Third-order nonlinearity vs. load impedance for CMOS low-noise amplifiers

The linearity of two 90 nm CMOS low-noise amplifiers has been measured and analyzed. The analysis is based on Taylor series expansion of simulated I–V characteristics. The two amplifiers are cascode amplifiers with transistors of the same size but with different loads. Even though the center frequencies of the amplifiers are as high as 15 and 20 GHz, respectively, the measured results correlate well with the low-frequency-based estimation of linearity. The analysis shows that for a low load impedance, the dominating source of nonlinearity is transconductance, while for a high load impedance the nonlinearity of the output conductance instead dominates.