A Wideband dB-Linear Variable-Gain Amplifier With a Compensated Negative Pseudo-Exponential Generation Technique

This article presents a wideband dB-linear variable-gain amplifier (VGA) with a compensated negative pseudo-exponential generation (C-NPEG) technique for high-speed applications. By shifting the concave and the convex function of an original negative pseudo-exponential generator (NPEG), two additional control signals are produced to manipulate two variable-gain Gilbert-cell-based amplifiers, respectively. Cascading these two stages with another variable gain stage controlled by the original NPEG, the multiplication of three different but mutually compensated control signals is realized. As a result, the dB-linear gain range of the overall VGA is extended, and the gain error is suppressed with the proposed C-NPEG technique. To further improve the VGA's gain while maintaining a wide bandwidth, a fixed-gain stage utilizing five common-source amplifiers with RC-degeneration and active feedback is implemented. Fabricated in 40-nm CMOS technology, the core circuit consumes 24.6-mA current (excluding the output buffer) under a power supply of 1.1 V and occupies an active die area of 0.038 mm2. The measured gain of the overall VGA varies from -34 to 17 dB, while the dB-linear gain error over the 51-dB gain range is within ±1 dB. Meanwhile, the -3-dB bandwidth is relatively constant from 7 to 8 GHz under different gain settings.