A 50-450 MHz Tunable RF Biquad Filter Based on a Wideband Source Follower With > 26 dBm IIP₃, +12 dBm P_1dB, and 15 dB Noise Figure

This paper reports a source follower-based active RC filter topology for RF front-ends in wideband systems. In the proposed active RC filter, the noise figure and bandwidth are improved through adopting the proposed sub-1 Ω Z_out source follower, which replaces the conventional operational amplifier-based unity gain buffer and through simply changing its location in the filter. Applying the proposed scheme to the conventional Sallen-Key filter also improves the stopband rejection performance. Furthermore, the proposed filter topology ultimately suppresses the DC offset voltage and flicker noise of the unity gain buffer, which is the only active circuit in the filter. The proposed biquad filter is implemented in 0.18 μm CMOS and the measurements exhibit > 26 dBm IIP₃, +12 dBm input P_1dB, and <15 dB noise figure over 50-450 MHz of a 6 bit tunable frequency range. The proposed biquad filter consumes 14 mA from a 1.8 V supply and the chip occupies 1000×500 μm².