Testing and Matching JFETS

By | March 3, 2012

Update: Matching JFETs – Revisited here.

Update: Kits available in the shop here.

Update: Matched sets of JFETs available for sale here.

We’re talking here about N-channel depletion-mode junction field-effect transistors.

  • “Field effect” because the current flowing between source and drain is controlled by the electric field strength at the gate.  Impedance at the gate approaches infinity so for practical purposes we assume that no current flows into or out of the gate – and therefore drain-source current is controlled by an electrostatic field rather than a current.
  • “Depletion mode” because if there is no electrostatic field at the gate maximum current flows between drain and source.
  • “N-channel” because the gate has to be negative with respect to the source to turn the drain-source channel off.

Quite interesting facts about JFETS:

1.  Current can flow in both directions in the drain-source channel unlike a bipolar junction transistor which only conducts in one direction.

2.  A lot of small-signal JFETS (of the types used for stompboxes and the like) are symmetrical.  This means that the drain and source are interchangeable.  This property is of practical use when designing a printed circuit board.

3.  When the gate-source is at zero volts a current can flow between drain and source.  In this state the JFET is acting as a constant-current source (and therefore the current flowing between drain and source is independent of drain-source voltage).  This current is termed Idss and is one of the parameters of interest when we match JFETs.

4.  To turn the JFET off we make the gate negative with respect to the source.  At some voltage – called Vgs(off), the “pinch-off voltage” – current in the drain-source falls to zero and the JFET is fully off.

5.  In between fully on and fully off there is a linear region where we can use the JFET as a transconductance amplifier.  Transconductance?  Well in a BJT (bipolar junction transistor) a small current flowing between base and emitter controls a larger current flowing between collector and emitter.  This makes a BJT essentially a current amplifier.  In the case of the JFET the voltage at the gate-source controls the current flowing between drain and source – and a voltage-controlled current-amplifier is called a transconductance amplifier.

6.  The nature of the manufacturing process for JFETs means that two JFETs with the same part number and from the same batch may vary wildly in important parameters.  Consequently JFETs don’t get used as amplifiers much in commercial designs if the manufacturer is not prepared to pay extra for tightly-matched (i.e. specially selected) parts.  Even when used as a switch the manufacturer may need to select devices with a Vgs(off) within a suitable range for the design.

I took a batch of twenty Fairchild 2N5458 JFETS and tested each sample for Vgs(10kOhm), Vgs(10MOhm) and Idss.

Firstly, R.G.Keen’s improved JFET matcher design was used to find the value of Vgs where Rds (resistance between source and drain) is 10k.  Here is a partial schematic of R.G.Keen’s design:

106SV

R1 and R2 form a potential divider that fixes the op-amp’s non-inverting input at 1/2 Vcc.  The JFET and R3 also form a (variable) potential divider.  The circuit will balance when the op-amp’s differential input voltage is zero.  So  if the voltage at pin 2 is the same as pin 3, the JFET/R3 potential divider must be equivalent to the R1/R2 potential divider.  Hence the output of the op-amp biases the JFET so that its Rds is 10k Ohm.

This test is useful for matching JFETs in phaser designs where a bunch of JFETs all get the same bias signal and need to be the same drain-source resistance for a given bias voltage.

Secondly, a digital multimeter was connected between gate and source to measure Vgs when the resistance between source and gate is equal to the input impedance of the voltmeter.

107SV

Here we are measuring the gate-source voltage when there is a very large resistance in the gate-source circuit.  Digital multimeters (DMM) typically have an input impedance of 10M Ohm so the DMM in the above circuit (left) is equivalent to putting a 10M Ohm resistor in the source leg and then measuring the voltage across the 10M Ohm resistor with a perfect voltmeter (i.e. a voltmeter with infinitely high input impedance).   This technique gives a value for Vgs(10M) which is practically the same as Vgs(off).

This test is useful for making sure that there is enough voltage to fully switch off a JFET when it is used as a switch (e.g. in an Ibanez-style effects pedal).

Thirdly the gate and source were connected together to set Vgs at zero volts and Idss was measured.

Measuring Idss

Idss is the current which flows in the JFET when there is no negative voltage on the gate with respect to the source.  If we are planning to use the JFET as a constant current source, it is important to know what the value of that current is.

The results of this testing on our twenty 2N5458 JFET samples revealed that:

  • Vgs(10K) varied between -0.99V and -1.75V
  • Vgs(10M) varied between -1.61V and -2.38V
  • Idss varied between 3.24 and 5.99 mA

According to the Fairchild datasheet for the 2N5458, Vgs(off) ranges between -1V and -7V and Idss ranges between 2mA and 9mA, so – whilst my batch of twenty is fairly well clustered in the middle of the range, the lowest Vgs(off) is still 50% lower than the highest and the highest Idss is 84% higher than the lowest.  These are significant differences.

