Matching Jfets

[littlegreiger]

I have a few questions about the Geofex Jfet matcher.  First, what setting do I use on my multimeter?  Second,  does it matter which probe goes to what test point?

[petemoore]

Volts, doesn't matter. switch leads if you're getting a - in front of the relevant number and you'd rather see a +.

[littlegreiger]

AC or DC volts?

[petemoore]

DC

[littlegreiger]

I've tested 5 so far and they all read 0.738.  Its that unusual?

[R.G.]

You probably have the pinout wrong on the JFET and you're seeing the approximately 0.7V forward drop of the gate-channel diode if they're all that close.

[littlegreiger]

I'm using the right pinout.  BTW i'm using J201's.

[R.G.]

Of course, it's possible that they're all 0.738V, since that is within the range of the Vgs you could get with in-spec J201's. However -

Almost no devices are ever **identical**, especially JFETs, which is what led me to think that there's an experimental error, especially with the questions about how to set up the test.

OK, let me be sure I understand the setup: you're using the "Improved JFET Matcher", correct?

If you set your meter to  DC, first range over 9V, and hook the black lead to the triangle ground symbol, what voltages do you read on all pins of the opamp and one of the JFETs?

[littlegreiger]

Sorry for not answering sooner but I have been busy.  I'm new to this, where do I put the probes from my multimeter?  Do I put the black probe to the ground (black) wire from the battery snap and then the red probe to the pin of the op amp?

[R.G.]

No problem.
Yes, that's the right way to do the testing.

[littlegreiger]

Heres the voltages:

Pin 1 = 6.73v
Pin 2 = 6.72v
Pin 3 = 6.12v
Pin 4 = 6.73v
Pin 5 = 6.73v
Pin 6 = 6.91v
Pin 7 = 8.13v
Pin 8 = 0.001v

Battery = 8.13v

JFet
D = 8.12v
S = 6.71v
G = 6.91v

Thanks for your help.

[petemoore]

Me>?
 I tried twice...nogo...then onthe third times a charm try, realized that the schematic shows how to wire the improved matcher...something to do with the 10k's wiring I was screwing up...
 I looked those things over 1000 times and couldn't catch it....'till I finally found what it wasnt'.

[R.G.]

Heres the voltages:

Pin 1 = 6.73v
Pin 2 = 6.72v
Pin 3 = 6.12v
Pin 4 = 6.73v
Pin 5 = 6.73v
Pin 6 = 6.91v
Pin 7 = 8.13v
Pin 8 = 0.001v

Battery = 8.13v

JFet
D = 8.12v
S = 6.71v
G = 6.91v

And here's your diagnosis - you have a wiring error in the matcher. I don't know what opamp you used, but no opamp I know of would operate correctly with those voltages. If you used a single, pin 7 is power, and should be at battery voltage. It is, but pin 4 should be at 0V, and it's up with all the others.  If you used a dual, pin 8 should be at battery, and again pin 4 should be at 0V. Neither matches your voltages, so something is wrong in the wiring.

The + input of the opamp is tied to two 10K's in series across the battery. If wired correctly, it *has* to be at half the battery, 4.06V in your case. No pin is there.

So - check the wiring again.

[littlegreiger]

Thanks i'll do that tonight.  I'm using a tl071 for the op amp.

[littlegreiger]

i just made a schematic for the way i wired it.  You can find it at http://community.webshots.com/user/littlegreiger could you please tell me what i'm doing wrong i just can't see.

[R.G.]

You don't show where your battery is connected.

Some obvious problems:

You say pin 4 is connected to the bottom of the two resistor bias string. If that's true, the battery is not connected correctly, because the "9v" is tied to the high side of the resistors, and the pin 4 is at the same voltage as the bottom of the string. But pin 4 and pin 3 are both quite high, 6+ volts, and pin 4 **has** to be at 0V, pin 3 **has** to be at about half the battery voltage to work correctly.

The simplest explanation is that the battery wire may not be connected to your "ground" point. Just a guess.

[littlegreiger]

I have it wired to the ground point on my schematic but i think it might be a bad solder joint.  I'll redo it tomorrow when i have some time.

[littlegreiger]

Well i finally got it to work.  It turns out that my opamp wasn't seated right in the socket.  I just pulled it up a bit and put it back in and all the voltages were right and i'm getting the right readings.  I want to thank everyone, especially R.G., for all their help.