[SOLVED] determine perfect value of W/L for designed circuit

[deepsetan]

Hi guys,

Currently, I've been working on with my design which is a MEMS oscillator that uses Silterra 0.13um technology. This is my parameter that I've calculated from the simulation (Id-Vds,Id-Vgs) and also some information from 0.13um design kit. From my simulation, I got Id=22uA. I actually obtained the Id from Id-Vgs simulation.

Vth=0.546V
Vdd=3.3V
Vds=0.5Vdd=1.65V
Vgs=1.546V
Cox=1.72m
Un=0.050542

From all of this information, how can I determine the perfect value of W/L for my design?. I knew that in order to determine the value of W/L, we can just use Id=K(Vgs-Vth)^2 (saturation formula). From the book, it says that if we use the minimum length, it will produce high speed design. Does it possible for my MEMS in real fabrication process? Can I use that minimum length for my design? Or I just can pick up any value greater than the minimum length?

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this is my simulation for Id-Vgs and Id-Vds

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this is my circuit that I've been working on.

 

[erikl]

In analog circuit design you shouldn't use minimum dimensions - if not necessary for speed reason -, but a factor of 2 .. 10 larger, depending on matching accuracy needs. Hence for the two PMOSes M2 & M3 - which should match their drain currents quite well - I'd recommend at least L=5*Lmin. For other transistors - which do not have to match their drain currents - a factor of 2 .. 3 should be sufficient.

 

[deepsetan]

thanks man, now i'more understand. From the kit,it says that the Lmin=0.28u. Let say I'm using L=Lmin*5=1.4u. For M2 and M3 transistors, the length should be the same because to maintain its drain current right? And for the others transistors such as M1 and M4, I will use any value such as factor of 2 or 3. Does it mean that if I choose any value for L for M1 and M4 then it will not effect the drain current for M2 and M3?

 

[erikl]

Let say I'm using L=Lmin*5=1.4u. For M2 and M3 transistors, the length should be the same because to maintain its drain current right?

Yes. And keep your original W/L ratio.

And for the others transistors such as M1 and M4, I will use any value such as factor of 2 or 3. Does it mean that if I choose any value for L for M1 and M4 then it will not effect the drain current for M2 and M3?

Right! Just keep the W/L ratio! And simulate your whole circuit.

 

[deepsetan]

thanks!! i really appreciate your answer..