A High-Linearity 264-MHz Source-Follower-Based Low-Pass Filter with High-Q Second-Order Cell for MB-OFDM UWB

Abstract

Source-follower-based (SFB) continuous-time low-pass filters (LPF) have the advantages of low power and high linearity over other filter topologies. The second-order SFB filter cells, which are key building blocks for high-order SFB filters, are often realized by composite source follower with positive feedback. For a single branch 2nd-order SFB cell, the linearity drops severely at high frequencies in the pass band because its slew-rate is restricted by the Q factor and the pole frequency. The folded 2nd-order SFB cell provides higher linearity because it has two DC branches, and hence has another freedom to increase the slew rate. However, because of the positive feedback, the folded and unfolded 2nd-order SFB cells, especially those with high Q factors, tend to be unstable and act as relaxation oscillators under given circuit parameters. In order to obtain higher Q factor, a new topology for the 2nd-order SFB cell without positive feedback is proposed in this paper, which is unconditionally stable and can provide high linearity. Based on the folded 2nd-order SFB cell and the proposed high-Q SFB cell, a 264MHz sixth-order LPF with 3 stages for ultra wideband (UWB) applications is designed in 0.18µm CMOS technology. Simulation results show that the LPF achieves an IIP3 of above 12.5dBm in the whole pass band. The LPF consumes only 4.1mA from a 1.8V power supply, and has a layout area of 200µm × 150µm.