TESTING AND USING THE J201 JFET
Introduction
The J201 has been a popular choice with guitar pedal builders for over 20 years and has what is like a
‘cult status’ with many builders. Its popularity appears to have begun in 2001 when Donal Tillman
used it for a guitar preamp. See Appendix B for details. His original design intended to use a
Motorola 2N5457 N-channel JFET but he changed it to a J201 as he found the J201 was a lower noise
device for his preamp. That was probably a sensible choice at the time for a preamp and as it was
intended to handle low level signals then the other limitations of the J201 would not have had any
detrimental effect on its performance. However from that point onwards it seems that many
builders started to use it and probably for no other reason than it had been used in a well-publicised
design. It has also achieved popularity through its mention regarding Runoffgroove’s Fetzer Valve.
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. I have shown some relevant extracts from his
book in Appendix A where you can also find a link to download a PDF copy of his book.
So exactly what is a J201?
For a start - nobody actually ’makes’ a J201 (or the MMBFJ201 SMD version for that matter). All
junction field effect transistors (JFETs) are manufactured using processes that make it extremely
difficult to determine the precise outcome. If you look at the ON Semiconductor datasheet for a J201
you will see it says it is an N-channel JFET that is ‘sourced from Process 52’. What this means is that
silicon wafers are manufactured with device geometries, doping and diffusion patterns in
accordance with those specified for the JFET manufacturing method known as ‘Process 52’. Then
during post production testing of the devices on those silicon wafers, devices that meet the
published characteristics of a J201 (as registered with JEDEC) can be selected, packaged and sold as
J201s. For comparison, a 2N5457 which is often used in similar applications as the J201 is sourced
from ‘Process 55’ and because of its larger geometry it has a higher saturation current and gain.
For more information regarding ‘Process 52’ please refer to page 22 of this ON semiconductor guide:
- AN-6609 Selecting the Best JFET for Your Application (onsemi.com)
So a J201 is a JFET whose characteristics meet those specified for the J201 type as registered with
JEDEC (https://www.jedec.org/), the global standards organisation for the microelectronics industry.
Any device that fully meets the J201 specification can legitimately be called a J201 and is therefore
‘genuine’ from that perspective. On the other hand a ‘fake’ may be a few different things.
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Firstly it may well be a JFET that fully meets the specification but is sold as being
manufactured by a specific company when it has not. In this case it would be entirely
useable in a circuit requiring a J201.
Secondly, although marked as a J201, it may actually be a JFET with characteristics quite
different to a J201 in which case it could be used in circuits requiring a JFET but would not
perform well when used in a circuit designed specifically for a J201.
And thirdly, although marked as a J201, it may not actually be a JFET at all and often with
fakes obtained from unscrupulous suppliers may well be a bipolar junction transistor (BJT).
CAUTION: Be very careful in how you choose to test and determine the legitimacy of a J201. This is
because certain cheap component testers such as the LCR-T4 type are unable to identify JFETs which
have a pinch off voltage of less than -1 volt such as J201s correctly. They will identify a good J201 as
a BJT rather than a JFET. Please see the next section on testing JFET characteristics for more details.
Testing JFET Characteristics
There are various ways to measure the characteristics of a JFET as well as other semiconductor
devices. For amateur and hobby use you can buy relatively cheap microcontroller based component
testers like the GM328 or more specialised but obviously more expensive semiconductor analysers
such as the Peak Atlas DCA55 and DCA75. Images of these are shown below. Semiconductor
manufacturers and other professional organisations use highly accurate and high performance
semiconductor curve tracers but the cost of these is way beyond most amateurs.
Typical GM328 Tester
Typical LCR-T4 (M-Tester)
The Peak Atlas DCA-75 Analyser
This is one of the only low cost component
testers that can correctly identify and
measure JFETs like the J201 although the
information it provides is very limited as it
only shows drain current at a specific G-S
voltage rather than showing the true pinch
off voltage.
NOTE: This tester is unable to identify JFETs
with low pinch off voltages such as the
J201. It shows a genuine J201 as a BJT and
so should not be relied on for testing JFETs
accurately. Upgrading the firmware may
give better performance but I would not
rely on it for J201s.
This tester will give the most accurate and
comprehensive set of measurements of the
J201s characteristics as well as being able
to plot characteristic curves on a PC via its
USB interface. The DCA-55 version is a text
only lower cost version.
By far the cheapest, simplest and most reliable (though not necessarily the most accurate) method
of determining the basic characteristics of a JFET like a J201 is to build a small circuit as shown below
using a 9 volt battery and a couple of resistors. The most fundamental characteristics such as the
pinch-off voltage and the saturation current can then be measured by using a digital multimeter
(DMM) set to the appropriate DC voltage range.
