Precision Current Sink - I found a Gem of a Circuit - Page 1
Forum section: electronics / projects

author: Benta
author_rank: Super Contributor
timestamp: May 24, 2025, 11:03:00 pm
content: Current sinks with BJT output always have the problem that the base current also flows into the sense resistor on the emitter. For small signal sinks normally not a problem, but power BJTs have low HFE, which poses a problem for high precision sinks.I found this little gem of a circuit that solves the problem with a couple of resistors. Hats off to whoever devised it, it's brilliant.I did not invent it, dunno who did, but I found it in AoE3, X-Chapters. Simulations show that it works perfectly.Perhaps it's useful for other people here.

author: D2236
author_rank: Regular Contributor
timestamp: May 24, 2025, 11:45:27 pm
content: Apparently, it was invented by Carl Nelson according to the late Bob Pease, where he talked about it on his Analog Design Show many years ago.https://www.electronicdesign.com/technologies/analog/article/21805566/an-interview-with-analog-guru-carl-nelson

author: mawyatt
author_rank: Super Contributor
timestamp: May 25, 2025, 02:58:55 am
content: Absolutely brilliant concept by Carl NelsonBest

author: Gertjan
author_rank: Frequent Contributor
timestamp: May 25, 2025, 07:07:32 am
content: Hi Benta,Thank you for bringing this brilliant circuit to our attention!For those who want to read more:- This can be found in the "Art of Electronics: The x-Chapters",page 356.- Horowitz & Hill are mentioning as their source: "Error compensation improves bipolar current sinks", EDN, 6 July 2006by Christian de Grodinsky.Horowitz & Hill are also noting that this circuit "was independently devised earlier (though not published) by Carl T. Nelson, working at National Semiconductor."regards, Gertjan.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 25, 2025, 07:36:20 am
content: Nifty when it was designed way-back, but these days MOSFETs can give a simpler solution that does not need 3 ratio matched precision resistor pairs.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 25, 2025, 09:05:03 am
content: Even back than they had the option to use a darlington configuration for more gain and better accuracy.

author: Gertjan
author_rank: Frequent Contributor
timestamp: May 25, 2025, 12:31:37 pm
content: Quote from: PCB.Wiz on May 25, 2025, 07:36:20 amI expect that this BJT based current sink is faster than a MOSFET version. Consider the big input capacitance of a MOSFET...

author: incf
author_rank: Frequent Contributor
timestamp: May 25, 2025, 01:13:48 pm
content: Tangentially related question:This uses a combination of positive feedback and negative feedback. Yet it is stable.Without performing an analysis of the loop transfer function in the frequency domain, is there a "rule of thumb" for determining how much positive feedback is acceptable before it starts to compromise stability? (for example something like "positive feedback must be 10 times smaller than the negative feedback")

author: kreutz
author_rank: Contributor
timestamp: May 25, 2025, 04:21:56 pm
content: Phase Margin and Gain Margin:Phase Margin:.The phase margin (PM) is the difference between the phase shift at the point where the loop gain is unity (0 dB) and -180°.Gain Margin:.The gain margin is the amount of additional gain that can be added before the system becomes unstable, measured at the frequency where the loop gain's phase shift is -180°.Stability:.A higher phase margin indicates a more stable system. Systems with a phase margin of 45 degrees or more are generally considered stable.In summary: Stability criteria are essential for designing circuits that operate reliably and predictably.

