How to plot JFET?

[spoontex]

Hi,

I need to match two JFET for a compressor build. I use BF545A that is the SMD version of BF245A. And I use this circuit to plot the fet's:




At 0 VGS I can read more or less 0.24V VDS, but when I increment the voltage to 0.25 VGS, the result of VDS voltage is the same of the power supply... so... How I can plot this jfet? I'm doing something wrong?

I post the results of the mason audio. They use BF245A, and the voltage between 0 and 0.25 VGS are very different and more logical for me.

[Rob Strand]

For a compressor or phaser that matching scheme might not be the best.  A compressor or phaser operates the JFET near VDS = 0 upto say 1V max, in the resistance controlled region.   The test jig produces a lot of the data points with VDS > 1V.

For accurate results ideally you need to use that jig with two multimeters as the DMM loads the gate supply.   The loading isn't much.  For a compressor you might get away with it but for a phaser you are pushing your luck a bit.

At 0 VGS I can read more or less 0.24V VDS, but when I increment the voltage to 0.25 VGS, the result of VDS voltage is the same of the power supply... so... How I can plot this jfet? I'm doing something wrong?

When VGS is near zero the JFET is on and pulls down VDS.  When VGS gets close or more negative than VP (Vgs off) the JFET will turn off and the VDS voltage will rise to just under the supply voltage.   This is to be expected.   VP is the point where VDS is near the supply.  Your VP is around -1.75V to -1.5V, which looks OK for these JFETs.

At VGS near zero the voltage you measure is representative of the minimum resistance of the JFET.   For example VD is around 0.25V,  rds0 = Vds/ Id;  Id = (9.58V-0.25V)/10k = 933uA.   Vds = 0.25,  rds0 = 268 ohm, which is very typical value for the minimum resistance.

Once you reach VP the datapoints aren't of much use.  In fact once VDS reaches about 1V they aren't of much use because the JFET is operating a mode different to the final circuit.    At VDS = 1V and Rd=10k the JFET resistance is only going to be around 1k ohm.   Depending on the actual compressor circuit that might be OK, however, you might want to match the JFET resistance out to 3k ohm or 5k ohm.   To match 3k ohm to 5k ohm and keep VGS < 1V you will need to increase the drain resistor to around 33k.

Instead of taking heaps of measurements you could take a few, for example,
- Vds @ Vgs=0, Rd = 10k   ;rds0 = rds_min
- Vgs @ Vds=1V, Rd = 10k  ;rds ~ 1.2k ohm
- Vgs @ Vds=1V, Rd = 33k  ;rds ~ 4k ohm

[spoontex]

This is my results.




This is enough for matching?

[Rob Strand]

This is enough for matching?

There's not enough information!

Firstly, the problem with your results is the JFETs have *much* lower VP than the common BF245A.   Some of the JFETs are turning off at -0.25V and others at -0.5V.   The BF245A's the VP values is around say -1.75V.    The low VP JFET are common on ebay these days, they are more like switching JFETs.  You can see the VDS voltage at VGS=0 is lower than the BF245A, that means the JFETs have a lower resistance to the BF245A's.

For a compressor a low VP means the compressor will distort for large signals.  In fact it will start to distort with input voltages around the VP.

The low VP also means stepping at 0.25V doesn't give you many useful points for matching.    The method I gave only finds useful points.

You have to ask yourself what is the point of matching? 

More often than not you want the levels on a two-channel compressor to track and that means you want the outputs to be within 1dB or 0.5dB over the entire range of inputs.

The range of JFET resistances that need matching depends on the circuit!

If your circuit is asking for JFETs with VP's of -1.75V and you throw in JFETs with VP's around -0.5V then you already have a problem with the range of input voltages the circuit can handle.  (and for some circuits the compression threshold is screwed up.)

The best of the bad might be Devices 8 and 20.  High VP and match on two points, although only single digit resolution on VGS=0.

[FSFX]

It may be easier to first test your JFETs for pinch-off voltage (Vgs(off)) and saturation current (Idss) to get potential pairs and then use a bridge type version of your test circuit to check how closely matched they are across the Vgs range. You can make and use quite simple circuits to do these tests using just a few basic components. If you are going to use a lot of JFETs in the future then it may be worthwhile investing in a Peak DCA75 semiconductor tester that will allow you to plot the characteristics curves on a PC.

For the time being, you may find the following circuits of some use for testing JFETs.

[ElectricDruid]

Honestly though, given the trouble getting hold of the damn things and the huge variability between supposedly-similar parts, why are we bothering with JFETs when we could perhaps be spending the effort researching better alternatives?

Seems to me that JFETs have had their day and we need to look elsewhere. I *know* there's lots of vintage circuits that use them, but they had those parts available in the 1970's and no other options, and now we *don't* have those parts available and we *do* have other options, so why are we trying to do the same thing they did way back then?!?

[Clint Eastwood]

Your bf545's are all at the very low end of the Vgsoff specification. That is either bad luck, or could they be fake? You never know these days..
What is the schematic of your compressor? maybe your Jfets can be used with some changes to the bias arrangement.

