LT1001 precision current sink:  Why the JFET? - Page 1
Forum section: electronics / projects

author: smoothVTer
author_rank: Regular Contributor
timestamp: September 07, 2018, 06:34:35 pm
content: From Linear's/Analog Devices website:(You can also find this example circuit in the LT1001 datasheet)I understand the concept of this current sink topology without the JFET, where the OA output goes through a series resistor directly to the NPN's base, and the OA servos the base current in order to achieve the same drop across the sense resistor as the non-inverting terminal.In the illustration above, why is the JFET used as such? What benefits does this provide?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 07, 2018, 06:40:21 pm
content: With just a BJT, there will be some base current that causes an error. The control current for a JFET or alternative MOSFET is smaller.

author: voltsandjolts
author_rank: Unknown
timestamp: September 07, 2018, 07:24:18 pm
content: Seems like an old design from the days when MOSFETs had a lot more limitations. I think today you would replace the JFET / NPN pair with a single MOSFET.

author: Benta
author_rank: Super Contributor
timestamp: September 07, 2018, 07:55:28 pm
content: It's actually a very nice design for a high precision current source. You could regard it as a sort of "JFET/BJT Darlington pair".The input current into the gate of the JFET is negligible, so current through the sense resistor mirrors 100% the BJT collector current.Without the JFET, the amplifier (and not the positive supply rail) will supply base current to the BJT, making the current through the sense resistor larger than the collector current.Replacing the JFET/BJT with a MOSFET is a bad idea, as they have relatively high drain-source leakage currents.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 07, 2018, 08:02:45 pm
content: Drain source leakage would not be such a problem unless the set current is smaller. The more critical part is gate leakage - which is low for most Mosfets.Due to the input current of the LT1001 the circuit is not good in the nA range anyway.

author: coppercone2
author_rank: Super Contributor
timestamp: September 07, 2018, 08:08:36 pm
content: how does gate capacitance of mosfet and jfet compare for high frequency ?The circuit can be improved by adding a hi-z buffer on the feedback path possibly. to make a like a improved hcp. 800mA dissipation on that part makes this a good circuit

author: voltsandjolts
author_rank: Unknown
timestamp: September 07, 2018, 08:21:20 pm
content: Quote from: Benta on September 07, 2018, 07:55:28 pmOK, same result as using a MOSFET.Quote from: Benta on September 07, 2018, 07:55:28 pmWell, the D-S leakage current also goes through the sense resistor and is compensated for.For very low currents, there are low leakage MOSFETs.

author: Benta
author_rank: Super Contributor
timestamp: September 07, 2018, 08:37:13 pm
content: Did you notice V+ to the LT1001? 2...35 V.Try doing that with a MOSFET.It's a nice design, JFETs are incredibly useful, but mostly forgotten and nowadays hard to find.

author: voltsandjolts
author_rank: Unknown
timestamp: September 07, 2018, 08:50:02 pm
content: Quote from: Benta on September 07, 2018, 08:37:13 pmI don't understand your point.Quote from: Benta on September 07, 2018, 08:37:13 pmThat's because modern MOSFETs ate their lunch in lots of applications.

author: Andreas
author_rank: Super Contributor
timestamp: September 07, 2018, 09:01:30 pm
content: Quote from: smoothVTer on September 07, 2018, 06:34:35 pmHello,the JFET acts as a level shifter.So you dont need a rail/rail OP amp.(Yes the cirquit is probably not the newest).with best regardsAndreas

author: coppercone2
author_rank: Super Contributor
timestamp: September 07, 2018, 09:14:32 pm
content: Quote from: voltsandjolts on September 07, 2018, 08:50:02 pmI believe he is saying a mosfet without weird parameters would have its gate fried if the signal is used at maximum amplification make maximum use of the low noise floor of the device and that a jfet can handle it. I think its gate-source breakdown voltage for JFET ?linear mosfets I see from IXYS have something like +-20V Vgs max.

