Using opamp saturation to limit voltage - Page 1
Forum section: electronics / projects

author: level6
author_rank: Regular Contributor
timestamp: October 13, 2023, 01:55:42 pm
content: There's a problem I'm trying to solve where I need to limit the incoming voltage on a signal. This could be AC or DC. I found a schematic that handles a similar problem by amplifying the signal with a TL074 so that the upper limit of expected incoming voltage, +-5V, saturates the opamp. Would this be a proper way to accomplish voltage limiting? Are they any downsides?

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 13, 2023, 02:40:05 pm
content: Some amps don't like saturation at all, causing all sort of issues.But if the amp you're using is fine with this, just take care to not exceed the input voltage limit.Another way would be to clip the signal with some clamping circuit, a TL431 should do it with decent accuracy (but at least 2.5V).

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 13, 2023, 03:45:53 pm
content: TI makes a couple of limiting opamps especially for this kind of application -OPA698and OPA699.CheersAlex

author: level6
author_rank: Regular Contributor
timestamp: October 13, 2023, 04:25:58 pm
content: Quote from: Alex Nikitin on October 13, 2023, 03:45:53 pmInteresting chips, but expensive.

author: TimFox
author_rank: Super Contributor
timestamp: October 13, 2023, 05:22:14 pm
content: Try to avoid saturating the output of "normal" op amps: it can take a long time for the amplifier to recover.In the old days of op amps running from split power rails (+/- 15V), in the inverting configuration, there were a lot of "bounding" circuits to limit the amplifier output to less than saturation.A simple method was back-to-back Zeners across the feedback resistor, but the diode capacitance was often too high.Other circuits used fast diodes along with Zeners.In a quick search, I couldn't find anything good online: I learned this stuff almost 50 years ago from Burr-Brown books on op amps.I'll try to post a possible circuit diagram later.

author: magic
author_rank: Super Contributor
timestamp: October 13, 2023, 06:01:02 pm
content: I can see the point of using TL074 for this because IIRC its inputs remain high impedance even when taken many volts above the positive rail.Beware that it has a nasty surprise at the opposite side: the output suddenly jumps positive if the noninverting input is very close to the negative rail, or below.With all that being said, maybe it would be faster to just solve your problem than think about a bizarre old trick.What signal are you talking about? How large, how fast?What's the source, can it tolerate simple clamping with zeners to ground, for example?Where is the signal going, what's the input impedance that must be driven?How much (in)accuracy can you tolerate?

author: level6
author_rank: Regular Contributor
timestamp: October 13, 2023, 06:22:20 pm
content: Quote from: magic on October 13, 2023, 06:01:02 pmSignal could be any here from DC -12V to +12V, so the input would be DC coupled. The goal is to clip any coming voltage that is coming in where it is > -+5V. It's possible that the signal input could also be low frequency under audio range. The VCC and VEE for any circuits would be _+12V and -12V respectively. The signal is going into specialized IC, the AS3360, a voltage controlled amplifier, that accepts a voltage on a control pin from -2V to +2.5V. Here's a link to the datasheet for the IC.https://alfarzpp.lv/eng/sc/AS3360.pdfThe signal of interest would be going into pins 5 and 10 of the chip, the linear control voltage.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 13, 2023, 07:22:17 pm
content: See if this idea fits your needs:https://www.eevblog.com/forum/metrology/diy-0-1-to-10hz-noise-amplifier/msg4907843/#msg4907843

author: TimFox
author_rank: Super Contributor
timestamp: October 13, 2023, 08:01:16 pm
content: If this helps, this is a circuit from the analog computer days that I often used to limit the output of an opamp (here shown inverting) without saturating its output.The two Zeners are always conducting, so their capacitance doesn't matter.The extra resistor and back-to-back diodes require a large current through one of the other two diodes before passing feedback current to the virtual ground (- input of op amp).The feedback current through the diode network keeps the output within limits, but without saturating the amplifier.

author: Someone
author_rank: Super Contributor
timestamp: October 13, 2023, 09:48:06 pm
content: so far grey beards with out of date methods, and beginners with no idea....clipping to the rails in 2023? 6c rail-to-rail input and output dual opamp. Inverting amplifier, gain set to ensure the input will never take the - input outside the rails.

author: TimFox
author_rank: Super Contributor
timestamp: October 13, 2023, 10:13:19 pm
content: And those who reject previous methods out-of-hand, yet cannot punctuate.

author: Someone
author_rank: Super Contributor
timestamp: October 13, 2023, 10:17:56 pm
content: Quote from: TimFox on October 13, 2023, 10:13:19 pmI believe; the only correct answer to that, would be, says you.Blankly criticising non-specific and off topic aspects, without any examples of how to correct that? Asshat.

author: TimFox
author_rank: Super Contributor
timestamp: October 13, 2023, 10:19:17 pm
content: Your punctuation actually makes your statements hard to read.

author: level6
author_rank: Regular Contributor
timestamp: October 13, 2023, 10:55:52 pm
content: Quote from: Someone on October 13, 2023, 09:48:06 pmHow would that solve my problem of clipping anything > +5V or < -5V?

author: Someone
author_rank: Super Contributor
timestamp: October 13, 2023, 11:03:58 pm
content: Quote from: level6 on October 13, 2023, 10:55:52 pmIt would help if you shared the schematic of what you are thinking of using. As mentioned above by DavidAlfa, saturating an opamp not designed for operation in that mode can cause undesirable effects (including inversion). Which is why there are rail to rail amplifiers which are specified for operation up to (sometimes a little beyond) the supply voltages without any unexpected side effects. Operating parts outside the specifications can lead to unexpected results, so best to avoid that.Operational amplifiers specified for saturation are cheap, so why not take advantage of them? What is special about the TL074 and circuit you are considering that you could change to something specified for saturation?

