Current clamp opamp operated outside of common mode range. - Page 1
Forum section: electronics / projects

author: Faringdon
author_rank: Super Contributor
timestamp: August 18, 2023, 10:54:48 pm
content: Hi,To what current, if any, would the attached current clamp regulate the current, given that it is being operated outside of its common mode input range?LTspice and jpeg attached.The opamp is actually a TL084CTL084 family datasheethttps://www.ti.com/lit/ds/symlink/tl084h.pdf?ts=1692381297170&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTL084H

author: moffy
author_rank: Super Contributor
timestamp: August 18, 2023, 11:24:16 pm
content: The spec sheet says that the minimum common mode range for +/-15V supplies is +/-11V but typically goes from -12V to +15V or the positive supply. So mostly you would probably get away with it but it isn't good design practice. The other problem is the output swing which might well not be high enough.

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 18, 2023, 11:52:47 pm
content: The DS gives VCM between (Vcc-)+1.5 and (Vcc+) (p.13) for the H version (here TL084H) but a recommended VCM between (VCC–)+4 and (VCC+)–4 for all other versions (p.11).Typical opamp DS.So if the device was an actual TI TL084H, the design would be essentially fine.For a TL084C, it *may* work as intended, but it's not guaranteed. The actual current is probably going to depend on each individual chip.Note that your annotated 18.25V rail is actually 18.75V.

author: magic
author_rank: Super Contributor
timestamp: August 19, 2023, 12:11:31 pm
content: TL072/TL082 is usually considered usable up to VCC and the internal schematic makes it fairly obvious it's good up to at least, say, VCC-0.5V.The current source will go all the way up to saturation, very close to VCC, and JFET gates are reverse biased.TL084H is a single-vendor CMOS opamp which has absolutely nothing to do with real TL084.

author: Faringdon
author_rank: Super Contributor
timestamp: August 19, 2023, 12:25:29 pm
content: QuoteThanks, the TL084H shares the same datasheet as the other TL084x's.Also, since TL084H is the same price as all the other TL084x's, and is pin-for-pin compatible, then wouldnt it actually be best to use the TL084H in that circuit above? (please forgive the unintended tone, of course i am not implying that you would take the opposite stance to this)The TL084 datasheet says that except for TL084H, the TL084x's only have common mode range up to Vcc-4V.....which means that the circuit of the top post is a bad one. Surely there is danger that it wont actually regulate the current at all?....and the current will fly off to approx 20V/115R?

author: magic
author_rank: Super Contributor
timestamp: August 19, 2023, 07:22:26 pm
content: Both should work, but you can surely pick the chip with guaranteed specs to CYA.Just a warning that it's not a generic part and you won't get it from anyone but TI.I wouldn't say they share the datasheet - all tables and plots are separate and different.Rather, the two datasheets are bundled in one PDF file to help you notice the new partIt was supposedly designed to be a drop-in substitute and most specs are similar enough.One notable exception I remember is higher noise over audio band and below.I don't know why TL072 doesn't have a wider guaranteed input range, but almost all non-R2R opamps have asymmetric input and output ranges and yet back then it was common to specify them as just ±N volts without going into details.

author: Faringdon
author_rank: Super Contributor
timestamp: August 21, 2023, 06:18:21 am
content: QuoteThanks, i am actually thinking, that maybe, that maybe the feedback loop has been mucked up, and it actually works with the TLO84CDT , but goes unstable when the TL084H is used.Thanks, this is always the situation, when someting is outside of limits, and what then happens......regarding the NON TL084H versions, ie the TL084CDT version...this is what a "Guru" on the offficial Texas instruments forum says...QUOTE >>>>>>The datasheet says that the common-mode input voltage range typically goes up to V+. So I'd expect the worst that would happen is that the electrical characteristics are not met, i.e., that the offset voltage could be somewhat larger than 6 mV.Just use the TL084H (or TL081H for a single channel) instead, which is guaranteed to go up to V+ (and is cheaper, and has a smaller offset voltage).<<<<<< UNQUOTESo, if we dont use the TL084H, but other TL084x , then, now with this extra info..... what would be the worst that could happen may i ask?QuoteThanks, but i wonder how you know this?...if this is the case, then virtually any opamps would be good to Vcc-0.5v, because a great many have output stages like the TL084C....so are we saying that virtually all opamps have good common mode range up to Vcc-0.5v?

