Matching Jfets

[NickC]

I need a circuit that can help me match Jfets to a specific current range of Idss 4-6ma and another circuit than can match them for gate source voltage. Can anyone point out a circuit that can do so. Thanks

[Netlist]

http://www.passdiy.com has a brilliant tool for that job.

/Hugo

[john curl]

I hope that Nelson has shown how to do this, BUT it is fairly easy with just a multimeter. You just short the gate to the source and add a 9V battery from the drain to the source. This gives you Idss. But how to measure? Put a 10 ohm resistor between the drain and the battery and measure across it. 0.1V=10ma and so forth. Matching Vbe is much more difficult.

[peufeu]

I use a multimeter set on 200 mA caliber and short grid to source...

I then take a sheet of paper, mark a scale from (for instance) 15 to 40 mA, and drop the transistors on the scale after measuring them. That's an easy way to group them without headaches...

[johnferrier]

Originally posted by peufeu
I then take a sheet of paper, mark a scale from (for instance) 15 to 40 mA, and drop the transistors...

Can you recommend good JFETs in the 15-40mA range? How many would it take to find 4 that match at around 20mA? Is 10% considered an adequate match? And how much better would 2% matches be???

What I have in mind is matching 4 JFETs for the outputs to a headphone amplifier (for 300 ohm Sennheisers). I'm thinking about 2sk170, but don't know if I would need to buy 20, 50 or a 100 (to find enough that "match").

Sorry for a barrage of questions (I don't know if someone is reading that knows such things "like the back of their hand"). TIA.


JF

[peufeu]

I have only used JFETs to cascode bipolars so can't comment directly on their sound qualities.

I have used J309 in my DAC and that works well (and is cheap, too).

I think J309 would fit your bill, maybe. Look at the IDss values...

[johnferrier]

Originally posted by peufeu
I have only used JFETs to cascode bipolars so can't comment directly on their sound qualities.

I have used J309 in my DAC and that works well (and is cheap, too).

I think J309 would fit your bill, maybe. Look at the IDss values...

: )


JF

[Jocko Homo]

If you really need that much current, assuming that you have the BL group, you can put 4 in parallel to get that amount of current. Maybe with 3, but 4 to be certain.

Jocko

[fdegrove]

Hi,

you can put 4 in parallel to get that amount of current. Maybe with 3, but 4 to be certain.

Current sharing resistors not included? Or are they?

Cheers,

[johnferrier]

What comes closest to what I have in mind is Borbely's "Super Buffer" which is figure 8 at the following link:

http://www.borbelyaudio.com/borb502.pdf

I'm thinking of parallelling 4, 5, 6...or whatever of the jsk170/jsk74 to get 100mA. It seems that violet parts would help me get there at a lower part count (and are just a little more cost). Maybe you're suggesting 5 ohm series resistors to allow for differences between JFETs. Figure 9 shows a MOSFET version (with higher current output), but Borbely suggests that an all JFET amplifier would be the best.

My question is how closely matched (10, 5, 2%) do the k170/j74 pairs need to be?


JF