[SOLVED] Overdrive voltage amount ?

[DharmaSlice]

Hi,

When designing a current mirror in tsmc18 process how much of an overdrive voltage (Vgs - Vt) should you have in the typical corner to ensure it stays in saturation when running corner simulations. Supply off 3V3.

 

[kemiyun]

Hello,

If you mean that you are biasing your current mirrors with a constant voltage they will not flow the current you want in corners. Using a biasing cell with a current reference is required. If you use a biasing cell with a current reference what you need to consider is not the saturation but the range of the drain voltage that is keeping the channel pinched off at all possible conditions. You can increase this range by lowering the overdrive voltage. However the overdrive voltage is crucial for high Monte Carlo performance. The statistical variation effects the current mirrors in a such way that they will deviate very much if their overdrive voltage is too low, regardless of their W over L ratios.