A low-voltage CMOS exponential function circuit for AGC applications

This paper describes how to implement a CMOS exponential function circuit, which can be used for Automatic Gain Control (AGC) and Variable Gain Amplifiers (VGA). The proposed exponential function circuit is based on Taylor's series expansion, and it has been implemented using a V-I converter and a squarer circuit, both obtained from the same core cell. The first order term of Taylor's series is implemented by a constant bias current. The low-voltage (less than or equal to 3V) low power CMOS core cell has an almost rail-to-rail differential-input swing. In the entire voltage control range, the dB-linear function achieves a 30dB linear range. For a 20dB range, the linearity error is less than +/-0.5%.