**Greatly Improved JFET Matcher**So you wanna build a Phase 45 or Phase 90 or one of its derivatives? Ahh, you need *matched* JFETs, so just read on... This was motivated by my latest experiments with a Phase 45 clone.

Current JFET matching techniques rely on matching at a single "point" along the response curve. In practice, accurate JFET matching needs to be done on *two* different points, since the drain-to-source resistance (Rds) is a function of *two* parameters related to the manufacturing process: Vp (pinch-off or cutoff voltage) and Idss (drain-to-source saturation current), as indicated by the following equation:

` Vp^2 Valid IIF:`

Rds = --------------- (a) Vds aproximately 0

2·Idss·(Vgs-Vp) (b) 0 >= Vgs >= Vp

Example: Typical values for an MPF102 could be Vp=-2V and Idss=6mA (note the negative sign on Vp!). At zero Vgs, we get the ON resistance, which is given by:

` -Vp -(-2)`

Rds(on) = ------ = ------- = 166.7 ohms

2·Idss 2·0.006

On the other hand, if we make Vgs = Vp, we get a division by zero, meaning infinite resistance (JFET fully OFF).

Where does this lead us? Well, if we can measure the two *key* parameters for two or more JFETs, Vp and Idss, then we can do the following:

1) Any pair of JFETs that have close Vp *and* Idss are *matched*. Coincidence on just one parameter is not enough!

2) Knowing Vp and Idss allows me to effectively calculate the required Vgs for a given Rds, meaning I can really know

*a-priori* where to adjust the control voltage for the JFETs. Manipulation of the first equation leads to:

` Vp^2`

Vgs = ---------- + Vp

2·Rds·Idss

Example: For the same MPF102, let's calculate the required Vgs for a 10k Rds.

` (-2)^2 4`

Vgs = ------------- - 2 = --- - 2 = -1.96 V

2·10000·0.006 120

Ok, now let's see the circuit that will let us read Vp and Idss:

As you see, the circuit is fairly simple. Only keep in mind the OpAmp MUST be MOSFET input to avoid altering the weak current through the 10Mohm resistor for the Vp reading, and must be capable of going down to GND (or the negative supply rail) for proper Idss reading.

After you characterize both Vp and Idss for each JFET you have, just find those with closest Vp AND Idss.

Have fun,

STM[/b]