Questions on JFET

[Alan0354]

1) What is the most popular JFET used a the first stage of effect pedal?

2) I have a question in reading the data sheet. Using 2N5457 as an example: ON Semiconductor - datasheet pdf, In Fig.2, Fig.4 and Fig.6, they are 3 graphs of VGS(0ff) = -1.2V, -3.5V and -5.8V respectively. Why having different VGS(0ff) specified?

3) The simplest circuit is self bias, how do you optimize so you get consistent operating point because you cannot control the VGS variation between parts.

[loudthud]

JFETs are of limited usefulness as gain stages if all you have is a single 9V rail. The spread of parameters is not as big in practice as the data sheet would lead you to believe, but the circuit needs to force them into conformity. That takes more voltage or a negative rail. Search for a Sunn Concert or Coliseum preamp schematic. Some Lab Series preamps use an opamp to servo the DC operating point.

1) Probably J201 if you can find them.

2) Real parts won't show that wide a spread. Buy some parts and get yourself a curve tracer.

3) Pretty hard to make a simple self bias circuit work without a trim pot. Some circuits bias the Gate to a positive voltage to make the source resistor look more like a current source or connect the source resistor to a negative rail. The circuit posted below is sort of an SRPP. It uses the JFETs near Idss where Vgs is near zero.

[Alan0354]

Thanks

How about a popular JFET that is available?

I am just puzzled why the data sheet show 3 different VGS.

I am trying to design pedal and the most common is 9V. I hate to go to 18V. Yes, if I use 18V, there would be no issue.

[loudthud]

How about a popular JFET that is available?

That's anybodys guess. TO-92 parts are going out of production every week and surface mount offerings are limited.

I am just puzzled why the data sheet show 3 different VGS.

Back in the 80's things were different. The best example is an old Siliconix JFET data book. Each part family had a single page data sheet with the test limits like Idss, Vgsoff, BVgss etc. Then you would be referred to a page that covered the Die Process. That page or two would cover ten or so different parts in different packages that used the same Die but might have different Idss ranges. So there might be graphs for Vd-Id-Vgs that cover different parts with different Vgsoff ranges. Notice that the 2N5457 data sheet also covers 2N5458. Today there is much tighter process control than there was 30 years ago even though transistors are made on old equipment in fabs that are too dirty to make digital MOS chips and memories.

[Alan0354]

That's anybodys guess. TO-92 parts are going out of production every week and surface mount offerings are limited.

Do you mean JFET is out of style? That's the only transistor I never designed with before, this is my first time!!!!

[Mick Bailey]

JFETs are still in plentiful supply, but the J201 has now been hijacked by the 'mojo pedal parts' guys who are hiking the price up. 2N5457 are cheaper and very easy to get hold of.

For ideas on design take a look at some of the circuits on Runoffgroove. Quite often a bias trim pot is fitted to optimize each stage. The Britannia pedal has such a trimmer on the first JFET.

[loudthud]

Do you mean JFET is out of style? That's the only transistor I never designed with before, this is my first time!!!!

Manufacturers look at the orders they get each quarter. If the quantities are not high enough, they stop making the part. They might still offer the same die in a different package though. You would think that they could just raise the price, but the pressure to keep prices low is too much. When OEMs stop ordering the parts, production is soon after halted.

[Alan0354]

Digikey still have U201 and 2N5457 available for cheap.

Problem is so far, I just never use JFET. The only reason I pick JFET is because for pedals, you want to maintain 1M input impedance. If you use lower terminating impedance, you muffle the sound of the guitar pickup. It is very obvious and people are very picky on the terminating impedance. This pretty much rule out BJT as the input impedance of BJT is lower than 1M and it draw too much base current using a 1M resistor as termination.

Most people use opamp to get the high input impedance for other applications, but you don't get the distortion of a discrete transistor that might give the organic sound in guitar amps and pedals. I think that's the reason the usefulness of JFET is not very wide. Music electronics is too low a volume to be on the radar of the transistor manufacturers.

I should look into small signal MOSFET to see whether the 1/f noise is low enough for front end applications. Usually they are really bad at audio frequency, the 1/f noise stretches into MHz range.

[Audiotexan]

You might also look into:

(US made, eg: DoD)
J111
J113
J201 (as mentioned)

(Japanese made, eg: Boss, Ibanez, Pearl, etc)
2SK30/30A/30ATM (tons of suffix variants: GR, Y, O)
2SK44SPC
2SK105
2SK118
2SK183
2SK184

AFAIK (and someone can correct me here) the suffixes were primarily OEM binning (using colors GR=green, O=Orange, Y=Yellow) -though not sure what spec they based it on.

Best of luck in the hunt/choice tho!

[J M Fahey]

Good (or at least realistic) design practice is to go check what's actually available at shop shelves and design based on that.

Minimizes heartburn, high blood pressure, insomnia, and a lot other side effects.