A High-Q Linear CMOS Digitally Controlled Accumulation-mode Varactor Array for Multiband RF Circuits

摘要

Electronically tunable capacitors are key elements for tunable and reconfigurable RF circuits. The most popular tunable capacitor topologies in CMOS are the analog varactor and digitally controlled switched capacitor array (SCA). The conventional analog varactor shows a high quality factor (Q-factor) in both the minimum capacitance (C_(min)) and the maximum capacitance (C_(max)) states with a wide tuning range, but it has a poor linearity performance due to the voltage dependent nonlinear capacitance. In case of the digitally controlled SCA, while it shows a very good linearity, its Q-factor in C_(min) state is strongly dependent on the substrate loss by the parasitic junction capacitances between P-well, deep N-well, and P-substrate and the substrate resistance. Especially, if the SCA is implemented in a standard digital CMOS process without deep N-well, it suffers from severe Q-factor degradation in C_(min) state at higher frequencies. In order to overcome the aforementioned drawbacks of the analog varactor and the digitally controlled SCA, the digitally controlled binary-weighted accumulation-mode varactor array (AVA) is proposed. Contrast to the conventional analog varactor tuned by continuous analog voltage, the proposed AVA uses only two states of C_(min) and C_(max) of the analog varactor by digitally on/off control. Instead of a zero voltage reference, the negative voltage (-VDD) is applied to the gate of the analog varactor in C_(min) state in order to maximize the tuning range, power handling capability, and linearity. The total capacitance varies by turning on (+ VDD) or off (-VDD) each branch of the proposed AVA. The proposed AVA keeps a high Q-factor in all states even if it is implemented in a standard digital CMOS process without deep N-well, while showing comparable linearity performance in comparison with the conventional SCA. In simulation, Q-factor at 2.4 GHz is greater than 70 over all states and the tuning range is about 3.1.