Digitally controlled linear-in-dB variable-gain amplifier for high-frequency applications

A design technique to implement a digitally controlled dB-linear variable-gain amplifier (VGA) for high-frequency applications is presented. The design approach is applicable to current-steering VGAs and is based on a digital gain control circuit that operates as a nonlinear digital-to-analogue converter to achieve a digitally controlled dB-linear gain control characteristic. Using the proposed approach, a variable-gain up-converter for 5-GHz wireless LAN applications has been fabricated with a low-cost 46-GHz-fT silicon bipolar technology. The adopted VGA, operating at a frequency of 1 GHz, provides a linear-in-dB gain control characteristic over a 30-dB dynamic range with a 2-dB step and a ±0.8-dB gain error.