Frequency response improvement of the two-stage opamp with indirect compensation and large load

Abstract

This paper presents a further development of an improved frequency compensation method for a two-stage operational amplifier with indirect feedback current, which is beneficial for large capacitive load applications. It describes the case when a nulling resistor is added at the output in such a way that it is residing outside the outer feedback loop of the amplifier. Compared to the case when the nulling resistor is placed outside inner frequency compensation loop but it is inside of the outer feedback loop of the amplifier, the driving capability can be substantially improved or for the same capacitive load it can improve the stability by gaining up to 45 degree of extra phase margin. A prototype has been implemented with the STMicroelectronics 28 nm CMOS technology and simulated with Cadence Virtuoso. Simulations at room temperature show an improvement of the unity gain frequency from 741.57 MHz to 1.047 GHz and an extra phase margin gain of 38 degree. The simulation of the temperature effect showed that the phase margin gain remains for temperature range [−20, 150] °C.