A comparison of npn vs. pnp SiGe HBT oscillator phase noise performance in a complementary SiGe platform

A comparison of cross-coupled oscillator performance is presented for a high-speed, complementary SiGe (C-SiGe = npn + pnp) BiCMOS platform with matched npn and pnp performance. Results show with all factors held constant, the pnp-only VCO design holds an advantage in white FM phase noise over its npn counterpart at constant current; a counter-intuitive outcome if one only considers absolute base resistance. The reduced noise in the pnp-only VCO is shown to stem from the device's decreased β compared to the npn, which reduces conversion of the thermal noise associated with the base resistance to the output node. Understanding this phase noise reduction effect has potential to influence oscillator device selection in any given technology family.