Performance Enhancement of Colpitts Oscillators by Parasitic Cancellation

The performance of conventional common-collector Colpitts oscillators is limited at higher frequencies due to the parasitic base-collector capacitance C_be and the base-emitter capacitance C_be. Due to the Miller effect, the parasitic capacitance C_be significantly reduces the negative resistance. A large collector inductor further reduces the negative resistance. This paper presents a solution based on parasitic cancellation to increase the negative resistance and improve the isolation between the RF output and the resonator. It is also shown that the technique allows designers to make a tradeoff between higher negative resistance and lower power consumption.