J201 JFET - fabulous friend, sneaky foe (comment/discussion)

[brett]

I'm building a booster with a J201.
Usual thing - Rd of 10k ohms, Rs 10k trimpot (to get Vd of Vsupply/2).  Gate grounded by 1 M ohm resistor.

It biased nicely, with the source resistance of just 1k ohms (plenty of gain).  That's the FABULOUS FRIEND part.

But the voltage at the source is just 0.25V - which leads me to believe it will clip with just -0.25 V p-p of input.  That's the SNEAKY FOE part (at least for me, looking for some clean solid state boost going into the valve preamp.

Maybe this is an old topic?  Maybe it's too obvious to be worthy of a comment?  Maybe I haven't plugged it in yet and realised that it's barely noticeable? (yes to that one).

Cheers and have a good day.

Any comments welcome.

ps I note that JFETs that "turn on" with the source at 1 to 2 volts (2N5854) only have half the transconductance of the J201.  I assume that means the gain is less in a similar configuration (at least until the source resistor is AC bypassed with a capacitor).

[idy]

If you are using it as booster and not buffer then you will take output from Drain, not Source, right? What is the voltage at D? That is your bias point. I think...

[brett]

Vsupply =6 V and Vdrain =3.4 V
About 0.25 mA of current.
The biasing resistance on the source  is about 1k ohms.

(Vsupply is low because it's the 6 V AC supply for the valve heaters, rectified to DC.  However, the low Vgs-on isn't affected much by Vsupply ).

[idy]

So looks good. Should sound good. Your signal output is floating close to 1/2 supply. Problem?

[diydave]

But the voltage at the source is just 0.25V - which leads me to believe it will clip with just -0.25 V p-p of input.  That's the SNEAKY FOE part (at least for me, looking for some clean solid state boost going into the valve preamp.

Can I deduce that the Vp of your fet is .5 V?
In that case, any inputsignal higher than .5V wil get clipped. If your guitar has high output pickups, chances are you will get some inputdistortion because of it.

A fet with a higher Vp can solve this. For this, I would go with a 2n5854 and a Vp of say 1V (input voltage swing just got doubled to .5 V p - p)
But you will have to lose on the voltagegain part of your booster - as you mentioned already.

[Rob Strand]

Can I deduce that the Vp of your fet is .5 V?

I'm getting Idss=0.6mA, VP=0.72V ; virtually typ. values

ID = (6 - 3.4) / 10k = 260uA
IS = 0.25 / 1k = 250uA         ; cross-checks ID
So ID = IS ~ 255uA

Calcs with above mentioned params:

ID = 0.6mA (1 - 0.25/0.72)^2 = 256uA   ; ignoring channel modulation

[FSFX]

The Pros and Cons of the J201 JFET:
The J201 is actually far from an ideal JFET for a lot of designs but it does have the benefit of having, by comparison with many other JFETs, an exceptionally low pinch off voltage, Vgs(off), which makes it ideal for use in circuits running from low power supply voltages such as the 9v supply used in guitar pedals. But it does come at a cost. By having such a low pinch off voltage, it means that it can only handle low input signal levels before it starts to distort. Of course this no good for linear amplifier circuits but can be exploited by those guitar pedal builders who deliberately want to distort the signal. If it is used in buffers such as source followers then the low pinch off voltage means that the input requires biasing correctly to provide sufficient dynamic range. The low pinch off voltage and its high on resistance or low saturation current also means that it is not really suitable for applications where it is used as a voltage controlled resistance such as a phaser or it is used as an analogue switch in the signal path. Often the J201 is cited as a high gain device but in reality there are many other JFETs with superior performance.

In Teemu Kyttälä 's book, "Solid State Guitar Amplifiers", he discusses the issues with use of the J201 in many types of guitar pedal circuits and amplifiers.

The following is an extract from Teemu Kyttälä 's book "Solid State Guitar Amplifiers" that discusses the issues with use of the J201 in many types of guitar pedal circuits.

Solid-State Guitar Amplifiers
Teemu Kyttälä
1st Edition 2008

Link to PDF of book:
https://www.thatraymond.com/downloads/solidstate_guitar_amplifiers_teemu_kyttala_v1.0.pdf

The problems with using devices like J201s:
One should avoid using JFETs with low IDS especially in buffering circuits. For example, J201 is a commonly used FET - probably because it was once used in the famous "Till" guitar preamplifier and "FET Preamp Cable", both designed by Donald Tillman. However, the gate cutoff voltage VGS (OFF) of a J201 is about the lowest amongst all depletion mode JFETs and with moderate source resistor values this device can't even handle input signals that are greater than few hundred millivolts peak-to-peak. This FET is a horrible choice for buffers and basically for common source circuits as well. In an equal circuit, a higher current FET, like J309, can handle input voltages higher than 1 VPP and even offer slightly greater gain.

