Quick and Dirty JFET Buffer, Rev. 2

As mentioned in the "JFET Study," convenient through-hole JFETs are going obsolete. We'll have to learn how to use surface mount parts. Fortunately, it's not too bad! Here's a pictorial guide to constructing a simple high impedance buffer circuit using a surface mount JFET.

Schematic

"Approved" JFET types include MMBF5486 and PMBFJ112.

R = "Ring" terminal of jack

T = "Tip" terminal of jack

S = "Sleeve" terminal of jack (grounded to metal enclosure if you choose)

The part values are super approximate.

Construction

I built the prototype on a piece of mini turret board, Keystone part number 15536. First, I laid some small pieces of copper foil tape on the underside of the turret board, and soldered the JFET as shown here:

The foil is soldered to the JFET, and to the bottoms of the turrets. Next, I flipped the board over and marked the JFET symbol for my convenience:

Now here are all of the parts. You can figure out the layout by associating the parts with the schematic -- a good exercise if you're thinking about building one of these:

Blue is the input, Green is ground, Red is power (from a 9-V battery connector), Brown is output. Now I've hooked it up to jacks:

It works! A quick diagnostic is to measure the voltage at the Source of the JFET to ground. It should be around 2 - 3 Volts.