Limiting op-amp output

[Jay_Diddy_B]

Sorry to be so negative on this but I still think it's worth taking a step back and trying to lessen the problem by changing the system slightly if that is allowable.


If (at my place of work)  I proposed a similar system that had an op amp with a +/-15V power rail interfacing its output to a stage that must stay inside a 0-5V range then I'd have a lot of trouble getting this past a preliminary design review even before anyone considered reviewing any of the addon 'limiter' solutions.

This odd interface requirement would either get killed at the design document review stage, or the preliminary system design review and I seriously doubt it would survive to a critical design review.

So although it's kind of interesting to think up clever 'sticking plaster' solutions to this issue using limiter circuits I think the real answer may lie in a rethink elsewhere in the overall system.

This is an approach that is much more likely to survive the design review process. I have used this technique many times for protecting ADC inputs from over-range inputs. It certainly works and it is robust.



The clipping level is set by the rail to rail op-amp.

Here is the circuit working at 10 kHz. The circuit does introduce artifacts in the waveform from over-drive conditions. If the over drive conditions only exist during fault conditions this techniques is probably o.k.



The circuit required to produce high-speed clipping without introducing artifacts are of interest (to me) from analog design perspective. Accurate clipping circuits can be used in waveform generation circuits.

Regards,

Jay_Diddy_B

[T3sl4co1l]

Another idea,



Universal inputs, but up to four transistors now, and R1-R2 have to be matched pairs.  The dividers can be removed if a negative current sink is available, or if the full range on the Max input is not required (with no dividers, Max has a "min" of about 2*Vbe, where the current sink saturates and the feedback stays off, so you can't get "max" action below that).  Does not include the positive supply; add another resistor from each diff base to GND to extend operation to both supplies.

Sharpness should be on the order of 50mV.  For better precision, it should work with a current mirror diff load, and a matched pair (to get Vos ~ mV).  Voltage gain will be very high, so compensation will be important to get right.

It could be done with a single transistor too (instead of the diff pair and CCS), but you'd have to get compensation feedback into an emitter, which isn't going to work very well.  Not that you'd care about performance with a whole Vbe offset in the first place.

Tim

[G0HZU]

This is an approach that is much more likely to survive the design review process. I have used this technique many times for protecting ADC inputs from over-range inputs. It certainly works and it is robust.

Yes, if it's allowed then a simple resistive attenuator would make more sense. I'd probably choose lower value resistors than 30k and 10k if high speed operation is required but I don't know what all the spec requirements are

[Chris Jones]

At the risk of disturbing a stale thread, is this what was wanted?

http://www.analog.com/media/en/technical-documentation/data-sheets/AD8036_8037.pdf

[T3sl4co1l]

At the risk of disturbing a stale thread, is this what was wanted?

http://www.analog.com/media/en/technical-documentation/data-sheets/AD8036_8037.pdf

Ah yes, that was the one -- I've ran across it before, thanks for adding it.

Cons:
- Low supply voltage (won't interface, at least easily, with 0/12V or +/-15V systems)
- Way more bandwidth than needed (VFB at 240MHz+ is starting to get squirrelly, and will only be burned off in actual application)
- Poor stock
- Almost $12 each in singles. http://www.digikey.com/product-detail/en/AD8036AR/AD8036AR-ND/611444

Even if it takes me $0.10 to hand-place the transistors and resistors, the above circuit comes out cheaper (though not really smaller unless using 0402s, which I wouldn't for most purposes).

So yeah, I'd love to have exactly the functionality of that part, with performance, cost and availability being comparable to TL071, TLV2371, OPA171, or etc.  Even if the cost were several times more, I'd be fine with that ($3 starts to look annoying but tolerable; a $6 LT/ADI part is on the prohibitive side).

So, yeah.  That's the right idea, but all things considered, it's just not good enough (or too good).

Tim

[Chris Jones]

At the risk of disturbing a stale thread, is this what was wanted?

http://www.analog.com/media/en/technical-documentation/data-sheets/AD8036_8037.pdf

Ah yes, that was the one -- I've ran across it before, thanks for adding it.

Cons:
- Low supply voltage (won't interface, at least easily, with 0/12V or +/-15V systems)
- Way more bandwidth than needed (VFB at 240MHz+ is starting to get squirrelly, and will only be burned off in actual application)
- Poor stock
- Almost $12 each in singles. http://www.digikey.com/product-detail/en/AD8036AR/AD8036AR-ND/611444

Even if it takes me $0.10 to hand-place the transistors and resistors, the above circuit comes out cheaper (though not really smaller unless using 0402s, which I wouldn't for most purposes).

