design of a ring oscillator

hi
i would like to design a five stage ring oscillator using CMOS differential amplifier by cascading .But the problem exist is when the differential output voltage is Vdd then at this point the load pmos transistor(which is controlled by a contolling voltage) will enter into the saturation stage but we are trying to model it as a resistor to obtain a good tuning range.
so please help me to design the differential circuit.

jithin, cmos load acts as resistor only in triode region, so when output vlotage is Vdd , then try to make Vgs i.e the controlling voltage more than Vdd by Vth.so that it will remain in triode region. i know if switching mechanism is used circuit will be really complex.

in this regard, cmos analog design by behzad razavi can help u.
hope u will get the solution soon.
i will try to solve it.if i get, i will reply surely.

Do you have a circuit that you can upload. I am not sure I know what you are asking exactly.

thank you praveen ,eventhough I have solved the problem by putting a pmos in parallel with another pmos in which gate and drain are shorted ie a diode connected pmos.
but during my analysis another problem risen such that the outputs of a differential amplifier is connected to the next stage and in anlysis we consider that only one mosfet will go in saturation and other will turned off due to more negative ,say if Vin1 is more negative than Vin2 then transistor M1 will turned off and M2 will allow the whole currrent to flow but my doubt is that evenif Vin1 is more negative than Vin2 but Vin1 is greater than its threshold then how it will turned off .

First of all let me thank you for your kindness
my doubt is that when we consider a ring oscillator made up of differential amplifier ,by cascading it, then in analysis we thought that when Vin1 is more negative than Vin2 then we consider transistor M1 will turned off and M2 will allow the whole current to flow,evenif Vin1 is more than its threshold so simply my doubt is that how it will happen.

Here is a picture