Analysis ofanActive Capacitance Circuit andItsApplication toVCO

Thispaperpresents anactive capacitance circuit using a LC series-feedback scheme. Thenoise performance is excellent andthegoodstability isaccomplished byadjusting the feedback elements. We analyze thecircuit performances indetail andapplyittoa voltage controlled oscillator. Theactive capacitance circuit provides negative resistance thatcan compensate notonlythelossofvaractor diodebutalsothe parasitic resistance ofinductor. A VCO employing theactive capacitance circuit isdesigned at2.179-2.191 GHzandthephase noise is-104.67dBc/Hz at10kHzoffset within thetuning range. IndexTerms- Active circuits, varactors, voltage controlled oscillators, bipolar transistors, negative resistance circuits.