Self biased JFET amplifier design complications

Thread Starter

Mozee

Joined Jul 23, 2016
87
Hello there I've been reading a lot about the J-FET transistors and I was trying to create a self biased JFET amplifier where there is drain resistor and source resistor in the design, I've read a lot and watched the lot YouTube videos but all seem to be explaining about an already made circuit where Rd and Rs are already chosen!

What I want is to start from scratch. and here are some questions:
1) what are the things that I should know about my design other than Idss and Vgs(off) so basically what are the other constants?
2) what are the thing that I am able to vary using my design for example can I choose my load resistance value even if I pe-set the max drain current of my design e.g. "Id or drain current"?
3) when I try following the graphical mode to find the Q-points I track the transition curve and when I choose the middle point of "Idss which is the max jet current" as Q-point (Id=0.5Idss) in the curve so that maximum swings in "drain current Id" could occur I end up setting Rs because[ Idq=(-1/Rs)*Vgsq where q means the q-point values ] and it's voltage so I cannot add "Rd" resistance any more because doing so will change the total series resistant and "Id" would also change as a result.
what am I doing wrong here? should I consider Vds to have 0.5 Vdd? or Id to be 0.5Idss for a J-FET amplifier for max voltage or current swings?

Note that I want to be able to control the LOAD so I want to be able to set my own load resistance also if possible the "Id" too
 

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crutschow

Joined Mar 14, 2008
34,827
Too many variables.
What exactly do you want the amp to do?
What are the values for gain, frequency response, power consumption, supply voltage, etc.
Without those you are just spinning your wheels.
 

Thread Starter

Mozee

Joined Jul 23, 2016
87
supply voltage will be 9V
gain could be 10 to 50 times the input
power consumption not an issue
my load is to be a 1k resistor

or you can assume any other values,
All I need to know is how to start and where from.
what should I select as a constant other than the supply voltage and the load
 

Thread Starter

Mozee

Joined Jul 23, 2016
87
I didn't think of that honestly, I was totally lost and didn't know where to start from.
one more question, Do it take the output across the Drain resistor or across the JFET's D-S junction as in the BJT amps?

after that, I will try to designing a simple self biasing Jfet amplifier on a paper and post it for you here as soon as possible.
 

crutschow

Joined Mar 14, 2008
34,827
Do it take the output across the Drain resistor or across the JFET's D-S junction as in the BJT amps?
For gain the output is taken at the drain-resistor drain junction, the same as across the collector-resistor collector junction in a BJT.
 
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