What Prevents the Self-Biasing JFET Current Source from Reaching VGS = 0?

[daskywalker]

Homework Statement

For the self-biasing JFET current source in Figure (http://sound.westhost.com/ab-f5-2.gif), what prevents this circuit from reaching the characteristic curve (showing ID vs. VDS) where VGS equals zero? The answer has to do with the self-biasing aspect of this circuit. What would happen to the value for VGS if the source resistor, R1, is changed to 100 ohms? Would it increase, decrease or stay the same?
(The "LOAD" in the figure is a 10k pot)

Homework Equations

The Attempt at a Solution

For the first part I said there is no guarantee that Vgs is zero because of R1. (There will always be a voltage drop) But I don't know how to link it to the self biasing aspect.
For the second question I think the Vgs value will increase since less resistance=more current.

Please correct any false assumptions/approach to this problem.

 

[Jhero]

Your response is generally correct, but here are some additional points to consider:

1. The self-biasing aspect of this circuit refers to the fact that the JFET is biased by its own source current, rather than an external biasing circuit. This is achieved by connecting the gate and source together and using the source resistor (R1) to provide the necessary negative feedback.

2. The voltage drop across R1 will always be present, even if the JFET is biased at VGS = 0. This is because the JFET requires a minimum gate-to-source voltage (VGS(off)) to turn off completely. As long as the voltage drop across R1 is greater than VGS(off), the JFET will remain off.

3. If the source resistor (R1) is changed to 100 ohms, the value of VGS will increase. This is because the JFET requires a certain amount of source current to maintain its bias, and a decrease in resistance will result in an increase in current. This will cause the voltage drop across R1 to increase, and therefore increase the value of VGS.

Overall, your approach is correct and you have a good understanding of the concept. Just remember to consider the minimum VGS(off) requirement for the JFET and how changes in resistance can affect the source current and therefore the value of VGS.