A Two-Stage CMOS Amplifier Performing High Degree of Stability for All Capacitive Load

This paper presents the conception, design and realization of a fully-differential two-stage CMOS amplifier that is unconditionally stable for any value of the capacitive load. This is simply achieved by sending a scaled replica of the output stage current to the amplifier virtual ground such as to create a Left Half-Plane (LHP) zero in the loop-gain that either cancels or tracks the output pole in all process, voltage, and temperature (PVT) conditions. Consequently, from a stability point of view the amplifier behavior resembles that of a single-pole OTA. Starting from an existing two-stage gain-programmable amplifier, designed in a 0.18 mu m Bipolar-CMOS-DMOS (BCD) process, that was able to drive only 10pF without incurring into stability issues, a simple circuit has been added to extend the stability to any capacitive load value. Measurements, given with loads ranging from 0pF to 100nF, show high degree of stability in any load conditions. In the used 0.18 mu m BCD technology, silicon area and current consumption of the extra circuit are only 0.0004mm2 and 2 mu A, respectively, with a 5V power supply.