Dear All, I have attached the three diagram of VCO .No.1 is VCO for NMOS only tailed bias VCO. No2 is differetial VCO . No3. is Oscillator from cross couple inverter. Question: why they use 4 terminal NMOS as varactor ??what is the advantage of this .? Question :In No.3 , I know the top PMOS is cross coupled and what is the function of NMOS below. It look like MEMORY circuit DRAM .Why he choose that layout and advantage? Thanks..