Symmetrical Load Ring Oscillator Design

[ccw27]

Can anyone offer me tips on how to size the transistors for the buffer stage shown, particularly the PMOS transistors because I am having a hard time keeping PMOS load in triode and PMOS diode in saturation.

I am trying to build a four stage ring oscillator. From my calculations I need at least a Gain of 1.4 in each delay cell and I am targeting an oscillating frequency of 250 MHz.

Any help or links or papers will be appreciated.

Thanks

 

[kkoko]

you can make bufer transistors with smaler width because you will increase parasitics capacitance and this can make frequency shift