Using Moderate Inversion to Optimize Voltage Gain, Thermal Noise, and Settling Time in Two-stage CMOS Amplifiers

This paper describes the use of moderate inversion for selected MOS transistors in a two-stage CMOS amplifier to achieve high voltage gain, low thermal noise, and fast settling time at minimum power consumption. A detailed circuit analysis, implemented in a MATLAB design tool, is presented to find the optimal inversion level for MOS transistors. The analysis and design tool are illustrated for a fully differential, two-stage, 0.5-mu m CMOS amplifier having voltage gain >80 dB, input-referred thermal noise voltage <6 nV/Hz(1/2), gain bandwidth of 100 MHz, phase margin of 58(o), and 0.1% settling time <15 ns for load capacitances of 6 pF and a supply voltage of 2.5 V. SPICE simulation results confirm that these specifications are achieved at a minimum supply current of 2.42 mA by operating first- and second-stage input transistors in moderate inversion compared to 3.82 mA for operation in strong inversion, resulting in a 37% decrease in power consumption.