Op amp gain limiting. - Page 1
Forum section: electronics / beginners

author: Greg Robinson
author_rank: Regular Contributor
timestamp: February 05, 2016, 07:39:27 am
content: Hi,I've been trying to figure out how to prevent a non-inverting op amp amplifier from clipping to its rails so as to avoid signal inversion or latching if it's gain is set too high for it's input signal.A little background, I'm hoping to use this as an input for passive electro-magnetic guitar pickups or other musical instrument signals, so the input impedance must be high (the traditional value is >1MEG). The input signal may be expected to be anywhere between about 100mV peak to up to 4V peak depending on the output amplitude of the passive pickups themselves, or any preceding effects/pre-amplification.The voltage rails will be +/-9V, I plan to use a TL07x, and the load impedance on the op amp will be 10k, so maximum swing that the TL07x should be able to handle is around 7V peak. I was hoping to limit the maximum output swing of the stage to somewhere below this using diode clipping in the feedback network to prevent the op amp from ever approaching it's rails.The obvious solution of "if it's clipping, turn it down" is not something I would like to be forced to accept, as musicians will abuse any sort of suggestions like that, and blame me for the results. Diode clipping is also subjectively "nicer" than op amp clipping. In fact, there is an entire industry devoted to doing just that.My problem is, as the gain of the stage is increased with the potentiometer, the capacitance of the zener diodes becomes a problem, and high frequency content is lost. I could use a string of small signal diodes such as 1N4148, but with their low forward voltage, it would take a long string to achieve an appropriate limit. Another issue that I have run into is that I have not been able to find any zener datasheets that specify their junction capacitance, which strikes me as a little odd, but means I am not able to find zeners which may have an acceptably low junction capacitance.Does anyone have any suggestions that might help me achieve my goals of output swing limiting while maintaining bandwidth?Thanks for any help.

author: mij59
author_rank: Frequent Contributor
timestamp: February 05, 2016, 08:08:38 am
content: You could put D1,D2 between the output of the opamp and ground with a small series resistor.

author: Greg Robinson
author_rank: Regular Contributor
timestamp: February 05, 2016, 08:35:42 am
content: Quote from: mij59 on February 05, 2016, 08:08:38 am*Headslap moment*Can't believe I didn't think of that first, don't know why it didn't come to me.Thanks for the suggestion!However, if anyone has any suggestions regarding diode limiting in the feedback loop, that would still be my preference, as clipping in the feedback loop is "smoother" / has a softer transition knee (more low-order harmonics) than "harsh" clipping on the output (more higher-order harmonics), and so would be my preferred solution.I was giving thought to using an LED string (two 3V LED's in series with an anti-parallel pair) as an alternative to zeners, but again have found datasheets to be lacking with junction capacitance info. Does anyone have any idea of typical 3mm or 5mm LED junction capacitance? Might have to dig around my parts bins to find a range of different ones to see if I can get some vaguely representative sampling...

author: mij59
author_rank: Frequent Contributor
timestamp: February 05, 2016, 08:54:51 am
content: You could use an array of diodes like in a triangle to sine wave converter, or if you want something that’s EMP save use a vacuum tube.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 05, 2016, 10:21:45 am
content: The easy way would be reducing the 1 M Ohm resistor to something like 10 K. Even without the diodes the 1 M Feedback is rather high for a fast circuit. Having a gain of 100 with a TL07x will not be fast anyway. So you may need a faster OP.For the diodes one could use zener diodes and 1N4148s in series. So at least for small amplitudes the capacitance can be reduced.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2016, 10:36:58 am
content: Quote from: Greg Robinson on February 05, 2016, 07:39:27 amOk.Well, "duh" solutions first:1. Use an amp that's fine with this. Even TLV2371 will clip without unexpected behavior (phase reversal, slow recovery). (It's probably too slow, noisy and low voltage to be directly applicable, but there are plenty of similar parts out there.)You don't need to use ancient POS amps like LM741 anymore! (Not that you are, but old and funky still matters.)TL071 is still rather popular, but there are better options without costing much more. Shop around!2. Clamp the input instead. Back-to-back diodes? Low level op-amp clipper stage? (Maybe with a fixed gain preamp in front, to keep noise down.)The clamping might have to be adjustable, or set to a worst-case range, to make this work for any output gain.QuoteAs shown is fine, but you should use an inverting configuration instead, so the diodes clamp against virtual ground. As shown, output is input plus gain, so you'll still saturate the amp on very large inputs.Quote1.http://www.mouser.com/ds/2/427/1n5221-240386.pdfFig. 6Note it's at Vr=2V and the varied parameter is the Vz rating. Actual C for an individual part will follow a C vs. DC bias curve typical for diodes, having quite high capacitance at low voltages, somewhat less by 2V (how much depends on the voltage rating), and a minimum value at just below Vz.2. Use a smaller feedback resistor. Which means you need an even smaller resistor against the divider, but that's okay. (Not as okay for the inverting configuration, but you can use an input buffer or gain stage to solve that.)3. Use smaller diodes. Although I don't know that you can really find much under 100pF, anyway.4. Put one zener inside a FWB of regular (1N4148 ish) diodes. Zener goes cathode to anode across the DC+ to DC- terminals, and the AC terminals go across the op-amp.This is frequently used when low average capacitance is needed; high speed data line ESD chips do this, for instance.Tim