Although popular the J201 is really not that marvellous a device. Note that Donald Tillman originally substituted a higher current model with a J201 solely because of improved noise performance. Always base your component selection principles on circuit theory – not to a fact that a particular component was used in some famous circuit! It might have worked there – likely it will not work as well in another application.

The following is an extract from Donald Tillman's article that popularised the J201 amongst guitar pedal builders.

A Discrete FET Guitar Preamp
J. Donald Tillman
6 May 2001, updated June 2005

Link to article:
http://www.till.com/articles/GuitarPreamp/
Link to additional 'FET Preamp Cable article:
http://www.till.com/articles/PreampCable/

Technical details:
It's a simple unassuming common source FET stage that looks and performs somewhat like the first 12AX7 stage in a Fender preamp. Q1 is a Siliconix J201 N-channel JFET. You can get a copy of the data sheet from the Siliconix web site. My original design used a Motorola 2N5457 N-channel JFET, which also works well, but the J201 is a lower noise device.

A note about FETs:
I should point out that FETs in general suffer from a serious lack of manufacturing consistancy. The FET VGS and IDSS, the parameters that determine the bias point, can be anywhere over a 5-to-1 range and still be within spec. That's pretty awful, but such is life. It's an engineering accomplishment to design a circuit that can function exactly the same over a wide variation of component parameters. But I can't guarantee that in this situation; there's not enough supply voltage headroom.
The upshot is that while this preamp circuit is designed to work with typical J201 FETs, it will not work with all of them and it would be a good idea to try a handful of FETs and throw out any that don't bias correctly. An easy test is to measure the voltage at the drain of Q1 and if it's between 5.0 and 7.0 volts things are fine.

[FSFX]

I'm getting Idss=0.6mA, VP=0.72V ; virtually typ. values

With those values of Idss and Vgs(off), then it looks like a drain resistor of 15k and a source resistor of about 680 ohms would bias it to a sensible point.

[cspar]

Is that a screenshot from an online calculator or an installed program FSFX? I'm not familiar with that interface or any jfet calculator that outputs that much data.

[FSFX]

Is that a screenshot from an online calculator or an installed program FSFX? I'm not familiar with that interface or any jfet calculator that outputs that much data.

It is a brilliant JFET bias calculator spreadsheet by Frank_NH, one of the members of this forum.
I did a few minor changes to it and loaded it up on my website for people to use.

You can download it here: www.fleetingspider.com/files/JFET_Calc.xlsx

[Clint Eastwood]

You can use voltage divider bias to get a higher gate and source voltage, to solve the problem of clipping when the input signal goes too high.
6 volts supply really gives very little headroom for a clean booster.  Maybe you can use a voltage doubler or tripler with your 6 volt AC supply?

[brett]

Thanks everyone.
I note that some people thought the Gate was at -5 V or -0.6 V.
That would be good, but it was at 0.25 V.  I "squeezed" it a little further and got 0.27 V without throwing Vd too far away from the often-recommended Vs/2.  A possible advantage of this is the associated "cold/cutoff" bias, which to my ears gives more pleasant distortion than "hot/saturation" bias.  (I may change my mind about that in the future  ).

I got good gain (unmeasured as yet, but approx 20x) with a mild source resistor bypass (680 ohm resistor and 22uF cap).  Output goes through a 50kA 'gain' pot and into a 12AX7 triode.

There's mild distortion with the guitar volume cranked.  Quite pleasant, to my surprise.  And close to clean with the guitar volume at 8 to 9.  With the output maxxed, it's capable of driving the 12AX7 triode quite hard, with somewhere between the tasteful Fender blackface breakup and a raucous Marshall 18 watter.  The 12AX7 is set up very much like a Fender blackface but lower supply voltage (300V supply, 100k plate resistor, 1.5 k cathode bias resistor).

Overall, I've quite happy.

[Rob Strand]

That would be good, but it was at 0.25 V.  I "squeezed" it a little further and got 0.27 V without throwing Vd too far away from the often-recommended Vs/2.

The 0.6V is the JFET parameter VP and in-circuit the JFET operates at a gate-source voltage less than that, like 0.25V.

If you measure across the gate and source the meter loading can drop the measurement by a factor of 1/2 for a 1M ohm input DMM.  However if you measure the source voltage and the meter loading has (virtually) no effect.

