A 173 GHz Amplifier With a 18.5 dB Power Gain in a 130 nm SiGe Process: A Systematic Design of High-Gain Amplifiers Above $f_{\max }/2$

A novel theory of stability for two-port networks is developed. Using this theory, a new method of designing amplifiers with a high-power gain working close to the maximum frequency of oscillation (f
_max
) is proposed. Contrary to the existing amplifier design methodologies, in this method, the transistor capability of power amplification is fully utilized. This becomes more important at frequencies close to the f
_max
where having a high-power gain is challenging due to the degraded activity of the employed device. The proposed method considers the modeling errors and process-voltage-temperature variations of the employed components in the design stage to ensure that the fabricated amplifier will be stable with a decent power gain even if the worst case variations and modeling errors happen. To show the feasibility of the proposed approach, a three-stage amplifier at 173 GHz, using bipolar junction transistors from a 130 nm SiGe process, is designed. The fabricated amplifier has a maximum measured power gain of 18.5 dB at 173 GHz. A similar three-stage amplifier using the same transistors with the same bias would give a maximum gain of 6.8 dB in simulation, assuming perfect lossless conjugate matching at input, output, and between stages. So it is clear that the fabricated amplifier achieves a significant improvement over the power gain.