10A current source with 10V compliance - Page 1
Forum section: electronics / projects

author: Terry Bites
author_rank: Super Contributor
timestamp: May 20, 2023, 12:53:47 pm
content: I'm labless for now. Moving house is such a pain.Maybe some one can try this simulation (maybe in copper!) in another app.It seems very, maybe too well behaved to me.My objective is ccs capable of 10A @ 10V at up to a kHz into a resistive load.It uses no exotic components. Its nicely retro.My sim was done in microcap12

author: David Hess
author_rank: Super Contributor
timestamp: May 20, 2023, 02:25:01 pm
content: That will certainly work with perhaps some adjustments for stability.Did you consider configuring the output transistors so that they drive the load with their collectors, which are inherently current sources, instead of their emitters?I would also ground the output of the operational amplifier through a resistor to turn the supply outputs into current sources. And then the output can be tied to the inverting input through a capacitor for frequency compensation. As shown the low output impedance prevents the feedback capacitor from doing much.

author: mawyatt
author_rank: Super Contributor
timestamp: May 20, 2023, 02:49:01 pm
content: Interesting topology, agree for a current source taking the output from the transistor collectors seems better. Regarding the capacitor C1, it works into the R1 10K resistor, so the low output impedance doesn't matter.Best,

author: David Hess
author_rank: Super Contributor
timestamp: May 20, 2023, 03:43:14 pm
content: Quote from: mawyatt on May 20, 2023, 02:49:01 pmThe low output impedance at the emitters divides the output impedance of the operational amplifier to attenuate the voltage change on the output, and that output impedance changes with output current, so the frequency compensation is changing with output current. As the output current increases, and as the load impedance decreases, the capacitor effectively becomes smaller so there is less frequency compensation.

author: Benta
author_rank: Super Contributor
timestamp: May 20, 2023, 09:11:34 pm
content: What's the purpose of Q1 and Q4? You already have current limitng.Those two transistors can cause excesive high frequency oscillation when they start being active.I'd suggest that you remove them alltogether.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 21, 2023, 03:39:15 am
content: Quote from: Terry Bites on May 20, 2023, 12:53:47 pmThat's a lot (> 100W) to ask from a single transistor ?The base drive transistors are also a bit feeble, in TO-92 packages.

author: duak
author_rank: Super Contributor
timestamp: May 21, 2023, 03:50:35 am
content: Interesting design. I've read over the years that using the op amp's power pins as outputs to the power stage is never quite as easy as it seems. I've never tried doing so myself so I can't say from experience. Anyhoo, I present the following for consideration:1.) If the power supply voltages are ramped up quickly enough, the voltages across R2 & R3 caused by the current to charge C2 & C3 will be high enough to cause common conduction of the Q3 & Q6. I come up with about 69 V/S but I'm not confidant in that number. In light of this, does C2 need to be that large?2.) I think there's a real possibility of thermal runaway in the output stage. The AD820 quiescant current over the temperature range is typically 700 uA with a max of 900 uA. 1K0 resistors R2 & R3 will develop 700 to 900 mV which is enough to bias the output stage on with significant idle current. Driver transistors Q2 & Q5 are quite small with a Pdmax of 1.5 W (or less) so they'll heat up pronto, their Vbes will drop, the idle current will go up, the dissipation will go up and so on. I don't know if the emitter resistors R4 & R5 are large enough to develop a counter voltage to limit the ultimate idle current.3.) some hp function generators from the 80s used a Current Feedback output amplifier. I understand some of designers left hp to form Comlinear that produced fast CFAs. The output stages of some of their products such as pin drivers were complementary current mirrors rather than voltage or current amplifiers. I wonder if the thermal runaway in 2. could be addressed by placing diode connected transistors in series with reduced values of R2 & R3 to make the output stage more stable thermally and also act more like a current mirror. The diode connected transistors would have to be thermally coupled to the driver transistors to track their temperatures.4.) I would consider some resistors across the base & emitter of Q3 & Q6 to absorb any leakage current from Q2 & Q5 and also speed up their turn off times.5.) I can see there's a possibility of misoperation caused by the current limit transistors Q1 & Q4 under extreme circumstances; maybe a reactive load break or connect? eg., Q4 detects an overcurrent condition and draws drive current away from the base of Q5 making that node voltage more negative. C2 & C3 pass this pulse to the base of Q2 which then turns it on harder, something that may not be a good idea.6.) Q2 & Q5 are perhaps a bit undersized powerwise if Q3 & Q6 are at the lower end of the hFE ranges. Q3 & Q6 will be operated outside of their SOAs with low values of load resistance at full current.Cheers,

