Jfet question

[lapsteelman]

OK, I am wondering why on some Jfet booster circuits they tie the gate to the Vcc through a resistor and on others they do not? I have played around with this on the breadboard and it works fine either way. So what is the advantage of having the resistor?

Thanks for your time.

[Suicufnoc]

What you're talking about is called noiseless biasing.  You can read about it on geofex. 

[Rob Strand]

If you want a specific drain resistor value, Rd, and you want a specific gain then that sets the value of the source resistor Rs.   With those two resistors fixed in value you then want to bias the drain at the optimum voltage (say roughly half the supply).  In order to do that you need to be able to adjust the gate voltage which means, in general, you will need a divider to bias the JFET.

The other way is you start with the a circuit with the gate grounded (via a gate resistor) you then adjust Rd and Rs until both the desired gain drain bias voltage all come into alignment (if possible).   The result is the Rd value might not give acceptable output impedance.

A lot of DIY circuits just set the gate voltage to zero, fix the Rs value then, adjust Rd for the optimum drain bias voltage.  When this is done the gain is what it is and hence will vary from JFET to JFET.

In other case say for a JFET buffer it is unlikely you will get a JFET with a large enough Vp value to bias it properly so for buffers you generally see the gate biased off a divider.

That's about the nuts of it.  There's other ways to tweak the gain in isolation like bypass Rs with a cap......