Compact Wideband Linear CMOS Variable Gain Amplifier for Analog-Predistortion Power Amplifiers

To predistort the signal of an analog-predistortion power amplifier system, this paper proposes a variable gain amplifier (VGA) topology that gives an optimized linearity and bandwidth performance while having a continuous, linear-in-decibel gain control curve. The design employs a self-biased differential amplifier with a dynamic current source to provide gain and linearity. The overall VGA gain is then controlled by a highly linear variable attenuator connected at the output of the amplifier. This separated controlling stage allows the VGA to have a continuous and linear-in-decibel gain curve without sacrificing the bandwidth and linearity of its amplifier stage. A VGA designed with such topology has been fabricated in a standard 0.18- μm CMOS technology. Targeted for wideband code-division multiple-access applications, the VGA has a maximum gain tuning range from -13.5 to 13.5 dB, a bandwidth better than 1.8 GHz (380 MHz to 2.2 GHz), and a worst case input P_1dB better than -5 and -3.6 dBm, respectively, at 850 MHz and 1.95 GHz. The post-attenuated structure also gives the VGA a noise figure as low as of 4.9 dB at 1.95 GHz. To our knowledge, the design demonstrates the best linearity performance and has other characteristics that are equivalent with other VGAs.