In what region (linear,saturation) does a MOSFET act as a variable resistor

In summary, the conversation discusses the use of a MOSFET as a voltage controlled resistor, and the behavior of the MOSFET in different regions of operation. It is mentioned that at higher VDS values, the MOSFET will no longer act as a variable resistor and may exhibit a short-like behavior. It is also suggested to consider using a digitally controlled potentiometer for this purpose. However, for high current applications, neither a MOSFET nor a digital controlled potentiometer may be suitable. The use of a transistor for voltage controlled resistance is also questioned.
  • #1
I'm trying to use a MOSFET as a voltage controlled resistor, and am under the impression that it acts this way when it is in the linear region. However, our VDS will be 2.5V-20V, and our VGS will be 3.6V-5V.

That means our VDS will most likely be greater than our VGS - VT, which means it will be operating in the saturation region and thus no longer operating in the manner we want (like a variable resistor), correct?
 
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  • #2
Yes with VDS much greater will take you out of the linear (ohmic) region. I don't know why they call it a linear region, when it is still non-linear.
 
  • #3
waht said:
Yes with VDS much greater will take you out of the linear (ohmic) region. I don't know why they call it a linear region, when it is still non-linear.

So with our VDS and VGS values, we can't expect the MOSFET to act as a voltage controlled resistor anymore, correct? What behavior does it exhibit at this point, instead?
 
  • #4
atlbraves49 said:
So with our VDS and VGS values, we can't expect the MOSFET to act as a voltage controlled resistor anymore, correct? What behavior does it exhibit at this point, instead?

With VDS as low as 2.5 V, and considering you have reasonable VGS-VT you could be in the linear region. But if you take VDS out to 20 V the curve will start leveling off in a saturation mode.
 
  • #5
waht said:
With VDS as low as 2.5 V, and considering you have reasonable VGS-VT you could be in the linear region. But if you take VDS out to 20 V the curve will start leveling off in a saturation mode.

At which point.. it will act like a short (no, or very small, resistance)?
 
  • #6
Perhaps you might like to download the data sheet for a typical MOSFET.
One would be the MTP3055V. You can find the data sheets free on Google although some of the sites will try to charge you for them.

What happens is that the FET behaves like a voltage controlled current drain. The Drain to Source resistance is high unless the Gate to Source ( "Gate" ) voltage is above about +2 volts threshold voltage.

If this threshold voltage is exceeded, the current through the Drain to Source resistance increases. However if there is more than 4 volts on the Drain, increasing the Drain voltage produces little change in the current.
Changing the gate voltage, though, produces large changes in the drain current. These devices have huge GMs measured in Amps per Volt !

It is still a voltage controlled resistor, but the resistance is very low and currents can get very high if there is no limiting of current. These devices can easily self destruct if currents are not limited.

You may be interested in Digitally Controlled Potentiometers. These are used in audio amplifiers. You rotate a knob which feels like a switch and somewhere else in the circuit the pulses from the switch cause the DC Pot to change its resistances. They are possibly more suitable for your purposes than MOSFETs.
 
  • #7
You all have gotten me curious. I've only used mosfet as switches.

Here are some low V_GS(th) mosfets. http://www.diodes.com/products/catalog/detail.php?item-id=5033" [Broken]

If this is intended as a production design--more than a one-off, atlbraves, they may be unsuitabe, with too much variation between parts, and too much temperature dependent resistance.
 
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  • #8
Hello All,
I would like to do voltage controlled resistance, but the current goes through it is varing between 0 to 15A, and the voltage across its terminals is variable between 0 to 50V. As mentioned in this post MOSFET will not work, and also digital controlled potentiometer will not work becuase of the high current. What about the transistor? or it will have the same MOSFET problem.
Many thanks
 

What is a MOSFET?

A MOSFET (metal-oxide-semiconductor field-effect transistor) is a type of transistor that is commonly used in electronic devices to amplify or switch electronic signals.

What is the region of a MOSFET?

The region of a MOSFET refers to the specific operating mode in which the transistor is functioning. The two main regions are the linear region and the saturation region.

What is the linear region of a MOSFET?

In the linear region, the MOSFET acts as an amplifier. This means that the output current is directly proportional to the input voltage, and the transistor is not fully turned on.

What is the saturation region of a MOSFET?

In the saturation region, the MOSFET acts as a switch. This means that the output current is not affected by changes in the input voltage, and the transistor is fully turned on.

In what region does a MOSFET act as a variable resistor?

A MOSFET can act as a variable resistor in the linear region. In this mode, the transistor can be used to control the amount of current flowing through a circuit, similar to a traditional variable resistor.

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