Design Of Rail-to-Rail Low Voltage Operational Amplifier

Operational amplifier(opamp) is widely used in analog circuits. With the development of thesemiconductor technology and the decrease of the integrated circuit(IC) feature size, it is a greatchallenge to the design of analog circuits as power supply voltage continues to drop. In addition,due to the rapid development of portable electronic devices and a sharp rise in market demand, thelow supply voltage, low power dissipation circuits become an inevitable trend of analog circuitdesigns.In this paper, a low-voltage low-power three stage rail-to-rail CMOS opamp is presented. Theinput stage of the opamp applies a p-channel differential input pair and a level-shift p-channeldifferential input pair to obtain rail-to-rail signaling, while the intermediate stage uses anoninverting amplifer in order to guarantee the entirely gain of the opamp and that Millercompensation is negative feedback. In the output stage, a simple classAB buffer which is suitablefor low-voltage applications is employed so that the opamp can acquire the current-handlingcapabilities to drive small resistive loads. The buffer applies an adaptive load to reduce thesensitivity of the quiescent current to the process variation. By nested-Miller compensation, theopamp can be operated stably. Moreover, a voltage bandgap is designed to assure the accuracy ofthe current mirror in the output stage.The proposed opamp is deigned in CSMC0.5μm double poly triple metal mixed signaltechnology and simulated by Cadence Spectre tool. The simulation results show that the opampachieves1.344mW static power dissipation,77.1dB dc gain,72.1degree phase margin and3.97MHz gain band width(GBW) when operating at1.8V power supply with100pF and2KΩoutput load. The positive and negative slew rates are5.9V/μs and-14.9V/μs, respectively. Thisproposed opamp can reach rail-to-rail signaling, either.