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 21, 2023, 04:34:18 am
content: Quote from: duak on May 21, 2023, 03:50:35 amGood point.I see ADi havehttps://www.analog.com/en/products/ada4622-1.html#product-overviewthey claim isNext generation of the AD820/AD822/AD824has slightly better Icc vs temperature stability, tho it still does not track a transistor Vbe.Icc still varies with Vcc, so a well regulated primary supply would be needed here too.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 21, 2023, 04:57:11 am
content: Quote from: Terry Bites on May 20, 2023, 12:53:47 pmI'd say it's a bit too retro.The circuit of an opamp power rails driving transistor-base was always a fudge, and it is prone to cross over distortion and production variations, and thermal instability.ie OpAmp (Icc * Rbe) does not track Vbe.10V is not too onerous, it may be better to find a less-variants design ?This approach could be simplerhttps://www.ti.com/product/OPA552Fig 30 shows a booster-transistor follower amp, where the power opamp provides the current up to where the emitter followers take over.

author: magic
author_rank: Super Contributor
timestamp: May 21, 2023, 08:35:24 am
content: Quote from: PCB.Wiz on May 21, 2023, 03:39:15 amWith 1Ω resistive load, worst case dissipation occurs at 5V and 5A, i.e. 25W.If the load can become shorted, then you've got a problem with 100W.Quote from: David Hess on May 20, 2023, 03:43:14 pmOutput impedance of the emitter isn't lower than impedance seen by the base divided by β. Here, output device bases are driven by high impedance of driver collectors.In practice, impedance at the output node will be simply 1.1Ω due to the load and current sense resistor. Indeed, an opamp IC can't meaningfully drive this load and C1 will be ineffective for bypassing phase lag in the power stage. However, if the power stage is fast enough, the chip will use its assist to drive the output at unity gain at high frequencies. In this manner, at least phase lag of the current sense amp is bypassed and maybe this was the reason for C1 inclusion.If the load is always exactly 1Ω, C1 could be replaced with a 10pF/100pF capacitive divider to ground. Then closed loop gain will be 11x, just like at DC. This means when a control voltage step arrives, the output will automatically move to the ballpark of where it needs to be, and the instrumentation amp will only deliver final corrections.

author: David Hess
author_rank: Super Contributor
timestamp: May 21, 2023, 12:42:43 pm
content: Quote from: PCB.Wiz on May 21, 2023, 04:57:11 amA different configuration is required to make the output class-AB. In the same circuit with a common emitter output, I use current mirrors instead of R2 and R3, so part of the operational amplifier's quiescent current controls biasing of the output transistors.QuoteThe advantage, albeit not in this particular implementation, is that current feedback into the output of the operational amplifier is faster while providing controlled gain in the output stage leading to increased slew rates and a rail-to-rail output even with a lower voltage operational amplifier.

author: David Hess
author_rank: Super Contributor
timestamp: May 21, 2023, 12:57:19 pm
content: Quote from: magic on May 21, 2023, 08:35:24 amThe output transistors are usually too slow compared to the operational amplifier so at high frequencies the emitter resistance decreases and becomes capacitive.QuoteFairchild showed an example in the early 70s with AC coupled current feedback instead of the resistor divider commonly shown later. Later dynamically optimized designs may shunt part of the resistive divider with a little bit of capacitance to adjust the local frequency compensation, which is why I would prefer it.Of course using local feedback would defeat the purpose of having a current output in this application, so I would leave it out with a resistor from the output of the operational amplifier to ground turning the output stage into a transconductance (current output) stage. Some curve tracers drive the base of the transistor under test this way.In case the above was not clear, for a current output I would use the circuit configuration shown below, without the AC feedback capacitor, with the emitter shunt resistors replaced with current mirrors, and without the voltage feedback from the output, however I am not sure about stability.

author: gbaddeley
author_rank: Regular Contributor
timestamp: May 21, 2023, 01:06:28 pm
content: Quote from: PCB.Wiz on May 21, 2023, 03:39:15 amFor robustness, I’d use say 3 of these pass transistors in parallel with 0.1 Ohm emitter current sharing resistors. Probably use something like bd139 / 140 as the driver.

author: mawyatt
author_rank: Super Contributor
timestamp: May 21, 2023, 01:49:53 pm
content: Quote from: duak on May 21, 2023, 03:50:35 amDavid Nelson invented Current Mode Feedback amplifiers, and Comlinear used a special complementary IC bipolar process, as did Burr Brown (both somewhat secretive) at Bell Labs which later became known as CBIC-V2. Rumor has it that ADI$ luredhired the process engineer from Bell to develop their high performance complementary bipolar version.Best,

author: Zero999
author_rank: Super Contributor
timestamp: May 21, 2023, 05:22:29 pm
content: There's no need for an additional op-amp. Put the buffer in the INA110's feedback loop. If an inverting buffer is added for the other side, the whole thing can be run off a single power supply.

