These resistance values seem to be dictated by termination considerations.


Bupesh

Raduga5,
 Peter is right, the resistance is dictated, generally by the application.  More specifically, your load resistance must match the impedance of the transmission line you are meant to drive, and terminate it (as Bupesh implied).
As Peter noted, video applications use 75ohms T-Lines (the type of coax or RCA cable that conveys the signals), most backplanes use 50 ohms trace routes, some chip to chip interconnect use 65 ohms (easy to design in a package).  If you do not impedance match the T-line line of your application, welcome to a signal integrity nightmare, and likely a failed design.
-Stephen

Hi Raduga,

Is this load device (MOS) on-chip? If so, then you may not have to worry about the termination
issues and can choose any convenient value, dictated by other design constraints (speed,power etc.).

Termination is generally required when interfacing to and from the external world. Tom Lee covers
the topic in his book on CMOS RF IC Design in the chapter on "The Smith Chart and s-parameters".

Regards
Vivek

Thanks Vivek and Bupesh ..

Bupesh, the modulator is a RF modulator module .. and there could be a filter before the modulator  
.. and i just know that the input it gonna see is  the gate of a MOSFET .. so thats the problem ..  
the typical block diagram of the system is like given below .