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

[atlbraves49]

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?

 

[waht]

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.

 

[atlbraves49]

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?

 

[waht]

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.

 

[atlbraves49]

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)?

 

[vk6kro]

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.

 

[Phrak]

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"

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.

 

[mawad]

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