author: David Hess
author_rank: Super Contributor
timestamp: May 21, 2023, 05:28:08 pm
content: Quote from: mawyatt on May 21, 2023, 01:49:53 pmBefore transistors, current mode feedback was known as cathode feedback, and was still popular intransistorized audio amplifiersuntil differential pairs became more popular. Of course with tubes, there could be no complementary design.A complementary process was required before it made sense for ICs, but discrete implementations existed in some applications. Once a complementary process became available, integrated current mode feedback amplifiers were inevitable.There are a couple of discrete fast power buffer designs out there which start with a diamond buffer and operate using current feedback. Using a voltage feedback operational amplifier like several examples here is the same thing but with the diamond buffer replaced with the operational amplifiers output stage.

author: mawyatt
author_rank: Super Contributor
timestamp: May 21, 2023, 06:14:53 pm
content: Quote from: David Hess on May 21, 2023, 05:28:08 pmDavid Nelson was grated a patent on the Complementary Current Mode Amplifier Method in 1983 (4,502,020), they were producing hybrid devices before this with the CLC103 first which we utilized in numerous applications when they first became available. Later they got early access to what became CBIC-V2 and produced many integrated CMF amplifier versions, which we also used, replacing the CLC103s. Independently in ~1980 we had developed an amplifier called "Summing Node", which was developed to act as a wide-band summing junction of multiple signals from custom CCD Convolvers utilized in a Real Time SA based upon the Chirp Z Transform, but wasn't complementary tho. We produced hybrids of these, and later integrated CMOS versions, classic example of Necessity being the Mother of Invention, and we needed low power wide-band precise signal summation!!Best,

author: David Hess
author_rank: Super Contributor
timestamp: May 21, 2023, 09:33:25 pm
content: Quote from: mawyatt on May 21, 2023, 06:14:53 pmI read the patent and know of prior art 10 years older, including the example I just gave, but the patent office is very selective about what they consider prior art and sure would not have known. They would not have read Fairchild databooks or Tektronix service manuals. (1) Draw the 741 schematic with the external output transistors and it is the same thing but with the transistors driving the diamond buffer replaced with diodes, at least on the schematic. (2) Someone was probably using National's hybrid diamond buffers to do the same thing.David Nelson might not have known that it had already been done, and nobody bothered to patent it because it was based on cathode/emitter feedback.(1) Tektronix used it in a couple of spots for no reason that I could discern. Maybe it was only to increase the output voltage swing with the common emitter output transistor.(2) Some Analog Devices current feedback amplifiers use this configuration with the input transistors to the diamond buffer shorted to make diodes. I forgot why one is preferred over the other.

author: mawyatt
author_rank: Super Contributor
timestamp: May 21, 2023, 11:23:26 pm
content: Quote from: David Hess on May 21, 2023, 09:33:25 pmHaving been a expert witness in both infringement and defense cases, inventor (mostly sole inventor) of 32+ US patents, the patent office does a decent job of prior art assessment considering the magnitude of filings they dealt with back then, and the filing attorneys and inventors(s) also are required to perform due diligence. IEEE articles, data sheets and service manuals are the first resource searched, otherwise the patent would be worthless (see below).When a patent is challenged in court (which I'm sure this patent has been subjected too many times over) the first line of attack is prior art, anything and everything is claimed as prior art, however this is quickly whittled down to just a few legitimate concerns which are extensively researched and examined in court and decisions based upon such. If a patent is found to have prior art, the patent may be revoked entirely, or at least amended to a lesser status, and we don't see any of this with the patent in question.Regarding this specific patent, we've found nothing that would be truly considered prior art, and even our own "Summing Node" which preceded this disclosure as we were not complementary, of course some folks might consider our work prior art, which it's not!!If you have substantial evidence of specific related prior art please disclose such (use a separate thread tho) , the reference you gave and the 741 doesn't hold IMO, and nothing National had done prior either, that we were aware of. Why do you think National acquired Comlinear?Anyway, this is deterring from the OP thread, sorry for the distraction.Best,