Typical Test Results
To demonstrate the typical results you get when testing J201 with different testers, I chose a
selection of three TO-92 packaged devices that all fully meet the J201 specification. I also tested a
typical value MMBFJ201 surface mount device for reference. One of the TO-92 J201s had similar
values to what the datasheet lists as typical. That TO-92 device and the MMBFJ201 reference device
were selected to be closely matched. The other two TO-92 J201s had values that are either side of
the typical values.
Unfortunately I didn’t have any J201s with values at the extremes of the specified values but the
ones I measured should be typical of what is commonly available.
I used a GM328 tester, the LCR-T4 tester and the Peak DCA75 tester to see whether they correctly
identified the J201 and what readings they gave. The results are as follows. The numbers in brackets
for the J201s are just inventory numbers of mine and have no other significance.
MMBFJ201
SMD N-Ch JFET
Symmetrical Src/Drn
Vgs(off)=-0.72V at Id=5.3µA
Vgs(on)=0.36V at Id=1.25mA
gfs=1.8mA/V at Id=0.8mA to 1.2mA
Idss=0.61mA at Vds=3.00V
Rds(on)=594.7Ω at Id=1.3mA and Vgs=0.0V
J201 (#133)
TO-92 N-Ch JFET
Blue-G, Symmetrical Src/Drn
Vgs(off)=-0.73V at Id=5.1µA
Vgs(on)=0.30V at Id=1.17mA
gfs=2.0mA/V at Id=0.7mA to 1.2mA
Idss=0.60mA at Vds=3.00V
Rds(on)=550.9Ω at Id=1.2mA and Vgs=0.0V
GM328 Display Id=.32mA @ Vg 0.22v
LCR-T4 Display NPN hFE 196 Vf 678mV
J201 (#121)
TO-92 N-Ch JFET
Symmetrical Src/Drn
Vgs(off)=-0.57V at Id=4.8µA
Vgs(on)=0.32V at Id=0.91mA
gfs=1.8mA/V at Id=0.5mA to 0.9mA
Idss=0.38mA at Vds=2.99V
Rds(on)=613.1Ω at Id=0.9mA and Vgs=0.0V
GM328 Display Id=.31mA @ Vg 0.22v
LCR-T4 Display NPN hFE 154 Vf 531mV
J201 (#115)
N-Ch JFET
Blue-G, Symmetrical Src/Drn
Vgs(off)=-0.86V at Id=5.3µA
Vgs(on)=0.28V at Id=1.36mA
gfs=2.1mA/V at Id=0.8mA to 1.4mA
Idss=0.77mA at Vds=3.00V
Rds(on)=523.3Ω at Id=1.4mA and Vgs=0.0V
GM328 Display Id=.21mA @ Vg 0.15v
LCR-T4 Display NPN hFE 121 Vf 523mV
GM328 Display Id=.38mA @ Vg 0.27v
LCR-T4 Display NPN hFE 172 Vf 528mV
Conclusions
The results show that the LCR-T4 (M-Tester) component tester failed to identify all of the devices
correctly whereas the GM328 tester was able to identify the J201s correctly as JFETs but it provides
very limited information about them.
The results also indicate very similar characteristics between a modern SMD version of the J201 and
TO-92 versions.
Additional Test Information
The graph below plotted using the DCA75 shows the Id/Vgs characteristics of the three TO-92 J201s.
From this graph you can see the typical spread of characteristic values that you get with JFETs such
as the J201s. The numbers on the traces indicate the pinch off voltage, i.e. the Vgs(off) for the
various devices. The Id value of those traces with Vgs = 0v indicates the saturation current, Idss.
This variability shows how important it is to measure JFETs to select suitable ones for a circuit with
fixed biasing values or to be prepared to adjust resistor values to correctly bias devices.
The updated Runoffgroove article on what they call the Fetzer Valve has a good discussion on how to
calculate values for correct biasing. It is well worth reading that Runoffgroove article as well as
looking at some of the excellent application notes from ON Semiconductor, InterFET and Vishay that
discuss JFET biasing techniques.
Useful Links:
Runoffgroove - Fetzer Valve
ON Semiconductor – Glossary of JFET Parameters
Vishay – JFET Biasing Techniques
APPENDIX A
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.
NOTE: This is the end of the extract from Teemu Kyttälä‘s book “Solid State Guitar Amplifiers”. All
rights of the original author and copyright of the original book are hereby acknowledged.
APPENDIX B
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/
Schematic
FET Preamp Parts List
Q1
J201 N-channel JFET
R1
3.0M ohm 1/4-watt 5% resistor
R2
2.2K ohm 1/4-watt 5% resistor
R3
6.8K ohm 1/4-watt 5% resistor
R4
51K ohm 1/4-watt 5% resistor
C1
4.7uF electrolytic capacitor
C2
10uF electrolytic or tantalum capacitor
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.
NOTE: This is the end of the extract from Donald Tillman’s article. All rights of the original author and
copyright of the original article are hereby acknowledged.
Author: Peter Singfield
Doc. Version: 1.0
Date: 27 June 2021