Bias pots vs. VREF resistors

[WhenBoredomPeaks]

There is a lot of pedals where you have to bias the FETs to half supply voltage with trimpots.

Wouldnt it be easier and much cheaper if we could replace the bias pots with the well known 1 resistor to supply voltage and 1 (same valued as the first) resistor to ground (half supply voltage can be tapped from between the two resistors) scheme?

Maybe some guys already use it but i missed it?

[frequencycentral]

But each FET has it's own unique bias point. 

[diydave]

Runoffgroove (fetzer) has a good explanation about what makes fet's tick.
It has to do with the pinch-off voltage (Vp) and the drain-source-current (Idss). Two fets have different Vp, which in return makes the Idss different, which in return causes you to use a different drain-resistor to bias your fets.

For what it's worth, I use diodes instead of source resistors. That way, I only have to 'search' for a correct drain-resistor to bias the fet to halfway supply voltage.

[Derringer]

Runoffgroove (fetzer) has a good explanation about what makes fet's tick.
It has to do with the pinch-off voltage (Vp) and the drain-source-current (Idss). Two fets have different Vp, which in return makes the Idss different, which in return causes you to use a different drain-resistor to bias your fets.

For what it's worth, I use diodes instead of source resistors. That way, I only have to 'search' for a correct drain-resistor to bias the fet to halfway supply voltage.

I haven't seen this done before (diode instead of source resistor). Is there a schem/circuit somewhere that illustrates this practice?

[MetalGod]

Runoffgroove (fetzer) has a good explanation about what makes fet's tick.
It has to do with the pinch-off voltage (Vp) and the drain-source-current (Idss). Two fets have different Vp, which in return makes the Idss different, which in return causes you to use a different drain-resistor to bias your fets.

For what it's worth, I use diodes instead of source resistors. That way, I only have to 'search' for a correct drain-resistor to bias the fet to halfway supply voltage.

I haven't seen this done before (diode instead of source resistor). Is there a schem/circuit somewhere that illustrates this practice?

my understanding is that you generally want the source to be at least 0.6v above the gate for the device to work, so by using a diode you're getting that 0.6v guaranteed as the voltage drop in a silicon diode is not going to change.  you could have two diodes in series for 1.2v, or three for 1.8v - you get the idea.  you can also try using LEDs, but of course they have a different turn on voltage (typically around 1.5v) and of course you could use a zener diode. 

[Derringer]

I just don't have the understanding of diodes yet that I need to acquire.

I'm still fuzzy on the exact physical layout of this concept.

Does the diode take the place of the source resistor? Does the anode connect to the Fet's source?

help please? picture (re: schem) would be awesome

[frequencycentral]

In a similar vein to the source diode is the cathode diode "has the same effect as a bias resistor with a perfect bypass capacitor":

http://www.freewebs.com/valvewizard2/OtherStuff.html

Scroll down to the bottom schematic, fascinating stuff.

[Derringer]

Perfect

thanks Rick