Novel Voltage-controlled Amplifiers for Multistandard Integrated Radio Receivers

This paper proposes a new architecture for transconductance (Gm)-based variable-gain amplifiers (VGA) that is better suited for multi-standard radio receivers than the classical two-Gm structure: its bandwidth and linearity are largely independent of the gain setting and both the gain range and the bandwidth can be programmed through digital controls. Unlike the classical structure that uses two Gm cells and modifies the gain by changing the transconductance of at least one of them, usually by varying the biasing conditions the new VGA employs three Gm cells with constant biasing; the gain variation is achieved through a differential current steering scheme. Two transistor-level implementations of the proposed VGA structure are presented, one using simple differential pairs and the other based on a more complex, highly linear Gm cell. New circuits for controlling the output common-mode voltage are proposed for both cases; these circuits do no require the placement of additional circuitry in the differential signal path. The VGAs were designed in a standard 0.18um CMOS technology to same specifications: to cover the gain range of 0dB-18dB while handling signals with amplitudes up to 800mVpkpkdiff without significant distortions; maximum bandwidth above 30MHz while driving 1pF; power consumption bellow 6mW. Simulation results show that both VGAs meet the requirements, thus validating the circuit implementations and proving the potential of the proposed architecture.