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# How VDS is calculated for stacked MOS transistors?

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#### Blackuni

##### Full Member level 4
Hi all,

How VDS is calculated for stacked MOS transistors??

Thanks,

Re: VDS

Hi,

any idea??

thanks,

VDS

can u plz expain ur question?

Re: VDS

Hi,

thanks,

If u stack 3 transistors. How the drain and source voltage of the middle transistor is arrived..

If u do .op analysis in hspice it will print vds value for each transistor. How this value is obtained.

Thanks,

Re: VDS

any inputs????

VDS

It depends on Vgs and Vdd. It is realized by Newton-R iteration algorithm

Re: VDS

With a stacked MOS transistors you must take care only to the fact that all transistors must be in saturation (for example if they are used like a current mirror).

For example for a Nmos structure:
Start with the one from bellow. The one that is connected directly to GND.
Choose L,W,Vds (base on the noise/matching/current). After that go to the other transistor.
Choose again the L, W and Vds. From equation you will obtain the gate voltage. Because usualy the bulk of the second transistor is connected to the same bulk of the first transistor you will notice that Vds is different for the same size.
And repeate the same procedure for the 3rd transistor.

Some remarks:
If this structure is a curent mirror you should set the noise/matching with 1st transistor. The other transistor that is on top of that is used just for Rout increase by aprox (1+gm2*Ro2). So for the second transistor you can use a bigger W/L ratio. This will improve your output voltage range.

### Blackuni

Points: 2
Re: VDS

Hi SwordFish

Thanks a lot for your inputs.

Is there a equation to show relation between VDS and dimension of a MOS(W & L)

Thanks,

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