author: mawyatt
author_rank: Super Contributor
timestamp: May 25, 2025, 05:40:33 pm
content: Quote from: incf on May 25, 2025, 01:13:48 pmMany "systems" employ both negative and positive feedback, for the "system" to be stable this requires the negative feedback to dominate over the positive feedback. So think anything over 3X of dominate negative feedback should be acceptable, the more the better obviously and one should always check for stability.In Carl Nelsons Precision Current Sink the negative feedback dominates since the negative feedback "moves" the effective differential side of the Voltage developed across the sense resistor, whereas the positive feedback doesn't "move" the Input Reference Voltage but only a small amount for the op-amp + input by means of the + feedback resistor ratio.A stability analysis shows that the system is inherently stable but can be marginally stable (Phase Margin <60 degrees) with very slow transistors, here one should employ typical stabilizing networks to improve the Phase (and Gain) Margins to acceptable levels.Here we show the LTSpice simulation with an output current of 100ma using a 2N2222 and 2N3055 transistor. C1 is the compensation capacitor (simplest method) and isn't required with the 2N2222 as the Phase Margin is ~69 degrees as shown, however with a much slower 2N3055 the Phase Margin Degrades to just 21 degrees and requires C1 to be 400pF to achieve acceptable similar Phase Margin. The Pulse Responses show the effects of the slower transistor and compensation with C1.Anyway, hope this helps.Edit: Added a Temp and Output Voltage Sweeps which help illustrate the pure genius of Carl Nelson's designBest

author: dobsonr741
author_rank: Frequent Contributor
timestamp: May 25, 2025, 06:30:48 pm
content: Time to build another electronic load now with 2N3055! No more worries about the flimsy DC SOA of the power mosfets. Nice!

author: incf
author_rank: Frequent Contributor
timestamp: May 25, 2025, 06:34:12 pm
content: Quote from: mawyatt on May 25, 2025, 05:40:33 pmThat's an excellent response.side question: when doing that type of simulation is C1 supposed to be on the other side of Vbode? (or maybe it doesn't affect the results?)

author: mawyatt
author_rank: Super Contributor
timestamp: May 25, 2025, 06:51:58 pm
content: Quote from: incf on May 25, 2025, 06:34:12 pmJust updated our response above, since we didn't directly respond to your question about "how much" negative feedback!!You are correct C1 should be on the other side (thanks for spotting this), since some current can bypass Vbode by means of C1. We show a Phase Margin of ~69 degrees when C1 connected to Vbode - and ~73 degrees connected to Vbode+, so in the case doesn't really matter.Best

author: mawyatt
author_rank: Super Contributor
timestamp: May 25, 2025, 06:56:07 pm
content: Quote from: dobsonr741 on May 25, 2025, 06:30:48 pmThose old steel can TO3 RCA 2N3055s were about as tough a device as one could get, and they were cheap!! As more sources began showing up, some weren't as "tough" as the RCA types, probably using a smaller die.BTW we all owe a debt of gratitude to Benta for bringing this topic upBest

author: mawyatt
author_rank: Super Contributor
timestamp: May 25, 2025, 07:25:20 pm
content: You can change the sense resistor to 1Ω and make a Darlington with the 2N3055 and a 2N3904, handles 5 amps without any issue. Now change Rsense to 0.1Ω and 10 amps are no problem with a Triple Darlington with Double 2N3055, tho you'll quickly run into the simulation/model limits!!Be sure to scale resistors properly.Best

author: MathWizard
author_rank: Super Contributor
timestamp: May 26, 2025, 03:37:44 am
content: The Array3011 E-load I've been modelling and thinking of making, uses this configuration, for it's mosfets. So there might not be much DC current to worry about, someday I'll try the AC model of it and see what that looks like.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 26, 2025, 08:33:37 am
content: A tripple darlington can be an issue as it does not work at low voltage, like discharging a single NiCd / NiMH or alkaline cell.Already a normal darlington configuration is not ideal but could be to maybe 0.8 V if the drop at the shunt is small.One could also consider the combination of BJT and MOSFET - so kind of the IGBT replacement circuit.When using the right (usually high voltage types) the SOA is actually not that bad with MOSFETs. BJTs also have there SOA issues above some 50 V.

