A 1.5 V Low-power Rail-to-Rail CMOS Operational Amplifier with Constant-gm

Operational amplifier is a basic device, most widely used in the analog integrated circuits. With the reduction of the system power consumption and with a low power supply voltage, the traditional operational amplifier can not meet the requirements of the large input common-mode range and the wide output swing under a low voltage. The rail-to-rail operational amplifier can meet these requirements. But the trans-conductance of the traditional rail-to-rail operational amplifier is not constant. A 1.5 V low-power CMOS rail-to-rail operational amplifier is designed in this paper. A minimum current selection circuit is adopted in the input stage to achieve constant-gm so that the trans-conductance is not independent of the ideal square law model, and the MOSFET transistors canwork not only in a strong inversion region but also in a weak inversion region. A feed-forward class AB output stage is used to ensure that the current of the output transistors to be controlled precisely and the output swing reaches rail to rail. In the operational amplifier, an improved compact circuit structure is adopted to reduce the noise and the offset of the operational amplifier. The circuit is simulated by using SMIC 0.18 μm process model and Hspice simulation software. The simulation results show that the DC gain is 83.2 dB, the unity gain bandwidth is 7.76 MHz, and the phase margin is 63° when the circuit is driving a load capacitance of 2pF and a load resistance of 10 kΩ. The input common-mode range and the output swing both reach rail to rail, and the variation of the gm is only 2.49%. The common-mode rejection ratio and the power supply rejection ratio of the circuit are both high. The circuit operates normally in a low voltage of 1.5 V, and the static power consumption is only 0.24 mW.