J201 vs 2SK30A

[Crontox102098]

Is it simple, just i want to know the differences between J201 and 2SK30A, hope you can help me.

Cheers!

[R.G.]

Datasheets.

[Crontox102098]

Datasheets.

I'm so bad to understand it... especially when we talk about transistors... That's why i'm asking.

[amptramp]

J201 short form datasheet:

http://pdf.datasheetcatalog.com/datasheet_pdf/interfet-corporation/J201_to_SMPJ202.pdf

2SK30A datasheet:

http://pdf.datasheetcatalog.com/datasheet/toshiba/1032.pdf

The major items are Idss and Vgs off and the 2SK30A offers Idss classification groups.  If the FET is never operated at positive bias, Idss tells you the maximum drain current it can consume.  If you are using it as an amplifier with a certain drain resistance, this tells you the worst case voltage drop across the resistor and the minimum voltage the output will be biased at.  Vgs off tells you how many volts the grid has to be reverse biased to cut the current down to a low value (and this value is given as part of the spec) or conversely, how high the drain can go if the device is biased from a grounded resistor.  These devices are also used as FET switches and Vgs off is a value the gate has to be reverse biased to allow the channel to turn off.

If you are building a phaser using JFET's as variable resistors, it pays to match the Vgs off values.

The transconductance gives you the gain in amps per volt for the device and transconductance times drain resistor (in parallel with any load resistance) gives you the voltage gain.  If you have a source resistor, you have to do the algebra that shows you what the source voltage will be to get the stage gain.

For these two devices, there seems to be more variability within each device spec than difference between the two of them.

[R.G.]

They're different many ways. The most common use for a 2SK30A is as an audio switch (although it can be an amplifier as well). It is convenient for pedals because its turn-off voltage is just a bit smaller than half of 9V, so it makes an easy switch in pedal circuits.

The J201 is an oddity in JFETs. It is intended for low level amplification, not switching. It has a very low gate cutoff voltage, as little as -0.3 and at most -1.5V, much smaller than the 2SK30A or other more normal JFETs. It has a quite low Idss (the maximum current the drain-source will let through) of 0.2 to 1.0ma, and a low gain, minimally 0.5ma/volt of gate voltage change.

The 2SK30A has a gate cutoff of -0.4 to -5.0V, Idss of 0.3 to 6.5ma, and a gain of 1.2ma/volt of gate change; all of these important parameters are significantly larger than the J201. The J201 datasheet at least says something about noise, the 2SK30A doesn't. I've never been able to measure a difference when they're used for low noise applications. Pedal applications are almost never set up properly for low noise.

I think the primary advantage of the J201 is that there are a lot of pre-existing pedal applications out there that use it. But I'm a cynic.

I've never been able to understand why the effects world likes the J201. I have had much better results from more mainstream JFETs.

And one way they're alike: they're both obsolete in the TO-92 plastic package.

[PRR]

I'd have to read between the line in Oxner, but I *suspect* the 20x series (which is VERY old) isn't "intended" for any specific thing. Rather I think they are sort-bins so that "any" working JFET of a certain die-size can be sold as one or another number.

Gate-Source Cutoff Voltage
201  -0.3  -1.5
202  -0.8  -4
203  -2    -10

IDSS Zero-Gate Voltage Drain Current
201   0.2  1.0
202   0.9  4.5
203   4    20

Any part from 0.3V to 10V will fall into one of the Vgs(off) bins. And for a given die-size and general layout, Idss tracks Vgs(off), so these parts are pretty-sure to also fall in the 1-2-3 bins 0.2mA to 20mA. (There's a little overlap between bins for slop and also for parts which fall on the edge of the Vgs/Idss trend.)

Yes, for a small-signal amplifier, you go for the low Vgs part, accepting its low Idss.

(And some hot-heads go too far, feeding not-small signals to J201, and it gets nasty, or just won't bias they way they think it should.)

JFET switches are always large-signal problems and the high Vgs part is the way to go (within power supply limits).

I thought there was noise-specs for 2SK30A ? Or at least that it is reputed to be low-hiss? This can only be true for specific situations. 2SK30A is a fat beast and can be run at HUGE current which modestly pushes-down voltage hiss. OTOH its fat gate leaks a bunch and can have high hiss in very high impedances (condenser mikes). OTOOH it has been used widely, either meaning it is good all-around or a lot of people listen to part-numbers instead of the actual signal.

> obsolete in the TO-92 plastic package.

Steve at SmallBear says he has plenty of TO-92 for individual builders, scavenged from odd-lots from the many millions made over the decades.

[J0K3RX]

You need to take note of the "Note" at the bottom of the Electrical Characteristics.. different letter markings = different IDSS  

Note: IDSS classification R: 0.30~0.75, O: 0.60~1.40, Y: 1.20~3.00, GR: 2.60~6.50

So, if you have a transistor with a minimum IDSS of 0.3ma you are not going to have a max of 6.5ma....

The transistor will be marked with a K30A and then one of the classification letters under that.

The 2SK208 is the exact same as the TO-92 2SK30A and and all of the specs are identical including the IDSS classifications.

The 2SK208-R are commonly used in almost all of the AMT preamp pedals... So, they are great for low noise medium to high gain pedals!


Finding these with the "Y" classification are very common but the "R" classification is very rare! R = Rare, easy to remember...

[Crontox102098]

So, i can use K30A in all those effects who needs J201?

[R.G.]

You can't even use all J201's in effects that specify J201's without the possibility of tinkering with biasing and gain resistors.

If you are willing to muck with the resistors, yes, probably. If you are willing to look for low Vgs 2SK30A, sure. Some 2SK30As get down to very low Vgs values according to the spec sheet. And 2SK30A has about 3-5 times the gain, on average.

[armdnrdy]

It might help to be specific. What are you trying to accomplish? Do you want to use J201s in a circuit that calls for 2SK30s?               If so...what circuit? Some solutions are dependent on seeing the complete picture.