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 22, 2023, 02:54:03 am
content: Quote from: gbaddeley on May 21, 2023, 01:06:28 pmYes, multiple packages helps in many ways.It spreads the thermal load, making heatsinking much easierIt moves most power devices higher up their HFE curvesPackages are interesting :TO-3 is fading, and really no new devices are released.Full-plastic TO3P variants exist, but not as consistent in naming as the TO220FPMore modern are the 5x6 flat pack DFN / LFPAK56 packages, where the design approach would be a multi-package PCB with a flat-pan fan/heatsink that cools the top of all devices.This one can take multiple TO-220FP packages on two sides under a pressure clamp, just add a fan.https://www.aliexpress.com/item/1005004182056495.htmlones like this include a fan, and have a flat plate area to extract the heat.https://www.aliexpress.com/item/32212557156.html

author: magic
author_rank: Super Contributor
timestamp: May 22, 2023, 05:43:32 am
content: Quote from: David Hess on May 21, 2023, 09:33:25 pmOffset voltage. The diamond buffer configuration has NPN followed by PNP and vice versa, good luck matching their Vbe.

author: jwet
author_rank: Frequent Contributor
timestamp: May 22, 2023, 03:14:58 pm
content: Have you considered a monolithic Audio power IC- I've used the LM1875 in similar circuits at lower power levels. There are higher power brothers of these parts that might be worth looking at- you could use it bridged or paralleled if you need more current. The nice thing about these is protection. You'd still have to sample current somehow, I would think you could just take feedback below your load though a sense R- not need for an INA probably. Art of Electronics has a bit to say on this topic- they have built some monster sources sometime.

author: David Hess
author_rank: Super Contributor
timestamp: May 22, 2023, 06:50:09 pm
content: Quote from: jwet on May 22, 2023, 03:14:58 pmVarious voltage regulators can be used as protected linear pass elements in place of transistors however their outputs are intended to be low impedance.What you can do however is wire a voltage regulator with its output tied to ground through a resistor, and use its input as a current source. With multiple regulators the output resistor now ballasts them so they can be used in parallel. Some old application notes show integrated voltage regulators used like this to make protected switches. You still need a bunch of them in parallel to handle the power dissipation. 20 watts each is typical so at least 5 regulators, but the circuit is still pretty simple.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 28, 2023, 07:39:52 am
content: I think you're right. I noted that at zero load impedance (simulated) there were severe stability issues- ringing and overshoot to the current limit, but even a few milliohms improved things dramatically. Its a bit of a mystery, I'm not exceeding any parmeters for the INAmp.Indeed, it doesnt seem to matter what sense amp I used. I had the same problem with a INA105 for instance.I've tinkered first some first order compensation to no avail. I can slug it and live with a lower slew rate, the 1Khz was notional and not an absolute requirement I noted that at the current (13A) the shunt will be getting very hot.So I thougth I'd use a .01R shunt and up the sense amp gain to 10. Fail...So any help on the compensation front wolud be greatly apreciated. Its fine at DCNew beasts for the output, MJL1302A and MJL3281A I've added a thermal shutdown circuit and some monitor outputs.

author: magic
author_rank: Super Contributor
timestamp: May 28, 2023, 08:15:07 am
content: This has been mentioned before. If the load is too stiff then the output node is roughly constant voltage and compensation by means of C1 is ineffective. I think it has also been suggested to place a resistor between opamp/C1 and R6.I think the suggestion of Zero999 is worth considering too. Less semiconductors in the loop - fewer problems. You could probably still use this funny power buffer, putting the INA110 in place of AD820.INA110 bandwidth and frequency response seems best at G=10. Higher gains may be okay too, particularly if it isn't inside the feedback loop of another opamp (see above).

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 28, 2023, 02:16:45 pm
content: I designed 10A AC (+-5 V power supply) current sourse with just OPA189 and couple of complementary TO220 Mosfets: classic Howland current source with 10k/2k 0.1% resistors, 0.01-0.02 R shunt (OAR5 series). It's much simpler to use Mosfets instead of bipolar transistors for currents more than 0.1-0.5 A. Gate bias (and source bias current) was set by simple regulated resistor divider. Such simple bias circuit with active cooling was fine for my case (80x80mm heathink plus 80mm fan), but of couse, it can be improved for more powerfull more hotter amplifier.You may want TO247 instead of TO220 Mosfets for +-10 VDC power supply (e.g. IRFP240/IRFP9240 ).This simple circuit works fine up to several kHz.THD depends on bias current and it was easy obtainable 0.01-0.1 % (OPA189 is not only ultra-precision but quite fast too).The good thing about OPA189 is that it is (or it was) quite cheap regarding to it's outstanding capabilities.I don't have my 'DaveCAD-style' schematics at my hands at the moment to post it here.