Design and Analysis of a Cascode Bipolar Low-Noise Amplifier With Capacitive Shunt Feedback Under Power-Constraint

A cascode bipolar low-noise amplifier (LNA) with capacitive shunt feedback has been developed to present a solution for simultaneous noise and power match when the real part of the optimum source impedance is not 50 Ω in order to keep high current density under power constraint. The proposed LNA also has the capability for the simultaneous improvement of noise figure (NF) and linearity. In addition, we analyzed and verified that the second-order interaction, which affects the third-order nonlinearity, becomes less sensitive to the low-frequency input termination at higher bias currents. The possibility of removing the low-frequency LC trap is investigated based on this analysis. We also show that LNAs with smaller base and/or smaller emitter resistances require lower source impedance at low frequencies to improve linearity by the same amount. Finally, prudent layouts for improving the performance of the LNA are considered. Eleven design examples of the proposed LNA, which individually operate at 880, 1575, or 1960 MHz, are fabricated in a low-cost 0.35- μm SiGe BiCMOS process to verify the design and analysis experimentally. The fabricated LNAs have excellent performances, especially in NF. For example, the 880-MHz LNA has an NF of 0.9 dB, a power gain of 16 dB, and an IIP₃ of +14 dBm with current consumption of 11 mA from a 2.8-V power supply.