tail current's action in LC VCO?

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Hi guys,
Would you tell me what's the tail current's action in LC VCO?

In some papers such as Marc Tiebout(JSSC July 2001),there are no other transistors besides LC tank and negative resistance.

In some papers such as Jan Craninckx(JSSC May 1997),there are two)transistors which provide tail current.What's the difference?

Thank you very much!
 

If no current source is used ,the output oscillation swings between -Vdd and Vdd. If a current source is added the oscillation amplitude will be equal to IbiasXRtank (for IbiasXRtank<Vdd) and Vdd elsewhere. Thus output amplitude can be controlled better and thus improving VCO linearity is possible.
Moreover , Hegazi et al. in "A Filtering Technique to Lower LC Oscillator
Phase Noise", JSSC dec. 2001, stated that the role of current source is to avoid resonator loading at zero crossing (since the input resistance of current source is high).
plz tell me if you need this paper
 

Thank u very much!
I can download this paper here.
Good luck to you!
 

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