how to decide W and L size?

[tok47]

Dear ALL,

Recently, I am designing a simple CMOS op-amp.

Here come a question in my mind. How to set the size of the Width and Length of the transistor? Is it any rules or formulas to help us decide on the size of the transistor? How W and L effect the circuit?

Do u guys see this formula before? -- WL = (Vsigma / deltaVt)2
where Vsigma = mV *um


thanks

tok[/code]

 

[jimjim2k]

Hi

Please seek for transistor re-sizing in this forum and google.

tnx

 

[djalli]

When designing an opamp in die area CMOS size of transistors matter for example how you come around to create a dominant pole when negative feedback is amplied to opamp? You need it to stabilize the ouput by creating a capacitance,Miller effect, etc. between/or adding among transistors.

The formula you are showing is some geometric optimization scheme formula in CMOS. WL are two variables (dimension actually) you play in design you are using. I think WL should become an indipedent variable for each transistor. Then from there you decide voltages, drain currents, etc.
I do not know what scheme you are using but formula must belong to some geometric optimization scheme formula in CMOS technology.

 

[pixel]

As a first you have to be sure that you have set DC voltages dc point at the output, and that all transistors can work in region where you want.
If you need higher DC somewhere you can make bigger W for upper transistors.
For differential pair it is good to be fast and have high gm(L small) but also they should be matched( L large). If they are not well matched there will be be some voltage offset (Grey Meyer)
Similary, for current sources L should be larger.

 

[tok47]

How about the phase and gain margin? normally, in what condition (in term of W and L) will make the op-amp have not enough phase margin?

If in my simple op-amp has no capacitor, what will determined the slew rate of the op-amp? Is it cause by the paracitic capacitance of the output stage transistor?

 

[anadigi]

its good to know the WL produt from ur formula..
Now to get the W/L u hav to start from ur specification of Gain BW and Swing..
U can find an opamp sizing example in Razavi's chapter of OPAMP.