Biasing JFET Question + Ohm's law?

[modsquad]

If I have for example 8.10v and I want the gate voltage to be 3.8v how do I determine what size resistor to use?

I guess a more general question is: How do I determine the resistor value for dropping voltage?  Ohms law says I need the current value of the circuit.  How do I measure the current in a circuit using a MM.  Mine goes to 200ma for DC current.

[GibsonGM]

You can use 2 resistors to do that, Mod.  Vout = R2/R1+R2 * supply voltage in.   MUCHO important.  Using that relationship, if you used 500 ohm resistors, you'd get .5*supply voltage.    The reduced voltage in this voltage divider is present at the connection between the 2 resistors, V+ at one end and V- (ground) at the other end.

I'd just assume 1/2 voltage for your problem; close enough! If you need more precision, make one of them a pot.     You could also do this job with a trim pot; take the new voltage off the wiper, one end to V+ and one end to ground.

What's actually more important is the current thru the voltage divider (and hence to the gate of your FET).  Maybe make the 500 ohm R's like 5K to make sure it's really low (in the mA's...)
To measure current, you need to disconnect one end of a point from the ckt, connect the MM between them.    Red goes to highest potential side, black to least. 
I'll post a pic, see this link:

http://members.aol.com/articfox34/vdiv.jpg

[modsquad]

What's going on is that using the following for building BSIAB II, the voltages are screwy on the FETS.  Since everyone talks about biasing the FETs correctly I am trying to check the voltages and make sure the resistors are right.  For information here is my previous post of the voltages on the FETs and the schematic that I used:

http://www.generalguitargadgets.com/index.php?option=content&task=view&id=150&Itemid=178

http://www.diystompboxes.com/smfforum/index.php?topic=48504.0

[DuncanM]

Since everyone talks about biasing the FETs correctly I am trying to check the voltages and make sure the resistors are right.

Luckily, the first two stages of the BSIAB (the Mu Amps) are self-biasing so no intervention is needed.
If you're not getting roughly half the supply voltage on the gates of Q2 and Q4 then you have a problem...

The third stage with the trim pot is where you can adjust bias for Q5 - I tune it by ear as it won't work at one end, works somewhere in the middle and then dies again at the other end.
Once you've got the region in which it passes signal, you can then fine tune.
On mine Q5 Drain sits at 5.1v

[modsquad]

When you say they are self biasing I am assuming that is because the gates are wide open.

[DuncanM]

Explained at:

http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm

http://www.geofex.com/Article_Folders/foolwfets/foolwfets.htm