Effects of resistive loading on unity gain frequency of two-stage CMOS operational amplifiers

It is known that the unity gain frequency of a CMOS operational amplifier reduces with heavy resistive loading. However, the analytical expression for this parameter as predicted by the classical theory does not contain any load resistance term. Therefore, the parameter is difficult to estimate when the load resistance is low. In this paper, this problem has been investigated for two-stage CMOS operational amplifiers employing two popular compensation schemes. The final outcome of this effort is the introduction of two resistive load dependent correction factors, which when applied to the traditional formulae predicts, reasonably accurately, the shifting of the open-loop poles and the unity gain frequency with any amount of resistive loading. The new formulae and many other useful results have been verified with simulation and measurement on four operational amplifiers.