author: magic
author_rank: Super Contributor
timestamp: August 21, 2023, 07:07:53 am
content: You asked about common mode input range, not output. So LM301 and TL072 seem to be about the only jellybean opamps whose input can generally work up to VCC, and despite lack of guaranteed specs, TL072 was sometimes chosen when LM301 is unsatisfactory. Few other opamps have similar input stages as either of them, and they aren't nearly as common.TL072H seems designed as a modern solution for this exact case and they named it "TL072" because that's the part people look for when they want input up to VCC. Other CMOS opamps generally work to ground but not to VCC, and the R2R input ones almost all have worse offset voltage near VCC.Now on the output side, new TL072H is R2R but old TL072 is not. Frankly, you may also have a problem here, because the guaranteed spec is only 3V below the positive supply and typical is 1.5V. The new part surely is better specified for what you want to do.That being said, I suspect you found this circuit somewhere and that it was in production with the original TL084 and still worked nevertheless.editThere is no problem with output swing because the 10kΩ shifts output voltage several volts up. It surely looks like somebody designed this for TL084 and wanted it to work even in the worst case.

author: AndersJ
author_rank: Frequent Contributor
timestamp: August 21, 2023, 11:47:01 am
content: What does C2 do?

author: magic
author_rank: Super Contributor
timestamp: August 21, 2023, 05:49:33 pm
content: The PNP has some parasitic input capacitance, which forms an RC lowpass filter with R3.Probably the time constant was low enough to cause significant phase delay through the loop and instability, or somebody was worried that this may happen.And R3 has the high value it does for reason explained right above.

author: Faringdon
author_rank: Super Contributor
timestamp: August 21, 2023, 08:23:38 pm
content: QuoteThanks, sorry but i think you are referring to hobbyists?....the non-TL084H ones are surely and simply not rated for the common mode input voltage that they see in the schem of the top post.Yes it may work in whichever individual production batch.....but i am certain that you would agree with me , when i say that it is simply a matter of time, before the horrendous (non TL084H) circuit of the top post gets made with that "bad batch" of TL084's (non TL084H) and the entire production run goes wrong in the field......and the company in question gets closed down.I know that this kind of "seat of the pants electronics" is popular with many electronics engineers...and i am sure you would agree with me that that is what it is......it kind of gives that twinge of excitement to say that you did it and got away with it....for now!............but any non TL084H based version of the top posts schem is a disaster waiting to happen....surely you would concur?The schem of the top post using TL084x, and not TL084H is simply an utter disaster.!

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 21, 2023, 08:52:57 pm
content: Is there even a spec regarding the output current? While the design sucks, it may be good enough for what it's supposed to achieve. (Although in that case I would have designed something simpler.)

author: Faringdon
author_rank: Super Contributor
timestamp: August 22, 2023, 05:30:40 am
content: Thanks, the TL084H version is good enough....the other is a disaster waiting to happen.As you know, the typical solution to this is a TLV431 into a NPN...but if youve got loads of spare opamps then its then worth using one as in the top post. (as long as TL084H)

author: Zero999
author_rank: Super Contributor
timestamp: August 22, 2023, 10:33:27 am
content: Quote from: moffy on August 18, 2023, 11:24:16 pmThat shouldn't be a problem, as it's driving a PNP transistor. The usual base-emitter resistor is all that's required to turn it off.

author: Faringdon
author_rank: Super Contributor
timestamp: January 12, 2024, 02:45:14 pm
content: From what i can see, this TL084C by st.com would be able to handle input voltages tight up to the vcc+(Our Vcc+ is +20V, our Vcc- is 0V)Would you agree?https://www.st.com/resource/en/datasheet/tl084c.pdfCurrent clamp in jpeg and LTspice attached

author: Zero999
author_rank: Super Contributor
timestamp: January 13, 2024, 02:13:30 pm
content: Don't use jpeg for schematics. Use PNG. Jpeg compression introduces artifacts which make it difficult for anyone to edit your schematic. Use PNG which has lossless compression and only jpeg for photographs. Fortunately you posted the LTSpice file which makes life even easier.You do know you can import any model into LTSpice.Data labels can be used, if you allow SPICE to calculate the steady state condition. The initial voltage on the capacitor can be set to zero to test the stability, as there will be a step change, when the circuit is powered up.R5 isn't needed. The bias current for the TL082 are so tiny, 100R won't make any difference.

author: Faringdon
author_rank: Super Contributor
timestamp: January 13, 2024, 05:23:07 pm
content: Thanks, i think as you know, LTspice models dont do the common mode range thing well, if at all. They'll never show it as a problem.

author: Zero999
author_rank: Super Contributor
timestamp: January 13, 2024, 07:19:00 pm
content: Quote from: Faringdon on January 13, 2024, 05:23:07 pmIt calculates the voltage at both op-amp inputs quite well and it's easy to figure out if it exceeds the CMR by referring to the data sheet. Don't expect it to wipe your bum for you!Refer to the simulation attached to my previous post. The voltage at the +input is about 18.75V and the op-amp is clearly working closed loop, in other words it's adjusting its output voltage to keep the voltage on its -input equal to the +input, i.e. both inputs are 1.25 less than the supply voltage.