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]

DMM

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]

Re: DMM

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]

JFETs

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]

Re: JFETs

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]

2SK170s........

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

 

[tmblack]

I have a bunch of 2N3819 JFETs with Vp from -1.3 to -2.97V.
How well do they need to be matched for a differential front end?

Also gathering 2SK170'BL's. How much variance are there and how much can I tolerate when paralleling 4 of them?

Tom

EE

 

[Turbon]

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.

Following this guide looking at the datasheet for 2sk170bl - 0,00v over 10 Ohms... Tried 4 of them now. Maybe the battery is old. Even moved everything down my experiment board - no dice...

Regards

 

[Turbon]

Following this guide looking at the datasheet for 2sk170bl - 0,00v over 10 Ohms... Tried 4 of them now. Maybe the battery is old. Even moved everything down my experiment board - no dice...

Regards

Battery broken...

Regards

 

[dchisholm]

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.

The Nelson Pass page with explanations and diagrams is "Matching Devices" at https://www.passdiy.com/project/articles/matching-devices

My atch diagram is taken from a circuit simulator (Multisim) but illustrates the concepts.

Dale

 

[vdi_nenna]

When checking for Idss, how long is the jfet left in the circuit and when is the measurement taken? Is it first few seconds or after 1 minute or until the jfet reaches a certain temp?

thanks,

Vince

 

[satoru]

Measure it at the quickest timing possible, as the temperature rise is dependent on the Idss which you have quite a variation in the lot.

 

[finwbu]

NickC try that

JFET Matcher Kit - Stompville Shop

 

[EUVL]

You can pretty much measure everything with a DMM and a sound card :
FET Measurements with Just A DMM


Patrick