Design considerations in high-frequency CMOS transconductance amplifier capacitor (TAC) filters

Design methodologies for HF and VHF TAC bandpass filters are reviewed and discussed. The factors limiting the filter performance are identified, and it is shown that nonideal effects may be minimized by adequate filter architecture, tuning method and circuit layout. It is concluded that the transconductance amplifiers for HF TAC filters are best when based on simple differential/balanced circuits and must be optimized at transistor level to achieve both linearity and good high-frequency performance. Improved modeling of the high-frequency behavior of the MOS transistor accounting for the distributed nature of the channel in nonquasistatic operation will be necessary for the design of VHF-TAC filters, since they are ultimately limited by the high-frequency roll-off of the transistor itself