MOS as a voltage controlled current source

I have just come across the condition for MOS voltage controlled current source below and I am a bit confused.
Could you explain why not Vds > Vgs - Vth?

I think you made a mistake, have a look:

vds-(vgs-vth)>=vsatmin == vds >= (vgs-vth)+vsatmin

Hi,
The one you've written is what you have from the excerpt above.
However, in all books I've read the condition for MOS transistor in saturation is Vds > Vgs - Vth.
That is what I am confused.

Now i got and you're right. Which book is this?

MOS in saturation condition:
https://www.google.com/search?q=mos...e=utf-8#q=mos+transistor+saturation+condition
And the above excerpt is in a paper I got from IEEE.

And the above excerpt is in a paper I got from IEEE.

Click to expand...

Can you tell me the title?

vds >= vgs-vth is ideal MOSFET characteristic

vds >= (vgs-vth)+vsatmin is apparently admitting an additional voltage drop, e.g. due to parasitic resistance, a step towards real MOSFETs

What's the problem with it? To assume an ideal MOSFET set vsatmin = 0.

johnjoe said:

Can you tell me the title?

Click to expand...

Hi,
It is "The Sizing Rules Method for Analog Integrated Circuit Design".
If can send you a copy if you can't get it.

FvM said:

vds >= vgs-vth is ideal MOSFET characteristic

vds >= (vgs-vth)+vsatmin is apparently admitting an additional voltage drop, e.g. due to parasitic resistance, a step towards real MOSFETs

What's the problem with it? To assume an ideal MOSFET set vsatmin = 0.

Click to expand...

I understand but is there a detail treatment of that? I'd like to read more about it.

When Vgs is > Vth it means above the threshold of saturation and must have a supply of current available between Vds so thus Vds is not zero and the voltage drop is the standard spec characteristic of RdsOn.

When Vgs = max i.e. >> Vth then it is fully saturated and reached the maximum conductance or best case low RdsOn.
In between these levels it is actually a Voltage controlled Resistor and thus with additional load resistance and a voltage source becomes a voltage controlled current source or sink depending on whether you are using Pch type or Nch type respectively.

Old MOSFETS were high threshold while modern MOSFETS are low Vth and often called Logic Level drive when used as switches.

In this case you are analyzing the "quasi" linear operation as a VCCS near or above Vth.

Of course it doesn't have the ideal qualities of a true current source since it is not infinite impedance and drops rapidly above threshold, but it does modulate the current.