Effects of the nonlinearity of the common-gate stage on the linearity of CMOS cascode low noise amplifier

This work presents the effects of the common-gate (CG) stage nonlinearity on the linearity of the cascode amplifier. Conventionally, the CG stage is assumed as an ideal current buffer in the CMOS cascode low noise amplifier (LNA) design, but the analysis shows that the CG stage limits the linearity of the cascode LNA as the gain increases, due to the finite output resistance of the CG stage and the parasitic capacitance at the interstage node. Therefore, the simple linearization of the CS stage has difficulties to enhance the gain of LNA as the operating frequency increases. To confirm the analysis, a 2 GHz CMOS LNA was designed and the gain and the nonlinearity were measured. The measurement results show that there exists optimum load impedance to achieve the maximum OIP₃ of the LNA as expected by the analysis.