JFET Shootout

[Chuck D. Bones]

Here's a comparison of most of the JFETs we use in pedal circuits.
I pulled 5 samples each of 15 different part numbers. I measured Vgs,off and Idss. From that, I used the JFET formula to calculate the Vgs and gm (transconductance) at Id = 200μA. I chose 200μA because that's a typical drain current for pedals. Some pedal circuits run the JFETs at a much lower or much higher drain current. It's easy enough to recalc the Vgs and gm for any drain current between 0 and Idss.

This is a small sample size and does not represent the full range of Vgs,off & Idss that we might experience for a given part number. All of these parts were in-spec, although a few were right on the edge. The test conditions for Vgs,off vary from one part number to the next. Some spec sheets measure at Id = 10nA, others at Id = 1μA. My measurement conditions were in the ballpark, typically around 150nA. The resulting error in my Vgs,off measurements are a few % at most. Close enough for Rock & Roll.

Some tips on reading the table below:
gm is negative because the JFET inverts the signal.
To calculate voltage gain, multiply gm (in mS) x drain resistor (in K). Example: Rd = 22K, gm = -1.691ms, then gain = 37.2 (31dB). This assumes the source resistor is bypassed by a large capacitor.
The source resistor can be calculated as Rs = -Vgs / Id. Example: 1.66V / 0.2mA = 8.3K.

 

[Chuck D. Bones]

Wow. No questions or comments. This has to be either the BEST article I've ever written or the worst.

 

[Big Monk]

Wow. No questions or comments. This has to be either the BEST article I've ever written or the worst.

It takes us yokels a little while to absorb what you post Dave.

I can hardly spell anymore so ima need a minute to process this...

 

[fig]

I’ve only now read it. That’s a nice cross-section you have there! Have you ever compared your readings to those from a DCA75 ?

 

[music6000]

Here's a comparison of most of the JFETs we use in pedal circuits.
I pulled 5 samples each of 15 different part numbers. I measured Vgs,off and Idss. From that, I used the JFET formula to calculate the Vgs and gm (transconductance) at Id = 200μA. I chose 200μA because that's a typical drain current for pedals. Some pedal circuits run the JFETs at a much lower or much higher drain current. It's easy enough to recalc the Vgs and gm for any drain current between 0 and Idss.

This is a small sample size and does not represent the full range of Vgs,off & Idss that we might experience for a given part number. All of these parts were in-spec, although a few were right on the edge. The test conditions for Vgs,off vary from one part number to the next. Some spec sheets measure at Id = 10nA, others at Id = 1μA. My measurement conditions were in the ballpark, typically around 150nA. The resulting error in my Vgs,off measurements are a few % at most. Close enough for Rock & Roll.

Some tips on reading the table below:
gm is negative because the JFET inverts the signal.
To calculate voltage gain, multiply gm (in mS) x drain resistor (in K). Example: Rd = 22K, gm = -1.691ms, then gain = 37.2 (31dB). This assumes the source resistor is bypassed by a large capacitor.
The source resistor can be calculated as Rs = -Vgs / Id. Example: 1.66V / 0.2mA = 8.3K.

View attachment 32190

View attachment 32193

View attachment 32194

 

[spi]

Is Vgs,off the same as Vp?

I always thought j201s were less than 2n5457 (well technically greater than, since they're negative). The j201s I have are around -0.7 to -1.

The 2n547s I used to buy many years ago are were around -1.3 to -1.5. The ones I bought more recently are -0.35 to -0.4 which seem way out of spec to me.

 

[szukalski]

Wow. No questions or comments. This has to be either the BEST article I've ever written or the worst.

I think it’s one of the best but it doesn’t fit seamlessly into my knowledge overview and takes time to digest.

 

[Chuck D. Bones]

View attachment 32288

Wait, what? I have dog feet?

 

[Chuck D. Bones]

Is Vgs,off the same as Vp?

Yes. We often drop the minus sign when talking about Vp, with the understanding that it's always a negative number with N-channel JFETs.

Is Vgs,off the same as Vp?

I always thought j201s were less than 2n5457 (well technically greater than, since they're negative). tThe j201s I have are around -0.7 to -1.

The 2n547s I used to buy many years ago are were around -1.3 to -1.5. The ones I bought more recently are -0.35 to -0.4 which seem way out of spec to me.

Spec for J201 is -0.3V to -1.5V.
Spec for 2N5457 is -0.5V to -6.0V.

Your 2N5457s that measure -0.35V to -0.4V are out of spec. They may be usable, but are also eligible for a refund IMHO.

The whole greater-than vs. less-than thing can be pretty damned ambiguous when discussing negative numbers. I usually default to the notion that then number that is closer to zero is the one that is less-than.