Here’s an integrated JFET tester/matcher to speed up the process of testing or matching JFETs:

JFET Matcher/tester Schematic

 The finished article:

JFET Tester/Matcher

Here is the PCB overlay and etch-resist pattern.  The etch resist pattern should be printed at 300 dpi.  The board is 55 x 38.5mm.

Overlay

JFET matcher kits are available in the store here and there are other JFET testing and matching articles here and here.

16 thoughts on “Testing and Matching JFETS

  1. BaldPaul

    Great article. I would love one of these PCBs or even a kit! Wonderful site. Just found you today and look forward to visiting often.

  2. SmudgerD Post author

    Thanks Paul.

    I’m looking into setting up a webshop to sell pcbs and matched JFETS. I have designed a MKII Jfet matcher in the form of an Arduino shield which enables me to do automated testing on large numbers of JFETS.

    SmudgerD

  3. dondejong

    This looks like a very comprehensive tester. I like that it can test all three parameters. Just one question, and maybe it’s because I’m a “Yank,” is LK1 a toggle switch? If so, is it left open for current measurements and closed for voltage measurements? Thanks.

    1. SmudgerD Post author

      LK1 is a ‘link’ or ‘jumper’. It could be a toggle switch. Its purpose is to allow the tester to be operated without having an ammeter in circuit. This is a convenience for when you may only have one multimeter handy and you are using it to measure the voltage. The toggle switch you see in the photo is used to switch the power.

      1. dondejong

        So, if the ammeter can be left in the circuit, the ‘link’ isn’t necessary for taking the voltage readings?

  4. RAD

    Hi, Thanks for posting this circuit – I’m in need of one of these, so I’m preparing to build it. One question though. As I’m more accustomed to push switches and toggles, could you please tell me what type of rotary switch that is (i.e., a 3P2-4T or 4P2-3T, or whatever else) ? Thanks for any insights!

    1. SmudgerD Post author

      The rotary switch is a 4-pole, 3-way (or 3-position) switch. Only 3 of the 4 poles are used. The prototype used a Lorlin type CK1052. You could use an alternative, such as an Alpha SR2612F-0403 part.

    1. JohnBS

      In the first op-amp circuit, 106SV, you are not actually setting Rds to 10k. You’re setting Id to match the current thro R1 / R2. In other words, if you plot Vds versas Id, you’re setting the chord to the curve, not the tangent.
      John

        1. JohnBS

          In Trump-world, you may be right. But we’re not talking about heair-spliitting here: the difference will typically be 2:1 or greater.

          1. SmudgerD Post author

            The difference in what?

            See image 106SV Rev1 above. Let’s assume the op-amp is ideal (infinite input impedance). Let’s assume the supply voltage Vcc is ideal and exactly 9V. Let’s assume the resistors are ideal and exactly 10k.

            By Ohm’s law, the voltage across R2 is 4.5V. The op-amp is going to set its output voltage so that the net input voltage (difference between inverting and non-inverting inputs) is zero. This means that the op-amp will bias the JFET so that the voltage across R3 is 4.5V. No current flows into the op-amp inputs and no current flows in the gate circuit of the JFET, so all the current flowing in R3 is flowing between the drain and the source of the JFET. Ergo, the equivalent series resistance of the JFET is voltage/current which equates to 10k.

    2. JohnBS

      Yes, what you’ve stated regarding the current flowing thro the FET is bang-on: if you substituted 10k for the FET, the DC condtions would be identical. But you’re using the term “resistance between source and drain”, which is defined as the delta current versas delat voltage applied, i.e incremental resistance, or hoe in semiconductor speak.
      I can’t post images here , but if you go to
      https://electronics.stackexchange.com/questions/176435/depletion-p-channel-jfet-thats-saturated-at-v-gs-0v
      and look at the curves for MMBF 5460, you can see what I’m trying to explain. The “resistance” on you definition at Vgs = 0, 5V Vds is 5/1.6mA = about 3k.
      But if you reduce Vds to 3V, the current only falls to 1.5mA, i.e the FET is behaving a like a resistor with value ~ 2/0.1mA = 20k.
      John

      1. SmudgerD Post author

        OK, so I think the point is here that my perhaps somewhat over-simplified analysis is nonetheless valid for matching JFETs (which, for our purposes here, basically only require one data point to match quite closely). In this instance, we are not trying to find JFETs with the same absolute characteristic, only the same relative characteristic.

        Furthermore I do clearly identify in later articles on the subject (and in the Instruction manual for the Stompville JFET Matcher Kit) the fact that any measurements made are dependant on the magnitude of Vcc.

        Also, please bear in mind that many of our readers are musicians and enthusiasts not graduate electronics engineers and this blog does not claim to (and is not intended to) give an exhaustive theoretical treatment of the subject.

        Thank you for your comments.

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