Electronics > Beginners
Mosfet as variable resistor?
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FriedMule:

--- Quote from: MrAl on January 23, 2019, 10:54:01 pm ---Hi,

Already been said.  You need to use negative feedback.

You can try it without feedback very easy.  Use two potentiometers and two mosfets, then adjust each pot and see what you get.  Check a few minutes later and see if the reading changes.  Get some real current flowing so the mosfets heat up a little and watch to see how much they drift as they heat up.  The output voltage will change if they drift.

--- End quote ---
Yes that is true, but not sure how to make one mosfet go up and the other go down, in syncron fairly precise, if we are talking about a volume control / voltage divider?
T3sl4co1l:
The definition of resistance is the ratio of voltage to current.

If we are only concerned with the instantaneous ratio, then yes, it's adjustable.  You could call that the average resistance.

But we know it is a nonlinear component.  What matters to circuits -- what affects dynamics -- is the incremental resistance, i.e., a small change in voltage divided by the corresponding small change in current, dV/dI.  This is normally very high, for a MOSFET in linear operation.

An ohmic device is one which obeys Ohm's law, R = V/I, for all V and I in its range.  That is, a linear device.

MOSFETs do have an ohmic region, but it's not very wide (fractional volt), nor very adjustable (a 2:1 resistance range is reasonable).  JFETs are somewhat better, but hardly ideal.

The next best option is an operational transconductance amplifier, which I won't go into detail about here, but you can read about if you're curious.

For greater ranges, we usually synthesize the resistance using a circuit to set I as a ratio of V, which works over a wide range -- compensating for the nonlinearity of the individual components used in the circuit.  For example, in the electronic load, if we have a control circuit to set output current as a function of an input voltage (which might be a potentiometer from a voltage reference, so we can set some constant current load), we can instead connect that input voltage to a fraction of the output voltage.

Tim
MrAl:

--- Quote from: FriedMule on January 23, 2019, 11:10:03 pm ---
--- Quote from: MrAl on January 23, 2019, 10:54:01 pm ---Hi,

Already been said.  You need to use negative feedback.

You can try it without feedback very easy.  Use two potentiometers and two mosfets, then adjust each pot and see what you get.  Check a few minutes later and see if the reading changes.  Get some real current flowing so the mosfets heat up a little and watch to see how much they drift as they heat up.  The output voltage will change if they drift.

--- End quote ---
Yes that is true, but not sure how to make one mosfet go up and the other go down, in syncron fairly precise, if we are talking about a volume control / voltage divider?

--- End quote ---

Hello again,

Well then you would need two control circuits, but your inquiry leads to a more general discussion of control circuits.

What is more normal to do is to control exactly whatever it is that has to be controlled, and that is usually current or voltage.  In other words, the current or voltage represents some other control level that has some other significance and so you try to control that.
For a simple example, the power supply circuit where we try to control the output voltage.  This would supersede the use of a voltage divider so you would not control the resistance of the mosfet you would control the output voltage itself.  In the case of a volume control you would control the output volume itself not any resistance.

The resistance of a mosfet is something that is not usually controlled directly.  The mosfet is a device that has such an unpredictable character that a feed forward system is not very accurate at all so some sort of feedback would be necessary.  This leads to a more complicated circuit of course although for DC control it's not usually too bad.  For an AC circuit though it could be more complicated because we have to measure AC voltages which is always a bit harder to do.

I dont think you need to do this to understand mosfets though, but if you still want to do it then i would suggest starting with a DC circuit.  You'd have to measure the current through the mosfet and voltage across it, then use that information to adjust the gate voltage.

Did you mention how you wanted to do this yet?  That is, microcontroller or by analog circuitry alone?

Just to keep in mind the reason we have to do something like this is because the mosfet characteristics change in hard to predict ways that affect the resistance drain to source.  In other words, it's not really like a digital potentiometer because it is less predictable.

This kind of problem is not new though in control theory.  The main idea is the real parameters are measured, then the control signal is changed to compensate, then a new measurement, a new control signal, etc., etc.  The non predictables are taken to be just disturbances in the system, but the feedback helps to compensate.  Sometimes the control can be quite precise.

The simpler problem of course it to try to control ONE mosfet alone.

Kasper:
If you search for "simple mosfet audio amplifier" you should see how changing resistance of mosfet can change voltage to speaker.

Yours will be a bit different (simpler) since you want attenuation, not amplification.
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