So yeah, I'd love to have exactly the functionality of that part, with performance, cost and availability being comparable to TL071, TLV2371, OPA171, or etc.  Even if the cost were several times more, I'd be fine with that ($3 starts to look annoying but tolerable; a $6 LT/ADI part is on the prohibitive side).

So, yeah.  That's the right idea, but all things considered, it's just not good enough (or too good).

Tim

Maybe someone should start a chip company that is more open to requests and petitions for parts. I think the existing ones only know how to sell (or even approve the development of) the types of parts they already sell, and maybe next year a slightly better version of the same part, but nothing too different!

Barrie Gilbert wrote a good piece about this in this book:
https://books.google.com.au/books?id=SPwqg7qpFWUC&pg=PA296

[dom0]

...Is "dur, use a diode" really the extent of original thought here?  Come on guys, let's see some brain storming!

Tek SG505 schematic sheet 1, Q1410, Q1411 and associated bias circuit. Note connection to negative input of U1401 (whose output is limited).

This is a brilliant circuit. It is simple and achieves it's goal with just a minuscule modification to a normal transistor/diode clamper: instead of returning the clamped current to ground, they are returned to the virtual ground of the op-to-be-limited. This way, the op will reduce it's output so as to nullify the added current.

edit: Attached image

Page 59 in this PDF scan: http://exodus.poly.edu/~kurt/manuals/manuals/Tektronix/TEK%20SG%20505%20Instruction.pdf

[TerminalJack505]

Nice circuit, dom0!     That should do for most of the situations where clipping is needed.

The one drawback it seems to have--at least as compared to Jay_Diddy_B's post #47 circuit--is that it isn't particularly precise.  So if you need precision clipping then Jay's circuit is better in that regard.

Jay's circuit, however, will allow the "control" op amp to go into saturation--which is something the OP is wanting to avoid.

[T3sl4co1l]

Ah, good ref.

Yes, that has two drawbacks: the threshold isn't precise (two diode drops -- about 0.2-0.4V "slop") and the input-to-clamp has to be within the clamp band.  Which it always will be, for an inverting stage (+in = GND), but not for general applications.

The voltage range restriction could be lifted if there were such a thing as a universal current mirror (one which communicates the current flow from the clamping transistors' collectors, over to the input pin that might be at a very different voltage), but alas, that would be difficult.  It would, of course, be easier to simply mirror each transistor to the opposite rail, then back, and then connect them in parallel; or mirror them just once and deal with the inversion (i.e., returning the clamp current to +in).  But these are rather tedious, bordering on inelegant.

When these aren't problems, it's a fine way to do it.  I'm sure they didn't need anything like speed (a 5532 isn't very impressive), and if it's just to constrain the output within, say, a vertical deflection band (maybe +/-6 div normal, or +/-2 div for a BEAM FINDER kind of function), it will do just fine; the "slop" might rather be considered a feature, as it compresses the waveform as it hits the limits, rather than hard cutting it off.

Tim

[dom0]

Yes, the somewhat "soft limiting" is good in their application ; it limits the output of an oscillator during amplitude settling without introducing too extreme distortion.  For some other applications it should be good/precise enough, too (driving ADCs for example).

If high precision and no saturation of the amp is required then I can only think of something like two fast comparators and a fast mux at the input of the amp.

[Kalvin]

How about using a unity gain, fast recovery RRIO op amp as a limiting buffer? Set the power supply to the limits. The output should swing within few tens millivolts from the power supply.

[Kalvin]

Or, use a fast recovery op amp driving a push-pull output creating unity gain buffer:

https://upload.wikimedia.org/wikipedia/en/thumb/1/1e/Pushpull.PNG/330px-Pushpull.PNG

Use MOSFETs instead of bipolar transistors. Set the MOSFET output stage voltages V+ and V- to the desired limits.

[T3sl4co1l]

How about using a unity gain, fast recovery RRIO op amp as a limiting buffer? Set the power supply to the limits. The output should swing within few tens millivolts from the power supply.

This was presented earlier (see #50).  It works if fixed limits are fine, but it's not practical to make it adjustable, and the inputs need to be within that band as well.

Or, use a fast recovery op amp driving a push-pull output creating unity gain buffer:

https://upload.wikimedia.org/wikipedia/en/thumb/1/1e/Pushpull.PNG/330px-Pushpull.PNG

Use MOSFETs instead of bipolar transistors. Set the MOSFET output stage voltages V+ and V- to the desired limits.

