The degrading effect of the circuit reactances on the maximum IIP3 (third order input intercept point) in the conventional derivative superposition (DS) method is explained using Volterra series analysis. The effect on the NF of the subthreshold biasing of one of the FETs in the DS method is also explained. A modified DS method is proposed to increase the maximum IIP3 at RF. It was used in a 0.25 μm Si CMOS LNA designed for cellular CDMA receivers. The LNA achieved +17.2 dBm IIP3 with 15.5 dB gain, 1.6 dB NF and 9 mA at 2.6 V power consumption.
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