I want to design an Output Buffer Amplfier for measureing the phase noise and frequency of the VCO. The output of the amplifier will be connected to 50 ohm probe for measurement. This means a load of 50 Ohm connected at the drain of output amplifier.
My questions are
1) What should be the schematic of the whole buffer amplifier. I think it should be
an intercconection (-->) as follows.
VCO output  --> Common Gate --> Common Drain ---> Common Source--> 50 Ohm Resistor at the drain of common source stage

2)How must should be the amplitude for phase noise and frequency measurement and how to design such an amplifier

purplewolf,

I would not load down the vco with a common gate amplifier (implies that the sources are connected to vco) this will de Q your VCO,  use a common source.  Also I am not sure there is such a thing as a common Drain amplifier or at least not one with any gain.    :)

Any phase noise measurement setup will have its own input referred noise so the amplitude you need to drive your signal needs to clear the noise floor with margin for the phase noise you are trying to measure.  at the same time not introduce noise above the vco's.  A common source stage with resistive load. provided that it switches fast enough will usually do the trick provided that the 1/f from the bias is minimal since it will be mixed up to the LO.  Finally the differential to single ended can be done with a totem pole stage.  

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