additionally, an operational transconductance amplifier (OTA) based on an inverter structure of the CMOS transistor is developed to design a sigma-delta modulator in order to decrease system power consumption. Moreover, a novel algorithm is also developed by rectifying the output bit stream of the sigma-delta modulator to detect the average waveform of the proposed AGC system. The proposed AGC system is implemented using TSMC 0.18 μm CMOS technology with an IC chip area of 600×680 μm2. Furthermore, the system power consumption is 42.3 μW with a DC supply voltage of 1 V. Measurement results verify that the proposed system can implement an IC chip that capable of providing a digital output signal of 6 bits, which is feasible for hearing aid systems.