author: Benta
author_rank: Super Contributor
timestamp: September 07, 2018, 09:43:27 pm
content: Quote from: voltsandjolts on September 07, 2018, 08:50:02 pmMy point? Try turning on a MOSFET with just 1 V at the gate.Concerning "ate their lunch": MOSFETs are great for power applications, but in small signal circuits far from optimal. JFETs are perfect voltage controlled current amplifiers. Only problem is, their parameter spread is wide, because they're all 40+ years old, and no semiconductor company has shown interest in optimizing their JFET offering.

author: coppercone2
author_rank: Super Contributor
timestamp: September 07, 2018, 09:56:23 pm
content: how do linear mosfets compare? Like ixis brand

author: KE5FX
author_rank: Super Contributor
timestamp: September 07, 2018, 10:11:26 pm
content: It won't have anything to do with errors associated with base current or collector current because the whole thing is a closed loop.My guess is they just don't want the opamp to source any current, perhaps out of a concern with the offset voltage tempco. With the circuit as drawn, if you want more current you just need to use a beefier BJT, no need to worry about the beta.This circuit probably came from an old Jim Williams app note or data sheet, possibly one even older than the LT1001. If the original source could be tracked down, it might have a good explanation.... but good luck finding that.Edit: actually, that's not correct -- I assumed this was a current source intended to drive R, but as the title of the thread itself says, it's a sink.For a source they wouldn't care about the base current error, only the voltage across R. But since the circuit's job is to sink current from the 2N2219 collector to ground, any current supplied via the opamp's power rails will bypass R and constitute an error term. So that's probably it after all.

author: voltsandjolts
author_rank: Unknown
timestamp: September 08, 2018, 08:28:47 am
content: Quote from: Benta on September 07, 2018, 09:43:27 pmHmmm, so if I am ever building a current sink where the power supply is so limited there is only 1V drive available I will consider a JFET.Quote from: Benta on September 07, 2018, 09:43:27 pmYes, JFETs still have their place in low noise instrumentation front ends but this thread is about a mediocre current sink that will work perfectly fine with an easy to source, low cost MOSFET. If you love JFETs and want to sprinkle them over your designs just for some audiophool type reasons, go ahead, knock yourself out.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 08, 2018, 11:50:52 am
content: Essentially the same circuit is sometimes used with MOSFETs. With a MOSFET one may not need the extra BJT, as there are MOSFETs for the 100 mA and higher range, while JFETs for more than 50 mA are rare.If the driving voltage is an issue, there are also depletion mode MOSFETs, that in many respects behave similar to a JFET. Too much gate voltage could be an Issue, but many modern MOSFETs include protection, which might need a series resistor. The JFET circuit could profit from a series resistor too, to limit the gate current is case of a fault (open drain).

author: David Hess
author_rank: Super Contributor
timestamp: September 10, 2018, 06:14:14 pm
content: Quote from: voltsandjolts on September 07, 2018, 07:24:18 pmThe JFET is lower noise than the MOSFET for a given capacitance. The capacitance also limits high frequency impedance although if this is an issue, then a JFET or MOSFET cascode will be added. Suitable small signal MOSFETs are rare and expensive compared to JFETs.For a lot of applications, a low input bias current bipolar design would be suitable and in a floating application, the discrete transistors are not required at all. The later is commonly used for biasing shunt references within the feedback loop of an operational amplifier.

author: exe
author_rank: Unknown
timestamp: September 12, 2018, 10:50:19 am
content: Quote from: David Hess on September 10, 2018, 06:14:14 pmDo you have any circuit in mind that uses this trick?

author: David Hess
author_rank: Super Contributor
timestamp: September 13, 2018, 03:10:48 am
content: Quote from: exe on September 12, 2018, 10:50:19 amFigure 19 and 20 ofNational application note 20.