author: level6
author_rank: Regular Contributor
timestamp: October 13, 2023, 11:57:23 pm
content: Here's the proposed input stage. The signal would be amplified by around -2, so that anything above 5 volts coming in would clip at 10V saturation. The next opamp would scale the signal down so that the max output would be 2V. Capacitors are there to filter out anything above 22 kHz.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: October 14, 2023, 01:07:47 am
content: My suggestion is the AD8036 clamp amp:https://www.analog.com/media/en/technical-documentation/data-sheets/AD8036_8037.pdf

author: mawyatt
author_rank: Super Contributor
timestamp: October 14, 2023, 02:13:17 am
content: What about a signal diode bridge with a Zener across the bridge in the feedback loop of a simple op-amp in inverting configuration. This would limit the +- outputs to equal magnitude levels if the signal diodes are somewhat matched.Edit: Here's the concept using a 1N750 Zener, 1N4148 & OP07, don't have a model for a ~4 Volt Zener (1N750 is ~4.7V) which should limit around ~ +-5V. Simple, but not fast.Best,

author: level6
author_rank: Regular Contributor
timestamp: October 14, 2023, 04:55:31 am
content: Quote from: dobsonr741 on October 14, 2023, 01:07:47 amThanks. Interesting chip, but too pricy.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 14, 2023, 07:29:33 am
content: The limiting scheme depends on the end use and precision required. Hard or soft limiting? Unipolar or Bipolar?Idle speculation belongs on TikTok.Opamp output headroom is not a well defined or stable parameter.A railed out amplifier willl have recovery time issues.As will the simple clamp shown here

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 14, 2023, 11:44:59 am
content: Quote from: level6 on October 13, 2023, 04:25:58 pmIf you want it cheap, here it is.CheersAlex

author: magic
author_rank: Super Contributor
timestamp: October 14, 2023, 01:47:59 pm
content: Not sure if your circuit will still be stable with high signal source impedance or capacitive output loading.I think I would put Miller around Q3/Q6, perhaps including the diodes as I think it would make reaction time essentially nothing.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 14, 2023, 02:40:58 pm
content: Quote from: magic on October 14, 2023, 01:47:59 pmAccording to LT Spice the circuit is stable with 100K on the input and 100nF on the output. Caps around Q3/Q6 don't work that well (I've tried in the sim).CheersAlex

author: dobsonr741
author_rank: Frequent Contributor
timestamp: October 15, 2023, 02:47:25 pm
content: QuoteIs it a hobby project, building one piece, or a commercial endeavor?Both TI and Analog have free sample programs.

author: level6
author_rank: Regular Contributor
timestamp: October 17, 2023, 04:40:27 am
content: Quote from: dobsonr741 on October 15, 2023, 02:47:25 pmLooking over the specialty ICs that have been suggested, the TI chip has a very low voltage it will control, and the Analog chip uses more current than I can spare. I'm building a device that could have as many as eight channels. With the Analog chip, being single channel and consuming ~20mA, that would be way more current than I can spare. Plus, costing $13USD a piece is beyond my budget.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 17, 2023, 01:10:57 pm
content: I've looked carefully on the data for the AS3360. The maximum gain is achieved at the control voltage equal to the reference voltage, 1.7V, so there is 0.8V to spare on top of it, and the lowest control voltage is actually at 0V so there is no problem whatsoever to use just a plain diode to limit the negative control voltage and a transistor to limit the positive (to reduce current required from the reference pin).CheersAlex

author: Kleinstein
author_rank: Super Contributor
timestamp: October 17, 2023, 04:54:50 pm
content: How bad a OP amp reacts to clipping at the output depends. Especially some rail to rail types can recover relatively fast from clipping and could be used limiting. This goes so far that I saw the suggestion to use it for a precision full wave rectifier.

author: magic
author_rank: Super Contributor
timestamp: October 17, 2023, 07:25:45 pm
content: Quote from: Alex Nikitin on October 14, 2023, 02:40:58 pmInteresting, you're right. I forgot that Miller comp falls apart when 2nd stage gm decreases too much without corresponding loss of 1st stage gm. High source impedance makes this happen by unloading the output transistor...BTW, for minimum offset voltage the collector load resistor should be chosen to conduct half diff pair current near the threshold. So maybe 1k, not 6k2.Quote from: Alex Nikitin on October 17, 2023, 01:10:57 pmAnd this is probably the sanest way out of this situation

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 17, 2023, 07:33:10 pm
content: Quote from: magic on October 17, 2023, 07:25:45 pm1K there will noticeably restrict the voltage ranges, both for the supply and the limiting levels. Ideally one should use a 2-transistors current source in place of 10K resistor, that will allow to balance the differential properly and provide a nice temperature compensation as well. Gets somewhat complicated though.CheersAlex

author: level6
author_rank: Regular Contributor
timestamp: October 17, 2023, 08:49:12 pm
content: Quote from: Alex Nikitin on October 17, 2023, 01:10:57 pmThis work very well. Thank you!I will need to scale the input, though, so that +5V scales to +1.7V. The expected external control voltage range is 0-5V, but anything above that, up to 12V, is possible.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 17, 2023, 09:00:20 pm
content: Quote from: level6 on October 17, 2023, 08:49:12 pmJust add 5K1 resistor in parallel to C1 in my second circuit.CheersAlex

author: level6
author_rank: Regular Contributor
timestamp: October 17, 2023, 11:42:51 pm
content: Quote from: Alex Nikitin on October 17, 2023, 09:00:20 pmPerfect! Thank you for this.