author: tszaboo
author_rank: Super Contributor
timestamp: May 26, 2025, 10:06:43 am
content: Quote from: mawyatt on May 25, 2025, 06:56:07 pmSure and the wires for it need to come through your heatsink. And the package has ~0.7K/W heat resistance, while TO220 goes down to 0.4K/W in some high power devices, TO247 to 0.2K/W.Most of the stability issues with FETs I've seen were relatively easy to solve by turning the driver OPA into a lossy integrator.

author: mawyatt
author_rank: Super Contributor
timestamp: May 26, 2025, 02:00:10 pm
content: Quote from: tszaboo on May 26, 2025, 10:06:43 amThe TO3 did required wiring to the heatsink which created additional unwanted and uncontrolled parasitics, whereas the TO220 (& later the TO247) allowed direct PCB connections. This not only reduced cost, assembly time, and parasitics, but improved thermal contact, and created a neater design. Don't think you can find many TO3 devices today, and if so, likely costly!!Best

author: mawyatt
author_rank: Super Contributor
timestamp: May 26, 2025, 02:15:35 pm
content: One of the interesting observations of this circuit by Nelson and shown by OP Benta, is the current sense resistor sets the overall accuracy, whereas all the other resistors influence the base current compensation. Because the base current is small relative to the output current (by beta), the accuracy/precision/TC of these resistors is much less important, and the absolute value is also less important, as the ratio of resistor pairs is the determining compensation factor.The more one studies this circuit, the more one appreciates the genius of Carl Nelson's designBest

author: David Hess
author_rank: Super Contributor
timestamp: May 27, 2025, 09:57:11 am
content: Quote from: PCB.Wiz on May 25, 2025, 07:36:20 amAnd before MOSFETs, JFETs.Quote from: Gertjan on May 25, 2025, 12:31:37 pmIt comes down to the ratio between the capacitance and transconductance, where bipolar parts are better for both.Quote from: mawyatt on May 25, 2025, 06:56:07 pmThe original 2N3055s used the mesa process which lacked secondary breakdown. These can be identified by their Ft of like 10 kHz minimum, or 60 kHz typical.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 27, 2025, 10:29:28 am
content: Quote from: dobsonr741 on May 25, 2025, 06:30:48 pmDon't know about how better SOA they have against MOSFETs, but they still need a lot of base current, which makes it larger and not so convenient.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: May 27, 2025, 01:46:40 pm
content: Quote from: dobsonr741 on May 25, 2025, 06:30:48 pmExactly my thoughts.For some strange reason I, and many others in this forum, find E-Loads a very satisfying circuit to play with. Always looking for an improvement.

author: mawyatt
author_rank: Super Contributor
timestamp: May 27, 2025, 02:41:53 pm
content: Quote from: UnijunctionTransistor on May 27, 2025, 01:46:40 pmThe E-Loads are great circuit for evaluating & learning about analog design concepts. They are simple, precise, and embrace higher currents/voltage with a single supply. With a large selection of available cheap parts, almost interchangeable, they serve a useful purpose on the bench, so not surprised they are a popular DIY circuit!!With base current compensation, this does seem quite inviting to "play around with"Best

author: Terry Bites
author_rank: Super Contributor
timestamp: May 27, 2025, 04:06:34 pm
content: I saw this in a design idea from Christian de Godzinsky in EDN in 2006.But I'm sure it pre-dates that.https://www.edn.com/error-compensation-improves-bipolar-current-sinks/It extends the idea with and improved compensation scheme. Works a treat as I recall.I had an idea for sampling Idss to correct mosfet sources/ sinksI'm sure one of you guys will have a go at that.

author: mawyatt
author_rank: Super Contributor
timestamp: May 27, 2025, 05:13:22 pm
content: Quote from: Terry Bites on May 27, 2025, 04:06:34 pmThe EDN article is referenced in post #3 above.The Idss leakage should be handled by the normal circuit configuration since the leakage from MOSFET Drain to Source flows thru the sense resistor and controlled by the "normal loop" behavior. Same should be true for a Bipolar Transistor with Collect to Emitter leakage.Agree, we're sure "Somebody" will play around with this, it's too intriguing a circuitBest

