VLSI Implementation of a Digitally Tunable Gm-C Filter with Double CMOS Pair

Abstract

This paper proposes a modified, inverter based transconductor using double CMOS pair for implementation of Gm-C filters . The advantage of this scheme is that, instead of varying the power supply, the bias voltages at high impedance nodes are varied for frequency (F) tuning. A current steering DAC is proposed for controlling these bias voltages. Another major contribution of this paper is the use of switchable transconductance cell for Q-tuning. This dispenses with the need for two separate biasing circuits (for F and Q tuning). To study the performance of proposed schemes, a bandpass filter is implemented on TSMC-0.18 mum CMOS process using Gm/Id design methodology. The simulation results show a good centre frequency (10 MHz-120 MHz) and pass band (10MH-80MHz) tuning. The proposed approach guarantees the upper bound on THD to be -40 dB for 1 Vw signal swing. The use of inverters with double CMOS pair results in 21 dB higher PSRR compared to those using push pull inverter.