A digitally generated exponential function for dB-linear CMOS variable gain amplifiers

The generation of an exponential function is required to attain a high dynamic control range and a dB-linear operation in AGC applications. In such applications, a DSP is usually an inherent part of the control loop. We present a simple way to synthesize the exponential function in the analog domain by using a DSP to generate the exponential curve values directly. For a 10-bit implementation, an accuracy of more than 0.05% and a dynamic range of 60 dB are obtained. An algorithm required for the generation of the exponential and a possible implementation are discussed. Simulation results of a 0.5 μm CMOS implementation show the validity of the proposed approach.