Research And Design Of Low-Voltage And Constant-g_m CMOS Operational Amplifier

Operational amplifier is an important and fundamental module in analog integrated circuit.Because many electronic products are powered by portable batteries,low-voltage and rail-to-rail circuits have became the inevitable trend of analog circuit design.When supply voltage is reduced,the input and output signal of the amplifier are reduced continuously,which affect the normal work of the whole circuit.In order to improve the signal-to-noise ratio of operational amplifier,input and output dynamic ranges of operational amplifier are urgently needed to reach rail-to-rail.Based on the study of operational principle of the domestic and international rail-to-rail operational amplifiers,this paper designs a 1.2V rail-to-rail three stage operational amplifier with constant transconductance.The biasing circuit of operational amplifier uses cascode current source by self-bias resistor,which provides the suitable quiescent points for all modules of the operational amplifier.The input stage adopts the complementary differential pair structure based on the improved maximum current selection circuit.It can enable the common mode input range to achieve rail-to-rail and ensure the constant transconductance of the input stage.The input stage also uses the folded-cascode structure to improve the gain of the input stage.The output stage uses the Class AB buffer with the adaptive load to achieve the quiescent current control.The output voltage swing can achieve rail-to-rail.The phase compensation circuit uses nested Miller compensation to ensure that the entire circuit is in a stable state.In addition,a bandgap voltage reference is designed in this paper.It realizes high-temperature curvature compensation by linearization compensation method,which can provide stable bias voltage for the operational amplifier.Based on the standard 0.18?m CMOS process,the bandgap voltage reference and operational amplifier are simulated and analyzed by using the simulation software.The simulation results show that the temperature coefficient of bandgap voltage reference is 6.95ppm/?,it has good temperature performance.With the parallel load of the capacitance 100 p F and the resistance 2k?,the dynamic ranges of the input and output of the operational amplifier can achieve rail-to-rail,transconductance rate of change is 4.6%,the open-loop gain can reach 109.6d B with unit gain bandwidth of 3.3MHz,phase margin is 61.1°,the common mode rejection ratio is 118.6d B,simulation results meet the predefined specifications.