MOSFET as variable resistor

[Denio]

Hi,all!
So I'am about to build a prototype of a madbean EgoDriver.The schematic uses a 1MA potentiometer for the gain and a 500kA pot for the volume control.Trouble is that I cannot find logarithmic potentiometers of those values in my country(Bulgaria).
This question is not only for this project but for other occasions when I can't find log pots of the appropriate values.
Since mosfets can be considered as voltage-controlled variable resistors,can I somehow control the gate voltage of one with a log potentiometer and use the drain-source resistance to control the desired effect parameter?I am guessing that the transistor will have to be biased in the linear region.
And also-how high can the drain-source resistance go?
Cheers!

[marcelomd]

Hi,
You can use one or two resistors to change the taper of the potentiometer.
Like this: https://www.diystompboxes.com/smfforum/index.php?topic=70732.0

[Denio]

Just checked out "The secret life of pots".I will try this approach but will still think of alternatives.Here are a few that came to mind:
1.Digital pots-convenient but can overcomplicate things when used in Analog pedals.
2.Range switch-a rotary switch connects one or more series resistors to increase a pot's value.
3.Transistors controlled with a different value pot.

[merlinb]

So I'am about to build a prototype of a madbean EgoDriver.The schematic uses a 1MA potentiometer for the gain and a 500kA pot for the volume control.

You can use a smaller value for the volume control, like 100kA if you can get it. Don't you have eBay in Bulgaria?

[Denio]

We do.But I would like to get the parts locally if possible or from some trusted dealer like Farnell,Transfer Multisoft electronic or something similar.For some reason I don't trust ebay.

[Danich_ivanov]

I just saw the schematic, and i think that unless you want to build a perfect clone, you can go with linear pots with next to no issues.

[amptramp]

A MOSFET acts as a variable resistor below the drain voltage at which it would be used as an amplifier.  Once the drain voltage is high enough, it goes into a constant current region where the resistance would be high but on the order of a megohm or so for some devices.  Obviously this varies with the device but if you are looking for anything above a megohm as a variable resistance, choose a different device.  Maybe an optoisolator would work for you.

[GGBB]

How does one get an accurate audio taper out of the MOSFET-as-variable-resistor?

[vigilante397]

There are a lot of things I don't trust eBay with, but for passive devices like pots I personally wouldn't worry.