 

[music6000]

Wait, what? I have dog feet?

Be thankful, I just finished putting your Pants & Shirt on!!!

 

[Chuck D. Bones]

I’ve only now read it. That’s a nice cross-section you have there! Have you ever compared your readings to those from a DCA75 ?

I don't have a DCA75, so, no.

I expect the Idss measurements to match pretty closely. The only trick there is to make the measurement quickly because the die heats up during the measurement. The conditions for Vgs,off require a specific drain current. The DMM method measures Vgs with a fixed resistance (10MΩ) between G & S. The drain current ends up being Id = Vgs,off / 10MΩ.

 

[music6000]

Be thankful, I just finished putting your Pants & Shirt on!!!

& Boots!!!

 

[temol]

The readings for J201 and 2N5457 also look a bit odd to me. I must admit that I have been using only the smd versions for a long time, but the lowest Vp reading I can remember is -0.5V for MMBFJ201 and -1.15V for MMBF5457. And I measured several hundred pcs. The vast majority of measurements for MMBFJ201 are between -0.6V and -0.75V.

 

[Dan M]

Wow. No questions or comments. This has to be either the BEST article I've ever written or the worst.

I've been conditioned by social media to not think for myself. I expect an article to tell me which one is The Best.
So. Which one is The Best?

But an actual question: Were these all through hole components? I've read on the internet (so it must be true) that new smd parts have less variation. Thoughts?

 

[fig]

The only trick there is to make the measurement quickly because the die heats up during the measurement.

Did you apply voltage to the drain? 4.5v? Does your method use 2 DMMs? <—sorry that’s 3 questions.

I've read on the internet (so it must be true) that new smd parts have less variation. Thoughts?

I measured 100 mmbf5457s and the grouping did seem a bit tighter, but not shockingly. I’ll look for the post where they are listed if you have an interest.

 

[Chuck D. Bones]

All thru-hole parts. Testing SMDs is a pain.

The drain supply was +9.2V. The was a 1K drain resistor for both Idss & Vgs,off measurements. That resistor was reduced to 178Ω if Idss was above 5mA. Nothing magic about those resistor values, they were just handy and kept from JFET from overheating or saturating.

My best guess is that because most of the thru-hole parts are OOP (out of production), some of the ones for sale are the "rejects" from cherry-picking. At the very least, they come from mixed lots. That's not even counting the vendors who sell fugazi parts. The SMD parts are readily available from authorized distributors (DigiKey, Mouser). When you buy 50 MMBF201's from them, they will all come from the same mfgr, have the same LDC and probably come from the same wafer. Therefore, they will have much tighter specs.

 

[fig]

Thanks, great takeaway.

 

[swyse]

Lots of interesting data here, I don't really know how to apply it for use in guitar pedals, but I'm sure it would be useful for people who understand how jfets work. I've bookmarked your jfet biasing and another jfet explanation of yours but havent gotten around to reading it, let alone understanding it yet.

I've been conditioned by social media to not think for myself. I expect an article to tell me which one is The Best.
So. Which one is The Best?

But an actual question: Were these all through hole components? I've read on the internet (so it must be true) that new smd parts have less variation. Thoughts?

Yeah, please tell us what one sounds like a marshall plexi so we can skip to the end of the line.

 

[Chuck D. Bones]

Yeah, if only.

To paraphrase Justice Potter Stewart: "I'll know it when I hear it."

 

[bowanderror]

All thru-hole parts. Testing SMDs is a pain.

My best guess is that because most of the thru-hole parts are OOP (out of production), some of the ones for sale are the "rejects" from cherry-picking. At the very least, they come from mixed lots. That's not even counting the vendors who sell fugazi parts. The SMD parts are readily available from authorized distributors (DigiKey, Mouser). When you buy 50 MMBF201's from them, they will all come from the same mfgr, have the same LDC and probably come from the same wafer. Therefore, they will have much tighter specs.

This would be my guess as well, most of the through-hole J201s I've bought have also tested higher than their SMD equivalents. Interestingly, my through-hole 2N5457s tended to test lower than the SMD version. If you want to peak around at a bunch of different JFETs I've tested, I've shared a spreadsheet here that has a bunch of SMD & TH measurements. It's a bit of a mess, but might help if you're looking for actual reference values when the specs are broad.

Here is a selection of J201s (TH & SMD) measured with RG Keen's Greatly Improved JFET Matcher II:


Fortunately, in most circuits you can tweak the resistors to accommodate the real-world parameters of your particular JFETs. The mass-producing something that didn't include a calibration step.

Thanks for uploading this, Chuck!