I was counting on you to have a solution! Both can be easily done, which one do you recommend?
JFET Shootout
- Thread starter Chuck D. Bones
- Start date
Chuck D. Bones
Circuit Wizard
Depends on the resistor values. Post the schematic you're using and I'll recommend something.
I'm using the rog tester with 100 ohm and 10meg resistors. My multimeter claims to have a 40MOhm input impedance. According to the 2n3819 datasheet, Idss can go up to 20mA
Last edited:
Chuck D. Bones
Circuit Wizard
That will work fine as-is. If Idss = 20mA, the voltage drop across the 100Ω will be 2V.
Measuring Vgs,off is always a crap-shoot because the datasheet specs Vgs,off at some arbitrary drain current that rarely matches the drain current in these simple test jigs. But it will be close enough.
Measuring Vgs,off is always a crap-shoot because the datasheet specs Vgs,off at some arbitrary drain current that rarely matches the drain current in these simple test jigs. But it will be close enough.
There you go:
Vp (V) | Idss (mA) |
2.91 | 6.6 |
2.32 | 4.3 |
1.64 | 2.1 |
2.75 | 5.9 |
1.82 | 2.8 |
2.94 | 6.7 |
2.55 | 5.4 |
1.82 | 2.6 |
2.70 | 5.9 |
3.01 | 7.0 |
Can you share where you plan to use them? I have quite a few of them that I'd be interested to use
Chuck D. Bones
Circuit Wizard
Quite a spread.
Vp varies 1.84:1
Idss varies 3.33:1
Vp varies 1.84:1
Idss varies 3.33:1
Sadly... I originally bought 40 of them in hopes of finding matching sets for some phasers.
I've been using them as buffers ever since
Chuck D. Bones
Circuit Wizard
Yeah, it's a crap-shoot.
Would you mind telling us where you bought them and the price?
Would you mind telling us where you bought them and the price?
Found the e-mail. I bought them at a brazilian store called Compomil (not sure if they do exports, customs clearance can be tricky here in Brazil), back in 2014. They were ~$0.25 back then.
Definitely showing the biggest downside of FETs
temol
Well-known member
Thank you very much for the measurements 
I plan to test some Mad Professor circuits and 2n3819s are used there as clippers.
I am not sure if I want to pay $1 each so I was curious if I could use a substitute I have in stock. Looks like MMBF5457 might be a good candidate. I have some 57s with high Vp/Idss. Another thing is that I don't know what Vp/Idss those used by Mad Prof have.
There's one or two local sellers with 2N38198 in stock but the price is too high as for me - close to $2. Ebay - China, China.. China.. and then expensive stuff from US/Canada/Australia. Looks like Mouser is the only realiable, not super expensive place to buy them.
I plan to test some Mad Professor circuits and 2n3819s are used there as clippers.
I am not sure if I want to pay $1 each so I was curious if I could use a substitute I have in stock. Looks like MMBF5457 might be a good candidate. I have some 57s with high Vp/Idss. Another thing is that I don't know what Vp/Idss those used by Mad Prof have.
I was going to ask the same
There's one or two local sellers with 2N38198 in stock but the price is too high as for me - close to $2. Ebay - China, China.. China.. and then expensive stuff from US/Canada/Australia. Looks like Mouser is the only realiable, not super expensive place to buy them.
Now you have me curious, is it necessary to select for Vp/Idss when using jfets as clippers?
szukalski
Well-known member
@temol Musikding is €1,25
electronic-studio have these funky looking ones, but shipping outside Germany is a killer:
electronic-studio.com
electronic-studio have these funky looking ones, but shipping outside Germany is a killer:
2N3819 JFET N-Ch. 25V 0,05A 0,35W
electronic-studio your electronics shipping! 2N3819 JFET N-Ch 25V 0,05A 0,35W Type of Transistor: JFET Type of Control Channel: N -Channel Maximum
temol
Well-known member
JFET as a clipper (shorted D and S) acts as a diode with forward voltage close to silicon diode. I've just measured several different JFETs. FV spans between 0.7V and 0.76V depending od the transistor model. There are rather small differences between same model transistors. Can you measure FV of the 2N3819 for a comparison?
In my opinion differences in FV of the same transistor model are small enough so there's no need to match them.
| Model | Vp | Idss | FV |
| MMBF5457 | 1.224V | 2.435mA | 0.735V |
| MMBF5457 | 1.283V | 2.58mA | 0.738V |
| MMBF5457 | 1.430V | 2.84mA | 0.739V |
| MMBF5457 | 1.478V | 3.15mA | 0.744V |
| MMBF5457 | 2.012V | 3.14mA | 0.739V |
| MMBF5457 | 2.1V | 4.93mA | 0.755V |
| MMBFJ201 | 0.600V | 0.341mA | 0.749V |
| MMBFJ201 | 0.530V | 0.264mA | 0.741V |
| MMBFJ201 | 0.828V | 0.686mA | 0.742V |
| MMBFJ201 | 0.945V | 0.833mA | 0.736V |
| MMBFJ201 | 0.740V | 0.564mA | 0.743V |
| 2SK208R | 0.606V | 0.506mS | 0.702V |
| 2SK208R | 0.613V | 0.52mA | 0.702V |
| 2SK208R | 0.613V | 0.52mA | 0.702V |
| MMBF4393 | 1.76V | 12.5mA | 0.712V |
| MMBF4393 | 1.99V | 14.5mA | 0.705V |
| MMBF5458 | 1.73V | 3.66mA | 0.731V |
| MMBF5458 | 1.6V | 3.31mA | 0.737V |
| MMBF5458 | 1.947V | 4.47mA | 0.734V |
| MMBF5485 | 2.427V | 5.9mA | 0.754V |
| MMBF5485 | 2.31V | 5.46mA | 0.752V |
| MPF102 | 1.785V | 6.44mA | 0.741V |
| MPF102 | 2.12V | 8.3mA | 0.742V |
| MPF102 | 1.99V | 7.7mA | 0.738V |
| J111 | 5.34V | 65mA | 0.683V |
| J111 | 5.241V | 63mA | 0.683V |
| J112 | 2.47V | 27mA | 0.720V |
| J112 | 2.894V | 34mA | 0.720V |
| J113 | 2.58V | 28mA | 0.722V |
| J113 | 2.270V | 24mA | 0.720V |
Forward Voltage | mV |
2sk246Y | 734 |
2sk246Y | 720 |
2sk304C | 726 |
2sk304C | 726 |
2n3819 | 752 |
2n3819 | 753 |
mpf102 | 758 |
mpf102 | 760 |
Added a couple other jfets, just because
Chuck D. Bones
Circuit Wizard
At what current are you measuring Vf? What matters is in a pedal circuit is the curve, not the voltage at a single point.
I found that JFETs have a Vf that is slightly higher than the usual signal diodes (1N4148, 1N914) and rectifiers (1N400x). I use BA482 diodes in the Mojito because their Vf curve is nearly identical to the 2N5457 (used in BJFe Model G) and 2N5952 (used in BJFe model H). Similar BJFe circuits use 1N4007 for the hard clippers. I think the differences are subtle at best.
I found that JFETs have a Vf that is slightly higher than the usual signal diodes (1N4148, 1N914) and rectifiers (1N400x). I use BA482 diodes in the Mojito because their Vf curve is nearly identical to the 2N5457 (used in BJFe Model G) and 2N5952 (used in BJFe model H). Similar BJFe circuits use 1N4007 for the hard clippers. I think the differences are subtle at best.
Should an arrangement like this (with the diode in place of the resistor, of course) work for tracing the curve? I'm not sure if it'd be precise enough, though, with my multimeters