What are the pros and cons of top biased and bottom biased LC VCO structures?

[davidwong]

There is a few basic structure of LC VCO.
1) Top biased NMOS
2) bottom biased NMOS
3) Top biased PMOS
4) bottom biased PMOS
5) top biased NMOS and PMOS
6) bottom biased NMOS and PMOS.

According to the paper "Design Issues in CMOS Differential LC Oscillators", JSSC, May 1999 and Feb, 2000.
5) and 6) would appear to achieve better phase noise.
But Top biased or bottom biased is better for phase noise?
What is the adv. and disadvangtage for top biased vs bottom biased?

 

[hrkhari]

Re: LC VCO

Hi:

I understand that LC-VCO in complementary form gives a higher gm for the given current consumption, moreover is draws less current compared to single cross coupled -gm LC-VCO.

In a top biased pMOS VCO the loading parasitic of the resonator is referred tovGnd hence enhances the loading performances, whereas top biased nMOS based LC-VCO gives higher speed of operation but with higher loading resonator.

Hope it helps

Rgds

 

[davidwong]

Re: LC VCO

to be honest,
I am always feel a little bit confusion.
PMOS will introduce a lesser phase noise due to flickering noise
Higer current will cause more flickering noise too.
NMOS introduce a higher phase noise but it have higher gain.
Thus I think the phase noise will be optimize due to the above factor and so on.
It seem to me, it is difficult to say which structure is good for phase noise performance.
it will totally depend on the process.
as the difference of 1/f noise between PMOS and NMOS may be difference in different technology.