Is the MOS always in saturation mode

[Osawa_Odessa]

Is this configuration always in saturation mode no matter how small/large Vgs and Vds are?
I think that because it is connected to current source that make Ids = I = const

 

[ArNvC]

"saturation" in mosfets means that change in VDS will not produce significant change in the Id (drain current).

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"saturation" in mosfets means that change in VDS will not produce significant change in the Id (drain current).

 

[Usama Siddiqui]

i think its in saturation,can u share some more detail of this circuit?

 

[rahdirs]

I don't think that it will always be in saturation .It would also depend on the load connected to the output node.

 

[godfreyl]

Even with nothing connected to Vout, it will not always be in saturation. In fact, for any given current, you will have to choose Vgs very carefully to get it into saturation.

 

[Osawa_Odessa]

Even with nothing connected to Vout, it will not always be in saturation. In fact, for any given current, you will have to choose Vgs very carefully to get it into saturation.

My intention is that nothing is connected to the output. Can you explain it in detail? Because it seems that Ids = I = const always!

 

[godfreyl]

Because it seems that Ids = I = const always!

Ids is constant, but that doesn't mean the MOSFET is in saturation. When Vgs changes, Vds will change, but for a wide range of Vgs the MOSFET will be in the linear region.

Can you explain it in detail?

Here's some idealized curves (swiped from Wikipedia). Consider what happens if Ids is set to 20 units and Vgs is varied.



If Vgs is set to Vth + 7V, Vth + 6V or Vth + 5V, Vds will be approximately 1.6V, 2V or 2.7V respectively. In each case the MOSFET is in the linear region, not the saturation region, as shown by the red dots.

If Vgs is set to Vth + 4V or lower, Vds will be extremely high, and the MOSFET will probably suffer breakdown.

The thing to remember is that if you look at the picture, the three red dots are all in the linear region, even though the currents are the same.

 

[Usama Siddiqui]

thats why i asked for the complete reference as by looking at this configuration you cannot determine exactly what it could be!

 

[rahdirs]

Consider what happens if Ids is set to 20 units and Vg

that is if current source is 20 units,if current source is 50 units & if output is open,then it has to be in saturation right?
So,it would depend on the current-source & output load.

 

[sarge]

Hi, i think that there we may have 2 ways:

1. Ideal current source in drain.
There are transistor in saturation until Vgs > Vth, because any changes in Vgs > Vth doesn't change drain current (ideal source). If Vgs goes below Vth we have open circuit and therefore no current.

2. Non-ideal current source (transistor current source, current mirror etc)
For this situation current of mosfet in saturation:
\[I=\frac{K}{2}\cdot{(Vgs-Vth)}^{2}\cdot(1+\lambda * Vds)\],
hence if current is constant and if Vgs goes to low, then Vds must goes to high to provide constant current.
If so, from some moment our current source begins to give smaller current.