Is this Mosfet in saturation?

Thread Starter

HarrisonG

Joined Aug 1, 2016
73
jM2NO.gif
Hello guys! In n channel mosfets saturation occurs when VDS>VGS-VTr
. When you turn on this mos nice, its drain to source resistance is less than one ohm. Then most if not all of the voltage will drop across RL. That means the drain will be near ground level. Then VDS wont be >VGS-VTr. In this circuit the mos can never be saturated, right? But if I were to replace the load with a led with say 4V turn on voltage, and I increace the supply voltage, since the voltage across the diode will always be 4V, there will come a moment when the drain voltage will be big enough and pinch off will occur and the mos will enter saturation. That right?
PS: Dont mind the red wire that is connecting the gate to ground, I didnt noticed the picture included it
 
Last edited:

crutschow

Joined Mar 14, 2008
27,010
Why are you concerned about the saturation point in that circuit?
Aren't you just using it as a switch?
Yes, if you increase the voltage (with or without the LED), you will reach a point where the MOSFET goes into saturation.
It's a matter of how much current goes through the MOSFET compared to Vgs.
 

Thread Starter

HarrisonG

Joined Aug 1, 2016
73
Why are you concerned about the saturation point in that circuit?
Aren't you just using it as a switch?
Yes, if you increase the voltage (with or without the LED), you will reach a point where the MOSFET goes into saturation.
It's a matter of how much current goes through the MOSFET compared to Vgs.
But the requirements for saturation is that VDS is large compared to VGS and VTr and if the load is some kind of resistor, with RDSon less than 1Ohm, that resistor is basically shorted to ground. That would mean all the voltage drop is across him and VDS is almost 0. How can it be saturated then?
 

crutschow

Joined Mar 14, 2008
27,010
You don't want the MOSFET in saturation if it's being used as a switch.
Only BJT's should be in saturation when used as a switch.
The definition of saturation for a MOSFET is different from that for a BJT (for no good reason other than to confuse the designer).
When a MOSFET is used as a switch it's in the linear region, not the saturation region, as shown below:

upload_2016-10-28_15-50-25.png
 

anhnha

Joined Apr 19, 2012
881
But the requirements for saturation is that VDS is large compared to VGS and VTr and if the load is some kind of resistor, with RDSon less than 1Ohm, that resistor is basically shorted to ground. That would mean all the voltage drop is across him and VDS is almost 0. How can it be saturated then?
RDSon and RD forms a voltage divider. As you increase VDD, there will be the time that voltage across RDSon is larger than overdrive voltage VGS - Vth and the transistor will be in saturation.
Also RDSon is small but with large enough VDD, the voltage across RDSon can be large and the transistor is in saturation.
 

Thread Starter

HarrisonG

Joined Aug 1, 2016
73
RDSon and RD forms a voltage divider. As you increase VDD, there will be the time that voltage across RDSon is larger than overdrive voltage VGS - Vth and the transistor will be in saturation.
Also RDSon is small but with large enough VDD, the voltage across RDSon can be large and the transistor is in saturation.
Thanks! That was exactly what I was thinking. It's nice to hear confirmation.
 
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