Evolution of and Perspectives on Differential‐Pair‐Based CMOS Variable Gain Amplifiers With Linear‐in‐Decibel Gain Control: A Tutorial

ABSTRACT A variable gain amplifier (VGA) is an essential building block used in the automatic gain control (AGC) loop to provide a continuous, adjustable gain for the signal strength. To maintain the stability and consistency of the AGC loop, it is necessary to ensure that the gain of the VGA has an accurate dB‐linear characteristic. This tutorial provides a detailed overview of the development of dB‐linear VGAs based on differential pair structures over recent years. We categorize various VGA cells based on differential pair structures, analyze their operating principles, and explain the key features of each topology. Based on observation and extraction of the dB‐linear implementation schemes used by these VGAs, we summarize three basic methods for achieving dB‐linear gain control. We also discuss specific methods for realizing the exponential function, using a true exponential and a mathematical approximation, and present the different types of approximation functions adopted in the works described here. Considerations in regard to the tuning range, gain error, bandwidth, linearity, process‐temperature robustness, and circuit simplicity are also discussed. Finally, we draw comparisons and give perspectives for future dB‐linear VGAs.