choosing a trimpot value for a JFET drain

[mordechai]

When working with a JFET as a circuit's final gain stage, what determines the value of the trimpot that is used to bias the drain?  For example, what makes a 50k trimpot the right choice for a J201 in that position as opposed to a 100k or 10k trimpot?

[midwayfair]

Just from my experience, J201s tend to bias into the "making sound range" between 7K and 25K, depending on the source resistor [edit: and the total supply voltage, e.g. 18v will require rebiasing]. But some could fall out of spec and require a larger trimpot.

2N5457 will pretty much always bias under 10K, so even a 5K trim works most of the time.

Ideally, you want the smallest trimpot you can get away with for any given FET. Makes it easier to dial in the bias perfectly.

[Gus]

I don't think trimpots are good design in the drain/collector leg of a circuit.  Noise, the wiper can move, someone might turn it etc and have you compared a trimpot specs to a nice 1% fixed film or 5% carbon film?

Sometimes what is done in simple jfet circuits is a trim pot to find the value of a resistor needed for proper bias then you remove the trim and measure the value then a fixed value resistor(s) is installed.  If a single fixed value can't be found from standard value resistors you will see parallel resistors used to achieve the required values.  Resistors have a tolerance so you can find resistance values above and below what is marked on the resistor.

Say you need 46K measure a 47K/51K etc value and find a higher value that when placed in parallel with the selected resistor the value = 46k.
http://en.wikipedia.org/wiki/Resistor
Another way to think about drain resistor value.  If you know about what drain current you want and the the power supply voltage and the drain idle voltage you can use Ohm's law to find the value.

Say you want .2ma drain current and you are using a 9VDC supply and want the drain at 5VDC.
9VDC - 5VDC = 4VDC
4VDC divided by .0002A(.2ma) = 20K


Here is a link to something I posted. 
http://www.diystompboxes.com/smfforum/index.php?topic=93817.0

[midwayfair]

Say you want .2ma drain current and you are using a 9VDC supply and want the drain at 5VDC.
9VDC - 5VDC = 4VDC
4VDC divided by .0002A(.2ma) = 20K

Gus, how do you know what you want the drain current to be for this equation?