How to confirm the W/L of a transistor in a circuit?

[honsong]

How to confirm the W/L of a transistor in a circuit?thak you!!!

 

[Teddy]

interesting question- what do you mean by that???

 

[honsong]

I want to simulate a circuit,but I do not know how to choose the best W/L of a transistor.

 

[srijesh]

I think you need to get your hands to a Circuit Design book. For starters, you decide this by keeping things like Current, process, and transconductance in mind..

 

[honsong]

Thanks for your reply!
Can you introduce some books about that?

 

[Frank_H]

The best W/L relation depende on your aplication.
By increasing W you will reduce the On resistance resulting in a reduction of the delay through the pass gate ( NMOS or PMOS) but in the other hand the Cox capacitance will increase, clearing this capacitance will be a problem so you need a compromise.
Another consideration is concerned to the expenses feeds, you must considere that every milimeter ^2 is very expensive.

P.D You need to keep L to the minimun value if you want a fast response of your device.

Regards.

 

[lianghuamo]

In general use, I design the W/L dependent on the Vdsat and the process.

 

[test_out]

In general use, I design the W/L dependent on the Vdsat and the process.

Can you explain more detail?

 

[sunjiao3]

In general use, I design the W/L dependent on the Vdsat and the process.

Can you explain more detail?

I think lianghuamo wanna get the correct operation point of the transistor. For example, with the drain current setted, the bigger the W/L is, the lowwer the Vgs-Vth, which is Vdssat would be. The more headroom one will get to insure the transistor works in saturation area.

 

[lianghuamo]

In general use, I design the W/L dependent on the Vdsat and the process.

Can you explain more detail?

I think lianghuamo wanna get the correct operation point of the transistor. For example, with the drain current setted, the bigger the W/L is, the lowwer the Vgs-Vth, which is Vdssat would be. The more headroom one will get to insure the transistor works in saturation area.

So we need a proper vdsat according to process. Different process has different proper vdsat.

 

[MSSN]

Razavi's book esign of analog CMOS integrated circuits is a very good book to start with

 

[avlsi]

just start writing your current and voltage equations,
You will automatically,get (W/L)

 

[jiveshgovil]

I want to simulate a circuit,but I do not know how to choose the best W/L of a transistor.

I can give you a quick way out. In most case W/L is kept as 6. So you can select W=60,and L=10. That works in most cases. And if the out put is somewhat as expected, then you can fine tune the value of W/L such as making W=120 and L=20. So go on.