Nonlinear Capacitance Effect on Stability and Stabilization of SiGe Power Amplifiers for 17.3–21.2 GHz SATCOM

This study investigates the impact of nonlinear capacitances of SiGe-HBT technology on the stability of power amplifier (PA) designs for 17.3–21.2 GHz satellite communication applications. Various nonlinear capacitances are considered and compared. The pronounced influence of the nonlinear base-collector diffusion capacitance (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${C} {_{\text {bc,D}}}$ </tex-math></inline-formula>) on large-signal stability of PAs is analyzed and derived mathematically using simplified circuit models. For more complicated circuitry such as transformer-based PAs, advanced stability analysis techniques based on harmonic balance are reviewed, clarified, and applied to detect the large-signal instability in simulations and to design stabilization networks. Two PA examples with different damping characteristics are designed and evaluated. Simulation results of the unstable PA example (PA_v1) validate the impact of the nonlinear <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${C} {_{\text {bc,D}}}$ </tex-math></inline-formula> on stability. In addition, a strong agreement between the measured spectra of the PA_v1 and simulations validates the efficacy of the adopted stability check methods. Moreover, the stable PA (PA_v2) achieves a saturated output power (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${P} {_{\text {sat}}}$ </tex-math></inline-formula>) of 22.8 dBm with power-added efficiency (PAE) of 37.7% at 18.8 GHz and 22.8 dBm with PAE of 37.2% at 19.25 GHz. At a noise power ratio of 15 dB, it delivers 18.6-dBm output power (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${P} {_{\text {out}}}$ </tex-math></inline-formula>) with greater than 20% PAE, at both 18.8 and 19.25 GHz. Meanwhile, for specific DVB-S2 16-APSK signal with 100 MBd, roughly 22-dBm average output power (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${P} {_{\text {avg}}}$ </tex-math></inline-formula>) with greater than 32% PAE is delivered, at EVM level of −16 dB (15.8%).