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srikanthp
Joined: 02 Jul 2005
Posts: 5
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 02 Jul 2005 6:18 how to choose length of the mos device in our design |
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hi i would like to clear a small doubt
suppose i want to design a CS amplifier with active load .
i will then use the eqn Id = 1/2*kn*(w/l)*(Vgs-Vt)˛
i would like to bias the ckt at 50 ľA and Vgs=1.5 V.
applying the eqn above i would get the value for w/l .
fine
just tell me how to choose the value for the length of channel first (l)
i find the simulation results vary as i change length keeping the aspect ratio constant .
pl help me out guys
lot of thanks in advance
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Btrend
Joined: 26 Dec 2003
Posts: 424
Helped: 55
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02 Jul 2005 12:05 Re: how to choose length of the mos device in our design |
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the equation u used was not the one used by the simulator.
the equation is more complex than the simple square law. actually, it will contain λ effect, body effect, .... , but from the simplest way (first order approximation) u should include the λ effect in ur equation. i.e. Id=1/2*kn*(w/l)*(Vgs-Vt)˛*(1+λVds).
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ezt
Joined: 14 Oct 2004
Posts: 105
Helped: 21
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02 Jul 2005 20:37 Re: how to choose length of the mos device in our design |
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Hi.
I agree with Btrend completely. But let me add something to make his word clear. λ is inversely proportional to "L", so with that equation he (Btrend) mentioned, Id will change with L, even if aspect ration remains constant. Besides, "L" specially in input devices will change input referred noise and in cascode devices (e.g. in a conventional cascode opamp) the magnitude of "L" will change ro but at the same time it will change the nondominant pole. So there are some trade-offs in choosing the right "L" for each device. Usually, we start our design with minimum "L" and if it is needed, we will increase it little by little.
Regards,
EZT
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