Best practices to ensure the stability of sige HBT cascode low noise amplifiers

This work provides a detailed examination of the stability of SiGe cascode low noise amplifiers (LNAs). The upper base is identified as a problematic node for stability. S-probe simulations are used to extract reflection coefficients internal to the circuit and provide insight on how to improve the stability of a cascode amplifier and thereby establish “best practices” for designers. These techniques are incorporated into a cascode LNA design fabricated on a 180 nm, 150 GHz f_T SiGe BiCMOS technology. The measured SiGe LNA has a gain of 16.5 dB and a noise figure of 2.1 dB at a center frequency of 9.2 GHz. A series of measurements using tuners at both the input and output confirm the LNA is stable for all impedances covered by the tuners (|Γ| <; 0.8).