NMOS LC tank with top tail vs bottom tail

[casual]

Hi,

I have seen two types of NMOS LC tank circuit. One with top tail (current source) and the other type, the current source (tail) at the bottom.

what are the advantages and disadvantages between them?
any related papers discuss this?

 

[Milad-D]

The first difference is that when the tail current is at top, the common mode dc voltage at each output node is determined by vgs and therefore you can design it in such a way that each node has a dc voltage of vdd/2. When the tail current is at bottom, the dc voltage at each node is vdd.
When you want to use accumulation mode mos varactors, having a vdd/2 at ouput nodes is desirable. But if you want to use diode varactors, the later is better ( for higher tuning range).

Also, when you use bottom tail current, you can get more swing from the circuit ~vdd at each node. But in another one ~ vdd/2.

And the last comparison:

When top tail current is used, the noise of the tail current has more effect on the phase noise of the oscillator.