A CMOS linear-in-dB VGA and AGC loop for telecommunication applications

In this paper, an Automatic Gain Control (AGC) loop which is based on a linear-in-dB Variable Gain Amplifier (VGA) is proposed. The VGA structure is based on simple nMOS differential pairs with variable tail currents. The linear-in-dB gain tuning schema is designed using a novel exponential current generator which also offers temperature compensation of the VGA's gain. The gain of the VGA is tuned by a control voltage with gain range about 28 dB with ±1 dB linearity error. The worst cases of the VGA gain, over process and temperature corners, are ±1.54 dB and ±2.45 dB for maximum and minimum gain setting, respectively. The proposed implementation is designed in a CMOS 90 nm triple-well process with 1.2 V supply voltage.