Ah, but that's the trick, isn't it?

If we always had "fast recovery" amps, like transconductance amps, we could just clamp the output directly -- this is what the AD8036 does internally.  There's a transconductance gain stage, which provides a constant current output and extremely high voltage gain from the main input; precision clamps (wired in a similar manner) set the max and min ranges.  The limits are freely adjustable, independent of both supply and input voltages, and recovery is rapid (with no overshoot or windup).  Finally, a buffer follows the gain node, so the output has a low impedance and high current drive capability.

If you use MOSFETs, you not only have the problem of windup (as the limiting follower clamps the output, the op-amp goes on to the rail, so it takes time for it to come back down, driving the gates hard to get there, too), but crossover distortion and biasing, too.  If done as shown, with enhancement mode FETs, there's a huge (>2V?) deadband where the op-amp output has no effect on the output.  Normally a class B or C bipolar output stage (e.g., LM324) will have a < 0.3V step at this point, but now you're talking huge slop, and no matter how fast your amp is, it will impact performance!

And if you add the bias components, now you have the issue of bias current drawn from the clamp supplies, which will cause a nonzero error, due to current drawn from the buffers setting those voltages, as well as the Rds(on) limitation of the FETs.  So they might not pull within mV anymore, and how much it's off by is dependent on supply voltages.

On a more subtle note, there's also feed-through from the op-amp output, via G-S capacitance.  So the class C biasing will "anticipate" the op-amp recovery before it finally happens, causing a pre-shoot effect.

It's a good building block, but it needs quite a bit of elaboration to polish into a precision circuit.  The foundation comes from RF mixer designs -- I suppose all of these do, ultimately: the basic idea is to use diodes and current steering, or saturated switches (usually FETs), to toggle an RF signal between "on" and "off" (two devices in series, single balanced mixer), or "plus" and "minus" (double balanced mixer, four devices in an H-bridge configuration).  The overshoot and preshoot issues, in RF terms, manifest as excess reactance and poor LO/RF isolation.

Tim

[Kalvin]

How about using a unity gain, fast recovery RRIO op amp as a limiting buffer? Set the power supply to the limits. The output should swing within few tens millivolts from the power supply.

This was presented earlier (see #50).  It works if fixed limits are fine, but it's not practical to make it adjustable, and the inputs need to be within that band as well.

Oops, I missed that one as I browsed through the messages. It was so close to the other messages with the simulation figures that I sort of skipped that by accident.

The limits can be made adjustable if the positive and the negative power supply rails are driven by a dual op amp. And if the RRIO op amp input has a 10k - 100k series resistor, the RRIO op amp should be fine although the input level is outside the power supply. However, I haven't tested or simulated this so this is just an educated guess.

Or, use a fast recovery op amp driving a push-pull output creating unity gain buffer:

https://upload.wikimedia.org/wikipedia/en/thumb/1/1e/Pushpull.PNG/330px-Pushpull.PNG

Use MOSFETs instead of bipolar transistors. Set the MOSFET output stage voltages V+ and V- to the desired limits.

Ah, but that's the trick, isn't it?

If we always had "fast recovery" amps, like transconductance amps, we could just clamp the output directly -- this is what the AD8036 does internally. (snip)

(snip)
If you use MOSFETs, you not only have the problem of windup (as the limiting follower clamps the output, the op-amp goes on to the rail, so it takes time for it to come back down, driving the gates hard to get there, too), but crossover distortion and biasing, too.  If done as shown, with enhancement mode FETs, there's a huge (>2V?) deadband where the op-amp output has no effect on the output.  Normally a class B or C bipolar output stage (e.g., LM324) will have a < 0.3V step at this point, but now you're talking huge slop, and no matter how fast your amp is, it will impact performance!

And if you add the bias components, now you have the issue of bias current drawn from the clamp supplies, which will cause a nonzero error, due to current drawn from the buffers setting those voltages, as well as the Rds(on) limitation of the FETs.  So they might not pull within mV anymore, and how much it's off by is dependent on supply voltages.
(snip)

This is very true. The idea of suggesting the MOSFETs instead of the bipolar transistors came from the fact that bipolar transistor saturation voltage is around 200mV and the base drive current (voltage) will leak to the output.

[Jay_Diddy_B]

Hi group,

I think that you will find it hard to beat the performance of this circuit, proposed in reply 47 of this thread.




The feedback loops around the op-amps remain closed at all times. This can be observed by looking at the differential voltage between the inverting and non-inverting inputs on the op-amps.

