The circuit above can be used to develop the characteristic curves. The Jfet characteristic curves are divided into two regions. Today I'm looking at the saturation region.
The family of curves you might find in the data sheet. You can use the math to define your circuit parameters are you can find it on the curves. The math may be better for 'real' design work but for home brew the curves will deliver fair results.
For the circuit I'm looking at today all I need to know is where the bias needs to be to create the 'flat line'. The best place to look is the datasheet for the device you intend to use. I will be using the 2SK2539.
Look at the -1.0V curve and you can see it is at about 1 ma. This make the bias point of the circuit very simple to establish. I want about 1 ma so I simply put a 1K Ohm resistor in the source circuit. I need to connect the gate to the bottom of the 1 K resistor. Since there is no current in the circuit I can use any value. I choose 100 K Ohm.
This is my circuit. Looking at J1 you see R1 sets the bias between the source and gate. R2 completes the connection for the gate. Q1 and J1 in series will have the same current ( 1ma ) when Q1 is at it Q point. If Q1 is biased on or off J1 will respond to maintain the 1 ma. As Q1s collector swings up or down C4 couples the change to the input of the second stage. The second stage operates in the same manner.
The gate of J3 will see changes applied thru C3 and its source current will vary. R13 sets the DC bias of J3. C7 couples the AC component to the load. Varying the load resistance will vary the current thru the load. It is much like a transformer. The voltage drop across R13 sets the maximum output so I can use a 10k headset or a 2k headset with the same voltage output. If I increase the load the voltage will drop as the current increases (as in a transformer). I plan to use a 60 Ohm earbud. The earbud is loud enough with 1 mv.
I vary the load from 100 Ohm to 10K Ohm. Once the resistance exceeds 1K Ohm the source resistor draws more (AC) current away from the load.
With 10 uv input I get 17 mv driving my 60 Ohm earbud. This is a must build. I'll get a board and some nails and tack it together.
Here is an AC sweep.
This sweep varies the load. Time for a build.
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