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: May 27, 2025, 08:39:47 pm
content: This circuit has intrigued me enough to simulate it. Works great!Of particular interest, is how to parallel at least two power pass transistors. On a real world implementation, for SOA considerations, one would very likely require two or more pass transistors. I am still in the “play with the circuit” phase.

author: mawyatt
author_rank: Super Contributor
timestamp: May 27, 2025, 08:51:15 pm
content: Quote from: UnijunctionTransistor on May 27, 2025, 08:39:47 pmThis circuit has intrigued me enough tobuildit. Works great!Terry knew we couldn't resistEven on a Protoboard with just 1% resistors, OP-07 and a TIP-41, using a 10-Turn Pot with Dial as the inputJust add a small emitter resistor to the parallel power bipolar devices for higher currents. You shouldn't need to "split up" the output devices with separate control if the emitter degeneration is designed properly. One could probably use a "Quasi" Darlington where one input transistor feeds a pair of power transistors with emitter degeneration. Just make sure that all the base "effective" currents flow thru the Op-Amp output series resistor, this is the only way to get proper base current compensation.Best

author: Benta
author_rank: Super Contributor
timestamp: May 27, 2025, 09:09:12 pm
content: A comeback from the OP.I'm gratified and a little bit surprised that this little circuit has inspired people this much.I played around with the simulations as well and found the same result as mawyatt: the sense resistor sets the precision (obviously), but the other resistor ratios are rather uncritical, as are their precision.The classical way of analyzing that (which I'm sure Carl Nelson did) is to find partial derivatives for the component sensitivities. I'm too lazy to do that myself today (was curriculum in the late 80s)Simulators rock for that.Thumbs up for analog electronics and circuit gems. I hope to find more.

author: mawyatt
author_rank: Super Contributor
timestamp: May 27, 2025, 09:32:00 pm
content: If one followed thru with the partial derivatives for component sensitivities, all the resistors except the sense resistor should have an output current sensitivity of at least 1/(Effective Beta) wrt the sense resistor. Effective Beta is Io/Ii of the output device type.The sense resistor obviously has a sensitivity of 1 to the output current, and also has the most temperature sensitivity, so this becomes the limiting component in this precision source.Anyway, thanks for posting this very intriguing circuit!!Best

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: May 27, 2025, 09:37:21 pm
content: Quote from: mawyatt on May 27, 2025, 08:51:15 pmYou mean like so?

author: mawyatt
author_rank: Super Contributor
timestamp: May 28, 2025, 01:58:06 am
content: Yes.If you need a Darlington for lower base current then you can use 1 transistor in front of the pair.Edit: Another option with higher current output transistors instead of a Darlington is to move the Darlington input transistor "inside" the control loop with the op-amp output with the collector connected to Vcc. This acts as a buffer for the op-amp output to reduce op-amp output current demands. With this configuration the output load current can be from a supply less than 0.3V and still maintains constant current throughout the output supply range.Best

author: RoGeorge
author_rank: Super Contributor
timestamp: May 28, 2025, 12:50:22 pm
content: Trying to avoid the 6 ratio matched resistors (with one of them being a power resistor), here's my take on it:Made a copy of the base current, then drained the same amount from the collector, thus making Ic=Ie.Problem solved, and easier to integrate (matched transistors are almost trivial in an IC).Might work much faster (but I didn't check), because all is current controlled, no M\$\Omega\$ resistors.

