A 250MHz Gm-C filter using negative current feedback OTAs

A CMOS fourth-order linear phase low-pass filter used for high speed wireless/wireline system is realized in 0.18-¿m CMOS process. The high speed filter is designed based on operational transconductance amplifier (OTA) biquad sections. As well known, large transconductance is required for high speed applications, and thus the conventional source degeneration topology, which operates with the trade-off of linearity and transconductance, is not suitable. In this paper, the proposed OTA use the negative current feedback topology to maintain linearity for high speed application. By using the proposed OTA as a building block, a 4-th order low-pass filter is realized. Fabricating in 0.18-¿m CMOS technology, the -3 dB filter frequency response at 250 MHz is measured. The measured HD3 performance is about -40 dB and the group delay variation is less than 5 ns at the filter -3 dB cutoff frequency.