ok i need to ask a stupid qusetion

[Ansil]

i dont' play with mosfets or jfets much and when i do i am usually cussing

so let me ask this.

if you have to adjust the drains to get 4.5v aprox on ti

right like in nearly every circuit i see

then tell me,, why not just make a vbias of 4.5. instead of all of the adjusting constantly


i mean i know i have to be missing somehting here

pardon my ignorance.  but i have read plenty of info on them but stil nothing explaining why we can't just do that

[Brian Marshall]

not sure where you want to connect the vbias...... the drain???

wont work, you need current, and voltage.

[Ansil]

ahh ok.. thanks

[brett]

Hi Ansil.  Not a dumb question at all.  

If I'd thought about it more, I'd have an answer for you.  But I don't - quite.

Instead, what I did was work out the relationship between Vgs as per RG's JFET tester and the optimum drain resistance.  What I got was a curve from about 4k at Vgs -3V (mostly MPF102s) through about 7k at -2V (lots of different JFETs) to about 18k at -1V (things like 5N5457s).  Because I test the Vgs on all my JFETs (looking for pairs), I can use this relationship to estimate what resistor to put on the drain.

If I'd also measured the voltage on the drain, I'd be able to tell you whether 4.5 V was optimum for all of them or whether it varied with type or Vgs.

Sorry about my lack of insight into your original question.

cheers

[Ge_Whiz]

It's more a case of setting the drain at half the supply voltage in order to get the maximum swing (headroom) out of what is a class A amplifier stage. If you were running the circuit at 12V, you'd be setting the drains at 6V (which would be about the same trimmer setting as 4.5/9V).

Am I right? Anybody?

[Jered]

For the most part I do believe you are correct about 1/2 supply V for max swing, but source resistance is also a factor.
 Anybody seen Jay Doyle?
  Jered