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
not sure where you want to connect the vbias...... the drain???
wont work, you need current, and voltage.
ahh ok.. thanks
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
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?
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