Design Of Automatic Gain Control Circuit In The Wireless Receiver

Automatic gain control (AGC) circuit is employed in the wireless communication systems where the amplitude of an incoming signal can vary over a wide dynamic range. The role of the AGC circuit is to provide a relatively constant output amplitude so the circuits following the AGC circuit only require less dynamic range and the wide dynamic range signal can be demodulated correctly.In order to maintain constant settling time with different input signal amplitude in automatic gain control (AGC) loop, the gain of VGA should have exponential gain characteristic. Due to the square-law characteristics of CMOS technology, it is difficult to generate an exponential gain only base on VGA itself. In this design, a new CMOS pseudo-exponential function is used to generate the required exponential function. By the proposed exponential function generator, the gain dynamic range of VGA can reach more than82dB with the linearity error less than±1dB. The VGA also has DC offset cancellation circuits and can tolerate an input offset voltage up to50mV. By improving the load, the constant-gm biasing amplifier has a temperature-independence DC output. Basing on this VGA, the AGC circuit is finished.The proposed AGC is implemented in TSMC0.18μm MOS technology. For the case of supply voltages VDD=1.8V, the current consumption is only4mA. The gain control range of the proposed VGA can be82dB with the linearity error less than±1dB, and the bandwidth is30MHz. The AGC outputs in the temperature range from-40℃to75℃are almost the same level of0.2Vpp with the error only less than±1dB.