author: mawyatt
author_rank: Super Contributor
timestamp: May 28, 2025, 01:46:35 pm
content: Interesting idea, some simulations should show how well behaved this is.Edit: You'll need a diode across the PNP Reference transistor to allow the op-amp to turn off the output device. Consider building and testing this, would be interesting to see how well it behaves and you could probably use some select 2N3904/6s or add some small ballast emitter resistors.WRT to the resistor "matching" and "value" in Nelson's circuit, as we mentioned the sensitivity of the matching is low and only ratios matter. Instead of 1MΩ/10kΩ, one might use 100kΩ/1kΩ, or other convenient ratios depending on the desired current range and sense resistor.We used 1% unmeasured and unmatched resistors grabbed from our old parts bin with excellent results.Best

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 28, 2025, 08:50:57 pm
content: Quote from: mawyatt on May 27, 2025, 09:32:00 pmThat's rather optimistic.Spice says it is far worse than that, as any skew in the bridge balance results in worse PSRR, (HFE varies with voltage) and if I measure PSRR, spice says* Input feedback resistor has a 0.5 sensitivity, unbalance that 1% and PSRR degrades to 0.5%* Most other resistors are about 0.15, so a 1% unbalance gives ~0.15% errorThat means if you want 10bit or 12bit accuracy, you will need ~0.1% resistors, or better, with good ppm specs too.

author: Benta
author_rank: Super Contributor
timestamp: May 28, 2025, 11:26:24 pm
content: Quote from: PCB.Wiz on May 28, 2025, 08:50:57 pmI cannot reproduce your results.Running sims on the circuit in my OP, and defining the sense resistor as a precision type (otherwise this exercise makes no sense) plus using a lousy BJT (MJD3055, Onsemi), I get the following results:Nominal ISINK= 100.02 mAR1, R2: 10k,R3: 525k, R4: 475k, R5: 500, R6: 10. ISINK= 100.17 mAR1, R2: 10k,R3: 475k, R4: 525k, R5: 500, R6: 10 . ISINK= 99.87 mAR1, R2: 10k, R3, R4: 500k,R5: 475, R6: 10. ISINK= 100.04 mAR1, R2: 10k, R3, R4: 500k,R5: 525, R6: 10. ISINK= 99.98 mAI've seldom seen a circuit with so low sensitivity to component tolerances, it impresses the h*** out of me. The resistor ratio skews are at 5%!The reference to HFEvs VCEis irrelevant, as this circuit cancels the effect of HFE. And how PSRR comes into the discussion needs an explanation. Are you modulating VCCwhile simulating?Cheers.PS: simulations run with ngspice in KiCAD V8.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 29, 2025, 12:40:22 am
content: Quote from: Benta on May 28, 2025, 11:26:24 pmThat's because you did not test PSRRQuote from: Benta on May 28, 2025, 11:26:24 pmIn the real world, you expectgood current sinks to not vary with voltage. (good PSRR or high output impedance)So I run a real world spice test, of vary of the current sink voltage span, (2-35V) and then see how that varies, as the ideal resistors vary from ideal.As the bridge unbalances, you expect PSRR to get worse, and indeed it does.Here are the LTspice resultsVC = 5-35V, dI(Re) = 17uA Io = 10.066343mA-10.083611mAdI(R7) = 5nA on Io = 10.000385mA, with matched resistors (very good)R5 +1% to 505 Ohms dI is -184nA Io = 10.001045mA 0.01%R2 +1% to 10.1k dI is +151nA Io = 9.9857915mA 0.142%R4 +1% to 505k dI is -145nA Io = 10.014834mA 0.148%R3 +1% to 505k dI is 17uA Io = 10.051133mA 0.5%R1 + 1% to 10.1k dI = 345nA Io = 10.015626mA 0.156%

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: May 29, 2025, 05:04:58 pm
content: Good enough for Australia.

author: Benta
author_rank: Super Contributor
timestamp: May 29, 2025, 06:31:23 pm
content: @PCB.Wiz:interesting. I'll try to rerun the sim while sweeping the collector voltage.

