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MOSFET active region condition

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hanhan

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Hi,
Please help me understand the active region for MOSFET from this section of my text book.
active-region-condition-jpg.75147

active-region-condition-2-jpg.75148


I am confused about the condition to keep MOSFET in active region that is used in this book.
Let's me explain about some notation used in the book:
We denote total variables with small letters and capital subscripts, average DC values using all capitals, and incremental values using all small letters.
Therefore:
vDS: Large-signal drain-source voltage
VDS: DC drain-source voltage
VGS: DC gate-source voltage
Vt: Threshold voltage of MOSFET

In the picture, it says that to maintain MOSFET in active region:
vDS > VDSmin = VGS - Vt
I am confused about this. Why not vDS > vGS - Vt?
large-signal-characteristic-cs-modified-jpg.75149
 

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Last edited:
Spectral regrowth is the term used to explain unwanted spectrum signals which can be due to a non linearity in an RF amplifier(among many other things like driving a modulator with square waves).

Running a MOSFET amplifier outside its linear region may amplify a signal but will introduce distortion, it will add spectral distortion (or growth).

A MOSFET used where Vds is close to Vdsmin will exhibit this effect as shown in the trasfer function you posted.

Try and bias the MOSFET close to the bottom of your shown signal where the device is in its linear region where an input voltage gives a linear output.

Difficult to explain without lots of pics.
 
Thanks WTP Peper,
Spectral regrowth is the term used to explain unwanted spectrum signals which can be due to a non linearity in an RF amplifier(among many other things like driving a modulator with square waves).

Running a MOSFET amplifier outside its linear region may amplify a signal but will introduce distortion, it will add spectral distortion (or growth).
Thanks for these new concepts.
A MOSFET used where Vds is close to Vdsmin will exhibit this effect as shown in the trasfer function you posted.

Try and bias the MOSFET close to the bottom of your shown signal where the device is in its linear region where an input voltage gives a linear output.

Difficult to explain without lots of pics.
yes, I see it now.
However, my confusion is the highlighted part in the picture.
In the picture, it says that to maintain MOSFET in active region:
vDS > VDSmin = VGS - Vt
I am wondering how can we know that VDSmin = VGS - Vt?
 
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