# MOSFET Transconductance Variation in different regions

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#### muheeb16

##### Member level 3
How is it constant in the saturation region and varies in the linear region.Please elaborate!

Maybe you are thinking about an ordinary transistor saturation and linear.
A Mosfet is the opposite:
1) It is turned on hard like a switch when it is a linear resistor.
2) It is an amplifier with plenty of drain to source voltage when it is saturated.

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Okay.How does that relate to my question?

In the "linear" region, the Mosfet is a linear voltage-controlled resistor with fixed transconductance.
In the "saturation" region the Mosfet transconductance is not linear.

In the "linear" region, the Mosfet is a linear voltage-controlled resistor with fixed transconductance.
In the "saturation" region the Mosfet transconductance is not linear.

* For sufficiently small VDS voltages (below the saturation region) the FET can be used as a voltage controlled resistor, however it is not a "linear" resistor. The transconductance is not a relevant parameter.
* Transconductance gm is defined as dId/dVgs. This relation cannot be identified in the Id=f(Vds) set of curves. Instead, you need the slope of the Id=f(Vgs) characteristics.
This characteristic has a quadratic form - and, hence, the slope (and the transconductance gm) is not constant but depends on the bias Vgs.

I have used as Jfet (not a Mosfet) as a voltage-controlled resistor for stabilizing the output level of a low distortion Wien bridge oscillator. It is linear (very low distortion) only when Vds is much less than only 100mV.

How is it constant in the saturation region and varies in the linear region.Please elaborate!
As explained above, it's neither constant. Do you mean conductance for a fixed Vgs or fixed Id value?

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