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 13, 2018, 04:34:43 pm
content: You will get less noise and a wider operating range with this arrangement (JFET+BJT) compared to a single MOSFET.The drawback is it requires a relatively large negative voltage.

author: Overspeed
author_rank: Frequent Contributor
timestamp: April 03, 2025, 05:35:23 am
content: HelloTI propose a new JFET in SOT 3 foot print , the JFE150DBVR which can provided a affordable solution at 2.5 Euro per unitRegardsOS

author: oran
author_rank: Newbie
timestamp: April 03, 2025, 06:06:22 am
content: Thanks everyone, this thread has been super helpful for my thermal camera project.

author: oran
author_rank: Newbie
timestamp: April 03, 2025, 06:07:04 am
content: thanks for that

author: Kleinstein
author_rank: Super Contributor
timestamp: April 03, 2025, 06:36:25 am
content: For the current sink the FET in question is inside the control loop of the OP-amp. So the FET noise would only be relevant for the higher frequency part (like > 10 kHz or 100 kHz), where the OP-amp part is slowed down. The extra slow down / compensation part is not shown in many of the circuits here, but it may be needed to avoid oscillation with a inductive "load". This is usually a capacitor as direct feedback at the OP-amp and if needed (high current versions) a series resistor for the DC feedback.

author: Gertjan
author_rank: Frequent Contributor
timestamp: April 03, 2025, 09:02:12 am
content: Quote from: Overspeed on April 03, 2025, 05:35:23 amThe JFE150 is NOT very suitable for this circuit. It's IDSS= 24mA....46mA is way to high. (JFE150 datasheet)The high IDSSwould result in the FET being almost in pinch-off. (It is only supplying the base current for the transistor...)Also, think of the fault mode when the power supply is removed, while Isinkis still connected. The FET will turn fully on, and I sink will be high. The higher IDSS, the higher this current will be. Another reason not to choose IDSShigher than necessary.When Max 30V for the sink is enough, BF245A is a good choice.For max 40V sink one can use J201 / MMBFJ201 (for small currents, <10mA), or J202 / MMBFJ202 (for currents up to 200mA)I used these FET's in combination with BD139-10 for current sinks 0,1mA...200mA.This is a very nice circuit, but it needs carefully dimensioning....regards, Gertjan.

author: Gertjan
author_rank: Frequent Contributor
timestamp: April 03, 2025, 10:50:04 am
content: An example of a design based on this idea, with low cost, easy available components.I needed a current sink for low currents (0,1...10mA). Most commercial loads are not very good in this range. Also I wanted to modulate the current.This design is a voltage controlled current sink, meant to be controlled by an ARB generator. Either with DC offset for constant current, or pulse etc. for all kinds of switched and/or modulated currents.As the ARB generator can be remote controlled, the combination is a Current Sink for use in automated measurements.mA-Current-Sink-v1,3-schematic.png(Sorry for some texts in Dutch, it was not intended to be published)DC accuracy is within 0,1% from 50uA to 200mA.Minimum Voltage for the sink is very low: only 0,75V @ 5mA Isink, 1,4V @ 50mA IsinkLED3 is limiting Isinkto 260mA when input is too high. R9 limits gate current when Isinkis missing. R8 helps to keep the gate a bit negative when the Power supply is missing, but Isinkis still there. in this situation Isinkis now <65mA.This current sink is fast and clean. I optimised it's response for having no aberrations. So any aberrations observed during a measurement must be caused by the DUT, and not by the sink:v1,3 1-7mA @ 10kHz-1200pix.pngv1,3 20-120mA @-10kHz-1200pix.pngAlthough I am happily using this Current Sink, i developed the design a bit further:mA-Current-Sink-v1,5-schematic.pngNow the FET is a MMBFJ202 for max 40V sink.D7 and R18 are added to clamp the opamp output when Isinkis missing. (To reduce gate current) So R9 could be reduced. (Did not make much difference in speed)R8 is smaller, to further reduce Isinkwhen the power supply is missing.The value of the input resistors is optimised to make the opamp input impedances more equal, to optimise DC accuracy at very small currents (10...50uA)regards, Gertjan.

