What is biasing?

[toddvirgil]

So another newbie question -

As I'm poking around the comments on a lot of the builds, folks keep talking about biasing chips and matching FETs and whatnot... So, I'm hoping someone can explain to me what you all are talking about? Thanks!

[GoranP]

As for jfet biasing, if you don't mind the technical explanation, this video covers a lot of ground

http://www.youtube.com/watch?v=zlW-gm62WBo

If you do mind it, I'll just throw in a (very large) blanket statement saying that biasing (through one of many possible variations (selfbias/fixedbias/potentialdivider...)) sets the transistor's operating point (see transfer characteristics) at a point where you achieve the desired function of said transistor.
FETs can also be matched according to various characteristics to again perform a certain function the way you see fit.

As for some middle-of-the-road real-world explanation, I'll let the native speaking folks take over...

[IvIark]

That explanation pretty much covers it, it's just basically using a resistor value (or trimmer) that puts a set voltage at a specific transistor pin, more often than not in these things that means round about 4.5V on the collector or drain for BJTs and JFETs.

Expanding on that and thinking out loud, I wonder why we mess around with resistor values and trimmers so much when you could achieve exactly 4.5V by simply putting the same resistor value from collector or drain to ground, that you used from collector or drain to supply?  Has anyone tried that?

[GoranP]

It's been almost 20 years that I'm out of school and I'm really hazy on most of the bjt and fet math (not that I was much good at it back then either) so hopefully this isn't going to turn into an embarassing conversation.

IvIark wrote

That explanation pretty much covers it, it's just basically using a resistor value (or trimmer) that puts a set voltage at a specific transistor pin, more often than not in these things that means round about 4.5V on the collector or drain for BJTs and JFETs.

Agreed. However there are weird circuits out there that defy this common practice. For example, Fairfield Barbershop is biased to 2/3 voltage instead of 1/2. I've seen Guillaume mention it himself someplace...

IvIark wrote

Expanding on that and thinking out loud, I wonder why we mess around with resistor values and trimmers so much when you could achieve exactly 4.5V by simply putting the same resistor value from collector or drain to ground, that you used from collector or drain to supply?  Has anyone tried that?

You mean supply - resistor- say drain - (same resistor to ground in parallel to fet)?

Also, are we talking fixed bias or self biasing?

[IvIark]

Say a Fuzz Face seeing as that is one of the common ones that many people aim for 4.5V.  If you use a 10K supply resistor and then a 10K resistor to ground you'll have pretty close to bang on 4.5V assuming a 9V supply.

[nocentelli]

You see that done on the base/gate quite often: Never seen it on collector/drain... There must be a good reason.

[IvIark]

Yes that was what I thought but I'm just not sure what it is.  

[GoranP]

How much of the Id would then go into the actual drain and how much would "divert" through that resistor to the ground?