[amz-fx]

Code Select Expand


   +------------------------+--- +9v
   |                        |
   <                        |
   <  6.8k                  |
   <                        |
   |     5uF                |
   +----||----+--- out      |
   |          |             |
|-+          |             |
    in ---+----->|            |             |
  |      |-+          |             |
  |        |          |             |
  < 10M    < 2.2k     < 51k        ---
  <        <          <            ---  10uF
  <        <          <             |
  |        |          |             |
  +--------+----------+-------------+--- gnd


  The FET is a 2N5457.  The voltage gain is subtle, 3dB or so. 


The original Till boost.

regards, Jack

[FSFX]

Jack, with respect, I had already posted details of the Till in my comment above.

For those interested in the original Till boost, here are the links.

A Discrete FET Guitar Preamp by J. Donald Tillman, 6 May 2001, updated June 2005

http://www.till.com/articles/GuitarPreamp/

Link to additional 'FET Preamp Cable article:
http://www.till.com/articles/PreampCable/

[Clint Eastwood]

The following is an extract from Donald Tillman's article that popularised the J201 amongst guitar pedal builders.

A Discrete FET Guitar Preamp
J. Donald Tillman
6 May 2001, updated June 2005

Link to article:
http://www.till.com/articles/GuitarPreamp/
Link to additional 'FET Preamp Cable article:
http://www.till.com/articles/PreampCable/

Technical details:
It's a simple unassuming common source FET stage that looks and performs somewhat like the first 12AX7 stage in a Fender preamp. Q1 is a Siliconix J201 N-channel JFET. You can get a copy of the data sheet from the Siliconix web site. My original design used a Motorola 2N5457 N-channel JFET, which also works well, but the J201 is a lower noise device.

My NatSemi Fet databook says that the 2N5457 was manufactured through a process very similar to that of the J201, just with a larger wafer geometry. For both devices a typical noise voltage of 10nV/Hz is given. These devices were marketed as audio devices, but they are not particularly low noise. Interestingly, the J113, sold as switch, is 6nV/Hz, and the BF245, RF device, is 8nV/hz.

So there isn't much difference in noise performance between these Jfets. I suspect, but that's a bit cynical maybe,  Jfets were marketed as 'general pupose or audio' if they were not good for anything else..

[FSFX]

My NatSemi Fet databook says that the 2N5457 was manufactured through a process very similar to that of the J201 . . .

The differences in JFET geometry, construction and application are fully described in the Fairchild Application Note. AN-6609.

https://www.onsemi.com/pub/collateral/an-6609.pdf

According to ON Semi, previously Fairchild, here are the different internal geometries of the J201, 2N5457 and J111 to J113.
The larger area (and more 'fingers') obviously give higher Idss and lower Ron for the different JFETs.

[Steben]

The following is an extract from Donald Tillman's article that popularised the J201 amongst guitar pedal builders.

A Discrete FET Guitar Preamp
J. Donald Tillman
6 May 2001, updated June 2005

Link to article:
http://www.till.com/articles/GuitarPreamp/
Link to additional 'FET Preamp Cable article:
http://www.till.com/articles/PreampCable/

Technical details:
It's a simple unassuming common source FET stage that looks and performs somewhat like the first 12AX7 stage in a Fender preamp. Q1 is a Siliconix J201 N-channel JFET. You can get a copy of the data sheet from the Siliconix web site. My original design used a Motorola 2N5457 N-channel JFET, which also works well, but the J201 is a lower noise device.

My NatSemi Fet databook says that the 2N5457 was manufactured through a process very similar to that of the J201, just with a larger wafer geometry. For both devices a typical noise voltage of 10nV/Hz is given. These devices were marketed as audio devices, but they are not particularly low noise. Interestingly, the J113, sold as switch, is 6nV/Hz, and the BF245, RF device, is 8nV/hz.

So there isn't much difference in noise performance between these Jfets. I suspect, but that's a bit cynical maybe,  Jfets were marketed as 'general pupose or audio' if they were not good for anything else..

If tubes would not have the high Usupply/filament issues, jFETs would be called utter crap.
Or am I very obnoxious now? 
Still, it is not hard to have a low to medium gain BJT stage with a high input impedance and comparable harmonic spectrum. You just need some extra components.

[FSFX]

If tubes would not have the high Usupply/filament issues, jFETs would be called utter crap.

I don't think it is just a matter of supply voltage as many JFETs work so much better at 24 to 30 volts anyway.
The only reason that the J201 is popular is that it runs OK on 9v supply.
Tubes will usually need output transformers.

[Rob Strand]

The J201 has a greatly reduced output swing on the Tillman preamp because the drain voltage biases high.
(No problem with the 2N5457, the original part used, despite what the website now shows.)



Here's a J201 version which performs very much like the 2N5457 version.







If I scaled the output 51k load the match is even closer.