Highly Linear RF CMOS Variable Attenuators With Adaptive Body Biasing

Several approaches to design a linear attenuator have been analyzed in terms of the transistor impedance variation, linearity, frequency responses, and circuit complexity. This paper proposes a novel method of using an adaptive bootstrapped body biasing. The method allows the attenuator to have maximum power handling capability and bandwidth without adding complexity to the circuit. A π-type variable attenuator for WCDMA transmitters has been designed and fabricated using IBM 0.18-μ m triple-well CMOS technology. The attenuator has a linear-in-dB controllability from 400 MHz to 3.7 GHz with an attenuation range of 33 dB. Its insertion loss is 0.9-2.9 dB and worst-case return loss is better than -9 dB within this frequency band. The minimum input 1-dB compression point (IP_1dB) is above 7.5 dBm, and the minimum IIP₃ is greater than 17 dBm at 1.95 GHz. To our knowledge, this design achieves the best linearity performance and frequency responses, and has the smallest area among similar CMOS works.