JFET buffer question

[tempus]

Hey all;

I've seen schematics for JFET buffers that are more or less identical other than the fact that some take the output from the source (e.g., http://www.muzique.com/lab/buffers.htm), and others from the drain (e.g., http://www.electronics-tutorials.ws/amplifier/amp_3.html, http://www.rason.org/Projects/jfetamp/jfetamp.htm). I know that the drain and source are interchangeable, and I suspect this is the reason for the different designs. Is there any advantage to using one over the other?

Thanks

[R.G.]

For JFETs that are symmetrical - that is, drain and source are interchangeable per the datasheet - there is no difference at all.

For JFETs not carrying that note, the device will act differently if "reversed" for the drain to act like a source, although it will probably act like a JFET. Just a different and unspecified kind of JFET.

For JFETs used as an inverting "buffer" with the signal taken from the proper/specified drain terminal, the output impedance will be higher than if the signal is taken from the source in the circuit. The high input impedance of the gate still makes it a high impedance input, as is useful for most buffers.

[blackieNYC]

Taking the output from the drain can give you gain. The source can only offer unity gain at the most. Here's a thread
http://www.diystompboxes.com/smfforum/index.php?topic=48917.0

RG's post - the buffer's lower output impedance makes it a slightly better choose to drive a cable, no?

[tempus]

Thanks for the replies guys. R.G., how much lower will the output impedance be if the output is taken from the drain? Why?

Thanks again

[PRR]

For N-type JFETs: which ever end is more-Positive will act as the Drain. The one more near Gate voltage will act as the Source.

For nearly all JFETs in audio use the D and S are utterly interchangeable with no significant difference.

(When "it matters" is in high frequency radio, where a pFd of capacitance matters. Some JFETs have G slightly offset between D and S, and you get the lowest G capacitance if you use it as indicated.)

Output from Source is gain of 0.99 at most.

Output from Drain "can be" 30 times larger voltage, though that would clip in guitar cord work.

> how much lower will the output impedance be if the output is taken from the drain?

Read again. He said "higher".

Working around 1mA, the Source output is near 1K impedance, the Drain output will be nearer 4K impedance. Both are "low" by guitar-cord conventions. If you need to obsess about output impedance, a 19 cent opamp will go well below 100 Ohms.

[GGBB]

Also - they provide opposite phase output signals -  output from drain will be inverted 180 degrees from input.

[tempus]

OK thanks for the clarification PRR

and GGBB - so technically, when taken from the drain it's not a follower, since the output is inverted.

[GGBB]

Right - afaik there is no such thing as a drain follower.

[PRR]

> thing as a drain follower.

A tube with unity NFB has been called a "plate follower".

This is going too far out in the weeds for the OP's interest.