author: Benta
author_rank: Super Contributor
timestamp: May 31, 2025, 09:46:33 pm
content: I've now done the PSRR sweep, and I'm still impressed with this circuit.The schematic has been modified to make a collector voltage sweep (5...35 V) of the output transistor:The results you see in the attached table. The methodology is different from that of PCB.Wiz, as I've only varied one parameter at a time, so this is the sensitivity of output current vs. each resistor.The sweeps all show monotonic behaviour, meaning there are no hidden current peaks or dips anywhere.As can be seen, the circuit is completely insensitive to collector voltage variations, meaning that PSRR is (very theoretically) infinite.But that's with using ideal resistors (0% tolerance).I've done the sweeps changing each resistor by +/-5%; output current change is lower than 1% in all cases, giving a sensitivity of <0.2, which is a very good value.More interesting is the PSRR sensitivity, which is below 0.1% in all cases.In conclusion: this circuit has excellent PSRR, sorry.PS: I haven't modelled changing R1 and R2, as only the resistor ratios are interesting here.

author: joseangel
author_rank: Contributor
timestamp: June 01, 2025, 06:35:07 am
content: Thank you all for the nice simulations and results report.It's not my intention to correct anybody, simply to state the terminology, to avoid confusion,- PSRR (Power Supply Rejection Ratio), is the variation of any parameter of interest wrt the Power Supply variations (including ripple, noise and the like). It is an important metric in many analog designs, even if you sometimes can avoid it altogether by providing very clean power rails.- What you have been simulating above is simply the Output Impedance (Zo) of the current sink. I have always liked the way it is done with SPICE, by simply sweeping a voltage source (burden voltage) in the load. The simulations above indicate Zo in the order of 1 Megohm, which is quite nice

author: RoGeorge
author_rank: Super Contributor
timestamp: June 01, 2025, 07:42:46 am
content: For the fun of it and to double check my idea, made a brief comparison.With green is Carlson's compensation for Ib, with yellow is mine.CMRR with frequency of Vx (-CMRR is Zout, because the chosen AC stimulus was Vx=1V).Transient response for a Vx swing between 5-25V, at 4 different CC values requested: 1mA, 10mA, 100mA, 1AChanged mine a little from my previous pencil draft, by subtracting the Ib from Ie, such that the current through the R sense is kept the same as Ic, and by moving the second mirror emitters below GND (to give enough Vc for Q6 when V on R9 approaches 0V).Pros:- easy to integrate in an IC (no high value resistors, no laser trimmed resistors matching)- has bigger Z, CMRR is with about 20dB better- slightly faster response and more stableCons:- doesn't behave well for very small currents (could be mitigated, but in this version it needs at least 3-5mA Iout)- requires from the opamp double the current (could be mitigated by using mirror BJTs with area ratio other than 1:1)- not suitable for discrete transistors (requires matched transistors, sensitive to thermal imbalance in the mirror BJTs)

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2025, 12:43:44 pm
content: Quote from: Benta on May 31, 2025, 09:46:33 pmThe limited output impedance of the transistor causes a variation in Vbe which is divided by the open loop gain to create an input offset error limiting PSRR. I think it simplifies to the output impedance of the transistor multiplied by the open loop gain, so high, but probably megohms.

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: June 03, 2025, 06:50:39 am
content: Quote from: Benta on May 24, 2025, 11:03:00 pmInteresting.I once did a driver circuit for a high-power LED in a biomedical fluorescence instrument. The circuit was very much like this, except:* no positive feedback (no R3);* added a capacitor immediately in series with R4, for minimum DC error and for rise/fall ramp control;* mosfet for Q1.It went into production and worked quite well. I didn't consider trying it with BJTs (no time; hurry, on to the next thing).

