Design and Analysis of a VHF OTA-C Cell for Optimum Phase Response

Here we discuss an OTA-C topology, suitable for high speed applications. In this topology the tuning circuit is separated from the main amplifier, which enables reduction in the number of stack transistors and hence, makes the circuit suitable for sub-micron technology. While smaller devices provide better speed, their larger channel length modulation makes the phase response more sensitive to the operating point and hence, to the Gm tuning point. A sizing guideline is adopted that helps to get an optimal frequency response over a large tuning range. The circuit was designed in a 0.18 mum CMOS technology. The phase error variation is within 0.5 deg as the unity-gain frequency is tuned from 250MHz to 1GHz