# MOSFET linear and saturation region operation

• 11th July 2019, 11:38
BoopathiS
MOSFET linear and saturation region operation
Hi,

N channel MOSFET Saturation operation condition is Vds > (Vgs - Vth).
But in saturation region, MOSFET will act like closed switch. hence Vds is approximately 0V(Rds*I). Examble VGS = 10V and VTH is 2V, and if switch is closed means VDS= 0V, but above formula is not satisfied.

Anyone kindly clarify this ?

And How to drive the N channel MOSFET to act as variable resistor ?
I saw the condition Vds < (Vgs -Vth), but how to calculate Vgs for required resistor ?
• 11th July 2019, 13:28
wwfeldman
Re: MOSFET linear and saturation region operation
i searched on "international rectifier fet gate charge" (google)

look at

International rectifier power mosfet basics
figure 13
Power MOSFET Basics - Infineon Technologies

International rectifier application note AN-944
figure 5
Use Gate Charge to Design the Gate Drive Circuit for Power MOSFETs ...

those figures show the behavior of fet as a function of gate charge
there are clearly indicated regions - off, turning on, linear, and on (saturated)

to make the fet act like a variable resistor, apply a feedback circuit to control the gate based on the desired ???
sometimes it helps to add a resistor across the drain source to be the maximum resistance
then, as the fet moves through the linear regions, the resistance drops
the saturated fet gives the lower limit of the resistance
• 11th July 2019, 13:43
Chinmaye
Re: MOSFET linear and saturation region operation
MOSFET behaves like a resistor in linear region with resistance = 1/(un*cox*(W/L)*(Vgs - Vth)) for small values of Vds.
• 15th July 2019, 15:58
sherline123
Re: MOSFET linear and saturation region operation
Quote:

Hi,

N channel MOSFET Saturation operation condition is Vds > (Vgs - Vth).
But in saturation region, MOSFET will act like closed switch. hence Vds is approximately 0V(Rds*I). Examble VGS = 10V and VTH is 2V, and if switch is closed means VDS= 0V, but above formula is not satisfied.

Anyone kindly clarify this ?

And How to drive the N channel MOSFET to act as variable resistor ?
I saw the condition Vds < (Vgs -Vth), but how to calculate Vgs for required resistor ?

MOSFET acts like a switch and it is opened when vgs < vth but closed when vgs > vth.
From simple MOSFET equation, ignoring body effect and etc.
When Vds < vgs - vth it acts like a variable resistor. Current allowed to pass through will be directly proportional to Vds.
But when Vds > vgs - vth, it will be saturated and the current allowed to pass through will be saturated, no matter how high is the Vds.
• 15th July 2019, 18:12
Akanimo
Re: MOSFET linear and saturation region operation
Hi,

In saturation region, with the constraint you provided, namely: Vg > Vth and Vds > Vds - Vth, the MOSFET is an amplifier.

This is a major difference between the MOSFET and the BJT.
• 16th July 2019, 17:27
crutschow
Re: MOSFET linear and saturation region operation
Quote:

N channel MOSFET Saturation operation condition is Vds > (Vgs - Vth).
But in saturation region, MOSFET will act like closed switch. hence Vds is approximately 0V(Rds*I). Examble VGS = 10V and VTH is 2V, and if switch is closed means VDS= 0V, but above formula is not satisfied.

Anyone kindly clarify this ?

Perhaps this diagram from Wikipedia will help clarify:

Attachment 154431

Quote:

And How to drive the N channel MOSFET to act as variable resistor ?
I saw the condition Vds < (Vgs -Vth), but how to calculate Vgs for required resistor ?
There's no easy way to calculate the Vds required for a required resistor value since there's a wide variation in the Vgs threshold voltage between MOSFETs, even those of the same part number.
One way is to measure the transistor you have to determine the resistance versus Vgs curve.
• 16th July 2019, 17:35
asdf44
Re: MOSFET linear and saturation region operation
One question I have when someone understands this stuff is why?

A mosftet goes from 'as off as it gets' -> partly on -> 'more on' -> 'mostly on' -> 'as on as it gets' as Vgs increases. But the Vgs threshold is so widely specified you can't count on anything in the linear region. So the exact relationship and the equations governing it are typically irrelevant. I've designed a lot of linear circuits and I've certainly never referenced any.
• 16th July 2019, 18:51
decaf14
Re: MOSFET linear and saturation region operation
The MOSFET acts as a zero-voltage drop device (ideally) IF it is connected to a load. If the MOSFET is by itself, it will of course have a voltage at the drain.
• 16th July 2019, 20:14
crutschow
Re: MOSFET linear and saturation region operation
Quote:

One question I have when someone understands this stuff is why?

A mosftet goes from 'as off as it gets' -> partly on -> 'more on' -> 'mostly on' -> 'as on as it gets' as Vgs increases. But the Vgs threshold is so widely specified you can't count on anything in the linear region. So the exact relationship and the equations governing it are typically irrelevant. I've designed a lot of linear circuits and I've certainly never referenced any.

Because that describes how the MOSFET responds to the gate-source voltage in the linear region.
Just because you've never used that info doesn't mean no-one does.

Besides, the info in the "linear region" is typically used when the MOSFET is used as a switch or variable resistance.
For linear (i.e. AC) amplifiers the MOSFET typically operates in the saturated region.
(It can be confusing because that's more or less opposite of the BJT definitions for the two operating regions).
• 16th July 2019, 21:05
asdf44
Re: MOSFET linear and saturation region operation
Quote:

Because that describes how the MOSFET responds to the gate-source voltage in the linear region.
Just because you've never used that info doesn't mean no-one does.

Besides, the info in the "linear region" is typically used when the MOSFET is used as a switch or variable resistance.
For linear (i.e. AC) amplifiers the MOSFET typically operates in the saturated region.
(It can be confusing because that's more or less opposite of the BJT definitions for the two operating regions).

Eh my point is that once you wrap it with feedback the exact equations become irrelevant.

And if you don't wrap it in feedback part variations mean you better not care about the exact relationship either.

Both cases are well served by understanding the shapes of the curves but don't necessitate plugging numbers into the equations.
• 17th July 2019, 19:53
crutschow
Re: MOSFET linear and saturation region operation
Quote:

Eh my point is that once you wrap it with feedback the exact equations become irrelevant.

And if you don't wrap it in feedback part variations mean you better not care about the exact relationship either.

Both cases are well served by understanding the shapes of the curves but don't necessitate plugging numbers into the equations.

Okay.
But that's generally true for any parts in a feedback loop, not just MOSFETs.
It you don't need the equations, don't use them.
But they are necessary for a fundamental understanding of the devices.
• 17th July 2019, 20:21
std_match
Re: MOSFET linear and saturation region operation
I have designed power outputs with MOSFET's where the short circuit protection was implemented using the Ids/Vgs diagram in the datasheet.
By not switching on the MOSFET to reach the minimum Rds, it will act as a current limiter.
To allow for temperature and individual variations, we adjusted Vgs so the maximum current in the MOSFET would nominally be 3 times the maximum load current.
When a short circuit occurs, the voltage will increase a lot over the MOSFET and that is very easy to detect, and Vgs can be switched off early enough to keep the MOSFET within the SOA (Safe Operating Area).