author: peter-h
author_rank: Super Contributor
timestamp: June 03, 2025, 11:14:47 am
content: Yes; if you use a MOSFET then it becomes trivial. I use such a circuit in production to sink 0-20mA (for 4-20mA sensor emulation).Interesting comments about old 3N3055 SOA. I used to work in another field where SOA was a big concern; in those days RCA had a big range of bipolar transistors, and I still have the ~1972 data book. I am amazed that SOA is a problem when you can get such huge MOSFETs.Thinking about this some more, this approach resembles feedforward compensation for load regulation, where feedback is not desirable e.g. high voltage power supplies where you want say 1% load regulation but not 0.01%, and you don't want 50kV-capable feedback resistors. You can achieve 1% by measuring the supply current to the oscillator which drives the multiplier stack and applying some % of that to the oscillator supply voltage. As noted above, for stability the feedforward fraction needs to be small. I was doing this in a previous life, in the 1970s.

author: David Hess
author_rank: Super Contributor
timestamp: June 04, 2025, 01:15:39 am
content: Quote from: peter-h on June 03, 2025, 11:14:47 amIt becomes a cost and sometimes performance issue; larger MOSFETs are more expensive and have more capacitance limiting speed.Early power MOSFETs were lateral parts and had square SOAs, and lower capacitance as well making them faster. They were quickly replaced with vertical parts with smaller die but the same voltage and current ratings.A transistor can always be derated to avoid exceeding safe-operating-area, but doing this increases cost. In some cases, multiple devices will have to be used in series and/or parallel.

author: incf
author_rank: Frequent Contributor
timestamp: June 04, 2025, 01:19:55 am
content: In series? huh... In parallel I've seen and done, but never in series.

author: David Hess
author_rank: Super Contributor
timestamp: June 04, 2025, 01:21:52 am
content: Quote from: incf on June 04, 2025, 01:19:55 amWhen dealing with non-square safe-operating-area, it can be more effective to operate two transistors in series rather than in parallel. At some lower voltage, somewhere between 15 and 30 volts depending on the transistor, the safe-operating-area becomes square.

author: EEVblog
author_rank: Unknown
timestamp: June 04, 2025, 02:18:51 am
content: Sweet circuit!

author: peter-h
author_rank: Super Contributor
timestamp: June 04, 2025, 11:29:10 am
content: You can have transistors in series but it needs more components because you need to not only drive them all but also ensure the total voltage is divided across them fairly evenly.

author: Zero999
author_rank: Super Contributor
timestamp: June 13, 2025, 05:48:25 pm
content: Clever circuit.Thinking outside of the box: how about floating the op-amp?It does have it's shortcomings: the minimum current has to be greater than the op-amp's quiescent current and there has to be enough voltage headroom to power the op-amp, but there are plenty of op-amps which will work down to a few volts.

author: David Hess
author_rank: Super Contributor
timestamp: June 13, 2025, 07:56:22 pm
content: Quote from: Zero999 on June 13, 2025, 05:48:25 pmCurrent loop powered 4 to 20 milliamp sensors do the same thing; their quiescent current is routed through the current sense resistor.

author: Zero999
author_rank: Super Contributor
timestamp: June 13, 2025, 09:24:59 pm
content: Quote from: David Hess on June 13, 2025, 07:56:22 pmWith 4 to 40mA sensors, the whole circuit is floated.https://www.ti.com/lit/ds/symlink/xtr115.pdf

author: David Hess
author_rank: Super Contributor
timestamp: June 14, 2025, 12:58:25 am
content: Quote from: Zero999 on June 13, 2025, 09:24:59 pmThat can be confusing because there are 4 to 20 milliamp transmitters which are floating while using an external isolated supply.

author: peter-h
author_rank: Super Contributor
timestamp: June 14, 2025, 07:32:48 am
content: They probably just use a DC-DC converter to power them.Otherwise, yes indeed you have to design it so the whole circuit draws < 4mA

author: David Hess
author_rank: Super Contributor
timestamp: June 14, 2025, 07:55:51 am
content: Quote from: peter-h on June 14, 2025, 07:32:48 amThat is more common now, but in the recent past they used transformer isolation on their current output.