Analysis and Design of a 17-GHz All-npn Push-Pull Class-C VCO

A push-pull oscillator topology that uses only one type of active device is proposed in this article. A magnetic transformer is leveraged to set positive feedback around a common-collector differential npn transistor pair, implementing the push-pull operation. This results in half the bias current for a given amplitude of oscillation, compared to more standard oscillator topologies. A thorough phase noise analysis of the circuit is carried out, emphasizing the crucial role of the magnetic transformer in the circuit operation and noise optimization. Proof-of-concept prototypes implemented in a 130-nm SiGe BiCMOS technology operate at 17 GHz and show a phase noise as low as -116 dBc/Hz at 1-MHz offset, while drawing 13.7 mA from the 3.3-V supply. The tuning range is 15%. While the circuit is demonstrated in SiGe BiCMOS technology, it lends itself equally well to implementations in other technologies where only one fast device is available, such as SiGe HBT, InP HBT, and GaN HEMT.