Compensation Method for Multistage Opamps With High Capacitive Load Using Negative Capacitance

It is shown that negative capacitance (NC) circuits can be systematically used to improve the gain-bandwidth product of the operational amplifiers (opamps). The NC circuit moves the nondominant pole of the opamp to higher frequency by decreasing the parasitic capacitance of the critical node. The impedance at the input of the NC circuits is neither purely capacitive nor negative at all frequencies. A design guide is presented by deriving the circuit model for a conventional NC circuit and investigating the extent of the improvement that can be achieved in a circuit by the use of the NC circuit. The model is then used to present the design guide for widebanding the multistage opamps with large capacitive loads. Based on the proposed method, a three-stage opamp with a 15-nF capacitive load was designed and fabricated in a 0.18-μm CMOS process. The measurements confirm that the opamp satisfies the design target of 1.5-MHz bandwidth at the power consumption of 75 μW.