Sub-THz SiGe HBT Cascode Power Amplifiers with Capacitive Feedback and its use in a Supply Modulated RF Transmitter Front-End

An alternate approach to boost the gain of SiGe HBT amplifiers operating above 250 GHz using capacitive feedback is demonstrated in a four-stage cascode power amplifier. Unlike the inductive feedback for MOS cascode amplifiers operating at ~200 GHz, the capacitive feedback provides higher gain (higher by ~4 dB) at frequencies higher than ~250 GHz due to the inherent parasitic inductance of interconnect between the collector of common-emitter (C-E) stage and the emitter of the common-base (C-B) stage of HBT cascode amplifiers. A PA using this technique is integrated in a transmitter with its fourth stage supply-modulated using a high-speed low-dropout regulator (LDO) to boost its peak modulated power by 3X and RF-to-DC efficiency by 2X at 250 GHz.