Design of variable gain amplifiers for wireless or wireline communication

Abstract

With the increasing demands for high data rate wireless and wireline communication systems, variable-gain amplifiers (VGAs) are more commonly used in 1) analog baseband circuits; 2) high-speed automatic gain control (AGC) circuits; 3) millimeter-wave (mm-wave) phased-array systems. For VGAs in the analog baseband and AGC circuits, a wide gain tuning range as well as an accurate dB-linear characteristic are desirable to provide a dynamic range for the transceiver chain and a uniform transient response for fast signal acquisition. For VGAs in mm-wave phased-array systems, except for the gain tuning capability, the phase-invariant characteristic under different gain settings is required to reduce sidelobes through amplitude tapering and the low-noise performance is important to ensure a low noise figure (NF) for the receiver front-end. There are mainly three research contributions in this work. Firstly, a power-efficient cell-based dB-linear VGA with MHz-range bandwidth is implemented. A negative gm cell is introduced to obtain a wider gain range and thus improves the power efficiency of the VGA cell. Furthermore, through tuning the substrate voltage of the subthreshold-region NMOS transistors, a well-compensated gain tuning characteristic is acquired by the proposed VGA cell. Secondly, to satisfy a higher speed requirement for VGA, a GHz-range wideband dB-linear VGA with a compensated negative pseudo-exponential generation (C-NPEG) technique is demonstrated. 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 also implemented. Thirdly, for mm-wave phased array applications, a low-noise as well as phase-invariant characteristic are required in addition to a gain tuning capability for VGAs. At the first stage of the proposed mm-wave VGA, a transformer-feedback based amplifier structure is utilized to operate under a low supply voltage and ensure a low NF for the overall VGA. Moreover, by employing a gate-to-drain capacitance neutralization technique based common-source amplifier and a control block with pre-differentiated signals, the proposed mm-wave VGA provides a wide gain tuning range with a phase-invariant characteristic. All the prototypes are fabricated in standard CMOS technologies to verify the proposed structures.