### JFET Biasing?

Posted:

**08 Oct 2013, 04:15**Hey everyone,

I have been running some simulations of JFETs in order to study a bit about how to bias JFETs properly and also to teach myself along the way. I'm having problem with some of my calculations though. I am currently able to accurately calculate the operating point of a simple JFET circuit so long as it only has a source resistor. However, I've noticed that as soon as the drain resistor gets large enough to limit the current to be lower than the expected current draw for some source resitor and Idss value, then the operating point of the transistor becomes elusive and strange to me and I'm unable to calculate it. I feel that it has something to do with changes in drain-source voltage drop (maybe gate-source too?) but I can't figure out exactly how to calculate it accurately.

Similarly, I am confused as to how to bias a JFET well enough as to basically negate the differences between individual components - similar to how you can by using a voltage divider bias network for a BJT.

Help?

I have been running some simulations of JFETs in order to study a bit about how to bias JFETs properly and also to teach myself along the way. I'm having problem with some of my calculations though. I am currently able to accurately calculate the operating point of a simple JFET circuit so long as it only has a source resistor. However, I've noticed that as soon as the drain resistor gets large enough to limit the current to be lower than the expected current draw for some source resitor and Idss value, then the operating point of the transistor becomes elusive and strange to me and I'm unable to calculate it. I feel that it has something to do with changes in drain-source voltage drop (maybe gate-source too?) but I can't figure out exactly how to calculate it accurately.

Similarly, I am confused as to how to bias a JFET well enough as to basically negate the differences between individual components - similar to how you can by using a voltage divider bias network for a BJT.

Help?