A VHF OTA-C Topology Having Low Phase Error With Gm Tuning

In sub-micron process, while speed of device is suitable for VHF applications, phase variation at unity gain frequency of OTA-C cell with frequency tuning is high. This is mainly because of high value of channel length modulation parameter of the devices. A technique which makes the phase response robust to frequency tuning is proposed here. Based on this technique a VHF CMOS OTA-C cell is designed in a 0.18 mum CMOS process. The phase error of the cell is within 0.3deg, over a range of unity-gain frequency from 300 MHz to 1.2 GHz. The small phase error over a wide range of frequency implies the use of just one Q-tuning loop for all the integrators having unity-gain frequency within this range. Also this makes this circuit suitable for application in programmable transfer function filters