How critical is source R for a 2n5457 buffer?

[Derringer]

I see source resistors ranging from 3k3 to 10K for J201 and 2n5457 buffers

Does the 2n5457 in a buffer scheme like higher resistance on its source than a J201?

[R.G.]

The answer is gm and Idss.

To back out one level:
- A JFET's gm (tranconductance, the ratio of output current to input voltage change) determines how much the current through the channel changes with input voltage changes. You want this to be  high because it's the Gm times the source resistor that determines how  close to unity gain a source follower gets. However, mostly, you live with what you get.

- Idss is the maximum current the JFET will conduct with its gate tied to its source. So Idss times the source resistor determines the largest possible voltage the source can get up to. However, if you tie the gate to ground, the source goes up as current in it goes up while the gate is tied to ground. This is the same as pulling the gate negative with respect to the source, and it cuts the current down lower than Idss. These two effects fight each other (that is, it's a negative feedback condition) until the JFET settles down with just enough voltage on the source to hold the gate-source at the voltage that lets that amount of current flow.

As you can imagine, the exact voltage needed to let that current flow is determined by both the Idss maximum current and the amount of current that the JFET lets through for a given change in the gate source voltage - the gm.  and both of these are different per type of JFET and per JFET within the type.

To answer your question then - it varies with the JFET. You can't get a JFET to let more current through than its Idss will let through, and you can't get more voltage range on the gate-source than Vgsoff.  So the source resistor sets the DC voltage on the source, and there is a range per JFET type to get to a specific voltage. The 5457 has Vgsoff of -0.5V to -6.0V, Idss of 1ma to 5ma, and gm of one millliamp/volt to five milliamp/volt. The J201 has Vgsoff of -0.1 to -1.5V, Idss of 0.2 to 1.0ma, and gm of 0.5 ma/V to (probably) 1ma/V.

That means that the J201 will only be able to let a smaller current through on average than a 5457, so to get the source voltage up into the middle of the power supply so it CAN follow an input voltage, the source resistor has to be higher for a J201. The J201 will have a smaller offset between the gate and source because of the smaller Vgsoff range on average. And there is a wide range of characteristics within both of them, notably the -6V Vgs off of the 5457.

[Derringer]

Thank you!