This paper presents a low-noise chopper operational amplifier using a lateral PNP input stage with bipolar junction transistor (BJT) current mirror base current cancellation. The BJT has a lower noise characteristic than the metal–oxide–semiconductor (MOS) transistor, where low-noise characteristics can be achieved by implanting the BJT to the input stage of the amplifier; however, the base current of the BJT input stage causes low input impedance of the amplifier. The BJT current mirror base current cancellation technique is implemented to enhance the input impedance of the BJT input stage by canceling the base current. BJT current mirror base current cancellation is implemented with a simple scheme using NPN transistors with deep n-well in a generic complementary metal–oxide–semiconductor (CMOS) process. For further noise reduction with the BJT input stage, a chopper amplifier scheme is adopted to reduce low-frequency components such as 1/f noise terms in the low-frequency range. The prototype chip is fabricated in a 0.18-μm CMOS process. The active area of the prototype amplifier is 0.213 mm2. The measured input-referred noise is 5.43 nV/√Hz. The measured input base current of the amplifier with base current cancellation is 67.971 nA. The total amplifier current consumption is 278.3 μA, with a power supply of 3.3 V.
A High-Gain and Low-Noise 0.9 µW Operational Amplifier