[spoontex]

Your bf545's are all at the very low end of the Vgsoff specification. That is either bad luck, or could they be fake? You never know these days..
What is the schematic of your compressor? maybe your Jfets can be used with some changes to the bias arrangement.

they are smd label 20, from nxp:

https://www.nxp.com/docs/en/data-sheet/BF545A_BF545B_BF545C.pdf

fake smd!?!? really?? It's for a 1176 compressor.

I've used this bf545a for a Dimension D clone also.

http://jhaible.com/legacy/subtle_chorus/jh_subtle_chorus.html

But with this build, the jfet are for switching, so I think that it's not crucial the specs of the jfet here.

[anotherjim]

By coincidence, I have recently been looking at this...
https://www.masonaudio.org/diy/comp1176
Matching is only so the metering has a better chance of indicating what it's doing. Would it be so terrible if it didn't always?

Anyway, there's a matching technique described but unfortunately, the link for the schematic of the jig fails 404.

[FSFX]

fake smd!?!? really??

Yes, there are loads of fake SMD parts including JFETs.

[spoontex]

By coincidence, I have recently been looking at this...
https://www.masonaudio.org/diy/comp1176
Matching is only so the metering has a better chance of indicating what it's doing. Would it be so terrible if it didn't always?

Anyway, there's a matching technique described but unfortunately, the link for the schematic of the jig fails 404.

The link is the shematic that I attach on the first post.

[Clint Eastwood]

Hmm, looking at the schematic I don't see an easy way to use low Vgsoff jfets like you have.
The j113 has a Vgsoff of around -1.6 volts, and in my experience they are pretty consistent. And she has legs! The transconductance is a bit higher than a bf245a, but I don't see why it would be a problem.

[spoontex]

Hmm, looking at the schematic I don't see an easy way to use low Vgsoff jfets like you have.
The j113 has a Vgsoff of around -1.6 volts, and in my experience they are pretty consistent. And she has legs! The transconductance is a bit higher than a bf245a, but I don't see why it would be a problem.

what scheme are you referring to?

So, I need to rechange all my fet's on my Dim D build. What jfet can I use? J113? Because found some NOS BF245A, but they are expensive... The original uses 2sk30, that can I found at reasonable price. If not, I can use 2n5457 smd version, for both, Dim D and 1176.

And I can use this "fake" BF545A as input buffer?

[anotherjim]

The whole thing (possibly?)...

[FSFX]

Seems to me that JFETs have had their day and we need to look elsewhere.

You could say that about tubes, germanium transistors and vinyl records. Someone always wants that 'less than perfect' sound.

[Clint Eastwood]

what scheme are you referring to?

So, I need to rechange all my fet's on my Dim D build. What jfet can I use? J113? Because found some NOS BF245A, but they are expensive... The original uses 2sk30, that can I found at reasonable price. If not, I can use 2n5457 smd version, for both, Dim D and 1176.

And I can use this "fake" BF545A as input buffer?

I was referring to the schematic posted above by anotherjim. The J113 has higher Idss and transconductance than 2sk30 and BF245a.   The 2n5457 would be a better replacement I think.
You can use your BF545A's as buffer, but you cannot use the auto-bias arrangement with just a source resistor. Your Vgsoff is only 0.5V, if your signal would go above that it will be clipped at the bottom. So you should bias the gate at least at +1volt or so with a voltage divider.

[spoontex]

what scheme are you referring to?

So, I need to rechange all my fet's on my Dim D build. What jfet can I use? J113? Because found some NOS BF245A, but they are expensive... The original uses 2sk30, that can I found at reasonable price. If not, I can use 2n5457 smd version, for both, Dim D and 1176.

And I can use this "fake" BF545A as input buffer?

I was referring to the schematic posted above by anotherjim. The J113 has higher Idss and transconductance than 2sk30 and BF245a.   The 2n5457 would be a better replacement I think.
You can use your BF545A's as buffer, but you cannot use the auto-bias arrangement with just a source resistor. Your Vgsoff is only 0.5V, if your signal would go above that it will be clipped at the bottom. So you should bias the gate at least at +1volt or so with a voltage divider.

And for the Dim D? I can leave the BF545A? or I need to change?

[Clint Eastwood]

Do you have a schematic of the Dim D?

[spoontex]

Do you have a schematic of the Dim D?

http://jhaible.com/legacy/subtle_chorus/jh_subtle_chorus.html

Main board:

PSU and LFO (HA1475 version not shown separately)
http://jhaible.com/legacy/subtle_chorus/subtle_chorus_main_board_sch_dwg1.pdf

Input Section (HA1475 version not shown separately)
http://jhaible.com/legacy/subtle_chorus/subtle_chorus_main_board_sch_dwg2.pdf

Output Section (HA1457 + Transistors version)
http://jhaible.com/legacy/subtle_chorus/subtle_chorus_main_board_sch_dwg3a.pdf

BBD Board:

http://jhaible.com/legacy/subtle_chorus/jh_subtle_chorus_BBD_Board_sch_page1.pdf
http://jhaible.com/legacy/subtle_chorus/jh_subtle_chorus_BBD_Board_sch_page2.pdf

[Clint Eastwood]

So you have built this, right? then the question is simply : does it work?

To be honest, I don't feel confident to tell you if it would work with your BF545's just by looking at the schematic.