The feedback loops only open if Vin is greater than the supply voltage to the op-amps.

Because the loops remain closed, the circuit does not have to deal with wind-up or overdrive recovery.

LT1224 op-amps will give even better high frequency performance.

Regards,

Jay_Diddy_B

[Kalvin]

Here is something I got using a fast a RRIO op amp as a clamp. The clamping voltage is fully adjustable within the op amp power supply minimum and maximum limits. The performance is good up to 100kHz and beyond. At 1MHz the performance is still good, although the op amp's delay is more obvious. Pretty simple if it works outside LTSpice

[Jay_Diddy_B]

Hi Kalvin and the group,

That circuit can be simplified to:



The diodes are not required.

The results of simulation are:




The key feature of the op-amp that makes this possible is the FAST OUTPUT RECOVERY

The main limitation is that circuit is restricted to the operating supply range of the op-amp:




Jay_Diddy_B

[Kalvin]

Hi Jay_Diddy_B,

Thank you for taking some time duplicating my simulation.

Diodes at the input: Originally I thought that it might be a good thing to pre-clamp the signal with the diodes so that the overloading input signal will not turn on the op amp's on-chip protection diodes. However, as you have observed, the diodes can be removed from the circuit.

The RRIO op amp can be selected according to the needs of the application. The LTC6247 is usable if the desired clamped peak-to-peak amplitude is between 2.5V ... 5V. The lower limit of 2.5V might be a bit of gambling for fast signals. I haven't taken a look whether there are other similar RRIO op amps with the higher maximum power supply.

Anyway, this was just a proof of concept of using a RRIO op amp with fast recovery as a signal clamp.

[T3sl4co1l]

The limits can be made adjustable if the positive and the negative power supply rails are driven by a dual op amp. And if the RRIO op amp input has a 10k - 100k series resistor, the RRIO op amp should be fine although the input level is outside the power supply. However, I haven't tested or simulated this so this is just an educated guess.

Of course, it invites still more questions: what if it needs to work down to 0V (an LV op-amp might go down to 2.7V or 1.8V, but not zero)?  Does the amp still need bypass caps, or what supply impedance does it require?  How fast can this method be -- ultimately, what's PSRR like (it should be exactly 100% in saturation, but it might not be as good at inbetween levels, especially at high frequency)?  Also, op-amps have bias circuitry and stuff inside, which might not appreciate relatively high dV/dt (if you're driving the max/min hard -- an unlikely scenario, but the circuit ought to be symmetrical -- think not so much of X limited by Y, but min(X, Y)).

This is very true. The idea of suggesting the MOSFETs instead of the bipolar transistors came from the fact that bipolar transistor saturation voltage is around 200mV and the base drive current (voltage) will leak to the output.

Bipolar are actually quite a bit better than that -- you'll get ~200mV (and up) for Ic near ratings, but more typically 10-50mV for smaller currents (like 2N3904 at ~1mA).  The residual is mainly due to built-in potential, a side effect of the collector being more lightly doped than the emitter (a situation which gives better hFE and voltage handing).

You can't get something for nothing, of course, so operating a transistor in reverse will, yes, reverse that built-in potential, but other things cancel out the negative excess.  That said, an inverted transistor (swapping C for E) typically has Vce(sat) in the single mV -- assuming you drive hFE low enough to compensate (e.g., the inverted hFE of a 2N3904 is < 5 or so).

Way back, in the days before cheap CMOS, they would build precision DACs this way: when you need 12+ bits, you went to the trouble of using inverted BJTs, switching them with precision matched currents (a pull-up for the base and a pull-down for the emitter, to keep it in that near-zero-Vce(sat) region).

Tim

[Kleinstein]

The ICL7650 OP offers internal clamping - though only to fixed output values, but in a way that can prevent an interator to wind up. The trick is to provide an extra feedback path from the output to the neg. input, once the output reaches certain limits.

 Adjustung to different Levels is possible with a variable offset/amplification stage afterwords. Hower Bandwidth is limited to 2 MHz only.

[Kalvin]

I played a bit with the clamping stuff. Here I use op amps as ideal diodes and use them as voltage clamps. Jay_Diddy_B had similar idea in post #18 but I tweaked the circuit a bit. I selected the same op amps Jay_Diddy_B used in his simulation so the results are comparable. At 10khz sine signal the circuit behaves very nicely. At 100kHz the op amps' slew rate limit the performance but it is still quite good. The positive and the negative clamping voltages are freely adjustable.

[T3sl4co1l]

If you need extremely sharp and accurate breakpoints, that's the way to do it!

Tim