Low-Power Digitally Controlled CMOS Source Follower Variable Attenuator

A low-power source follower variable attenuator (SFVA) is proposed and implemented using standard 0.18 mum CMOS. Using a PMOS source follower, a bandwidth of 200MHz is achieved with 500 muA current from a 1.8V single supply. Accurate attenuation can be varied with a digital control signal. Dc coupling at the input is made possible with a self-biasing technique. A single stage SFVA with a 2-bit control is demonstrated for 0dB to 6dB attenuation. The number of control bit and attenuation is expandable. The active area is merely 120 mum times 60 mum which is very cost effective.