author: Overspeed
author_rank: Frequent Contributor
timestamp: April 03, 2025, 11:08:16 am
content: Quote from: Gertjan on April 03, 2025, 09:02:12 amHelloGertjan , thanks I am not an expert in transistorBF245A is now obsolete but some dealer still list itmy purpose is to reach 1 amp current as source ( I don t built a sink but a source as the two circuit are available ) with voltage between 5V and 12V , now I have a voltage source in 5V .Question do you an advice for a possible transistor in 2 or 3 Amp to have safety margin ? BD139 is 1.5 Amp onlyRegardsOS

author: Gertjan
author_rank: Frequent Contributor
timestamp: April 03, 2025, 12:05:02 pm
content: Hi Overspeed,Yes, BF245 is obsolete, but still available. MMBF5484 is the SMD alternative. Unfortunately only max 25VDG. The J201 / MMBFJ201 and J202 / MMBFJ202 are current products, 40VDG, and the SOT-23 versions are cheap.But.... for a 2A current source I do not think that this is the best design. Realise that 12V x 2A = 24W. You will need an additional power transistor and good cooling.The nice features of this design will be lost (fast, clean, low Udrop). And its negatives amplified, especially the big Isinkthat will flow when the power supply is missing.I think that a design with OpAmp + MOSFET will be better suited for your purpose. Lots of ideas for those around here on EEVblog.regards, Gertjan.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 03, 2025, 01:32:11 pm
content: For the BJT one would need a higher gain (ggf. darlington like BDW93) or a high current JFET (e.g. 4393 or similar).The circuit as shown is more for small currents (e.g. 0.1 µA to 10 mA) like for the ohms current source in a DMM.One could also just use a large MOSFET (e.g. IRF540 or IRFP250). Here older and higher voltage types can have a better SOA - avoid modern low voltage types made for switching. They may not withstand much power in linear mode.The MOSFET version is the more common solution for electronic loads.

author: Overspeed
author_rank: Frequent Contributor
timestamp: April 03, 2025, 02:03:15 pm
content: Quote from: Gertjan on April 03, 2025, 12:05:02 pmHelloDoes the solution to use transistor in // can works ? even matched transistor ? on a lot of PSU they use multiple transistor setup .Additionnal question is between transitor or Mosfet wht is the best for 1 amp .I have a batch of IFR540 ( as Kleinstein advise in his answer ) but as the original circuit is with a JFET and a transistor why not try this setup , my purpose is to reach the best possible precision with the limitation to be able to locate the components at reasonable costs I don't want a '' cheap as dust '' solution or '' the miracle solution'' but I want to avoid the quest of rare components or aerospace grade / export limitedI agree that the original circuit goal was low current levelRegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: April 03, 2025, 03:10:32 pm
content: The advantage of the JFET is a more predictable gate leakage and this way good precision at low currents.Higher current need the extra BJT and beyond some 1 A one would run into the issue that the JFET has to provide enough base current. So one may need a darlington and than loose even more voltage (like 1.2 V).The advantage of a MOSFET is the ability to get to higher currents without the need for an extra BJT. This way the voltage drop can be low, mainly set by RON. This can be an advantage when doing things like single cell battery testing.The main downside of the MOSFET solution is the gate leakage from the usually included protection zeners. This may be a few nA (maybe more) and there are usually no strict specs for the leakage. So one may need the check the gate current to be on the safe side.An advantage is that without supply the current source is usually off.If really needed one can drive the MOSFET from a PV OK and this way get around the gate leakage, but this is a more excotic solution.A single IRFP250 or IRF540 can usually still be driven directly from an OP-amp. The capacitance is is bit higher than with the JFETs, but usually not too bad.