Q about fet Biasing!!!

[Minion]

Hi, I am wondering about Fet Biasing .....I am building a DR Boogie which has 4 fets that you have to Bias to half Supply Voltage useing 100k Trim pots....

Is it normal for one fet to need say 20k resistor to bias and the next Fet to need 120k to bias it???

Is it OK for a fet to be Biased a Little high or low??

Cheers

[Gus]

Minion
 
Why ask? try it and find out for yourself.

[MartyMart]

I'd be a bit surprised at THAT much difference for 4.0/5.0v bias !
Recently tried the "use a pot-bias-remove and measure" idea, so you end up with fixed
resistors.
Keeps any "drift" at bay and is also less noisey.
Depending on the input and source r , drain r will be different, normally between say 10k and 50k.
Fet's  are all over the place .
I had good success with more expensive 50k pots, but they are a "buck" a piece !
MM.

[Minion]

Minion
 
Why ask? try it and find out for yourself.

"I Did Try It" and It is true but I don"t see "WHY" it is true, Hence My Question.....

I have some Fets that Bias with a 20k resistor and others that Bias at several Hundred K......

[johngreene]

Minion
 
Why ask? try it and find out for yourself.

"I Did Try It" and It is true but I don"t see "WHY" it is true, Hence My Question.....

I have some Fets that Bias with a 20k resistor and others that Bias at several Hundred K......

In the same location? The same part number?

[Minion]

Yes...the same Fets (J201) but not in the exact same location but with the exact same surrounding curcuitry....If you look at the dr.boogie schematic it has a bunch of seperate but Identical fet stages.......

I actually did have to try a few different fets in the same spot to find one that would Bias with the 100k Trim pot as some Fets needed much more than 100k to bias them......


Cheers