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MOS in digital design

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Full Member level 2
Nov 30, 2005
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Are MOS transistors in digital circuits operated mostly in the linear region? If yes, what are the possible reasons?



No.. MOS Digital Circuits operate mainly in the Saturation and Cut-off Regions. Except for the Pass Transistor Logic where it might operate in the linear region.

We use the MOS circuits mainly in the saturation and cut-off regions because at any point in time, we want the output or the voltage at an internal output node be connected to either of the supply rails with a very low resistance to insure strong logic output and increased noise margin.

Sometimes, we have to make trade-offs, but mainly, even in ratioed logic, we want a strong output. So, one of the MOSFETs used must be in saturation to insure minimum impedance.

i ever meet the situtation mentioned by you ,but in analog circuit which digtial circuit function were implemeted by analog structure .But in pure digital circuit mos
can't work in linear-region because digtial circuit only need voltage level .

I think most of the MOS Digital circuits work in linear (sometimes saturation) and cut off region. As we use MOS digital circuits in ON and OFF, we can draw the analog of a switch. If switch is OFF, implies there is infinite impedance (open ckt.) between two nodes, which is cut off region. If the switch is ON, there is zero impedance between two nodes (short ckt.) , which is linear region. As the fact is saturation region has much more impedance (in mega ohms order) than linear region (in kilo ohms order). Therefor switch ON can be better modeled as linear region.
Does this make sense?


definition of "linear" in linear circuits and digital [MOS] cicuits have not same meaning.
as above guys said, in MOS when turn on it going to saturation or linear section.
it depend on VDS.
when VDS is smaler than VGS it is in linear section.
but it mean it is in switch on state!
and when VDS > VGS it is in saturation!!! but it means MOS is in current source state.
saturation , pinch off , linear definitions are respect to transistor physical behavior.

MOS in digital circuits is operated only in cut off or saturation regions. In linear region we have high resistance which is necessary to have gain. Thus for operating in analog ckt. MOS should be used in Linear region while in digital circuits we need very low resistance and no gain, hence should be operated in cut off or saturation regions in order to have full voltage swings.

I dont agree with elec_student
all his/her description is wrong !!!!!!!

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