author: orolo
author_rank: Frequent Contributor
timestamp: February 05, 2016, 11:10:00 am
content: You can limit the output instead of the input, making it to clip rather softly.From the classics: "I.25. Bound Circuits: Why and How":http://www.analog.com/library/analogdialogue/archives/philbrick/020-024.pdf. Part of the magnificent book on Computing Amplifiers by Philbrick:http://www.analog.com/library/analogdialogue/archives/philbrick/computing_amplifiers.htmlSeveral options there, from the classic diodes, to low leakage options. There is also the "I.27. Fault protection" section, with diodes and zeners at the input.

author: Greg Robinson
author_rank: Regular Contributor
timestamp: February 05, 2016, 01:12:36 pm
content: Thanks for the responses everyone.Quote from: T3sl4co1l on February 05, 2016, 10:36:58 amThat is an option of course, but I do like a bit of belt-and-braces/suspenders.Quote from: T3sl4co1l on February 05, 2016, 10:36:58 amThere's a certain satisfaction in using "jellybean" parts, but I will look into it.Quote from: T3sl4co1l on February 05, 2016, 10:36:58 amInput is already clamped with zeners. I believe this should be more than adequate? This is not intended to be a distortion effect as is so common with guitar products, I'm just hoping to make all potential settings not so "ugly" as they otherwise could be.Quote from: T3sl4co1l on February 05, 2016, 10:36:58 amHuh, that's odd. I checked out quite a few datasheets on element14 (Farnell/Newark) and none had capacitance graphs. Thanks for clearing that up for me.Quote from: T3sl4co1l on February 05, 2016, 10:36:58 amThis looks like it will work out perfectly! Relatively small added parts count and significantly reduces the capacitance.Quote from: orolo on February 05, 2016, 11:10:00 amLooks like I was on the right track with the bound circuit, although I wasn't aware of the term. Thanks for the links.Quote from: Kleinstein on February 05, 2016, 10:21:45 amThis project will not be full audio bandwidth, the upper -3dB point will be around 5.5kHz, so even with a gain of x100 a TL07x should be well inside its bandwidth. But thanks for pointing that out.I'm going to call this one answered for now, I think Tim's suggestion of the zener inside a bridge rectifier solves all my design goals.Thanks again!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2016, 12:46:30 am
content: Another option is to limit the signal without introducing distortion at all: compression. You can have whatever threshold, attack and decay you like; of course, that happens to be yet another guitar effect, so isn't strictly true...Your only true option is building a 1000V amplifier that can support the dynamic range.Tim

author: uncle_bob
author_rank: Unknown
timestamp: February 06, 2016, 01:08:42 am
content: HiI thought that distortion was the main objective of most guitar circuitsYou can get diodes that work at GHz frequencies. Their capacitance will have zero impact on the signals you are working with. The normal "best way" to deal with overload is to clip it before you do any amplification. With limiter diodes, that involves setting up a pair of DC levels to clip against. If you want to do it another way:Start off with a voltage follower (gain of +1) input stage. It will swing rail to rail with many modern op-amps. That's as much signal as you can possibly handle. Drive that into an inverting amp. Put in a bridge circuit (4 diode rectifier bridge) across the feedback resistor. Use good / fast / low leakage diodes. Stuff the zener diode where you have DC on the bridge. No more loading, no more trouble. You also likely now have feedback resistors in the K and 10's of K range rather than meg ohms. Your input impedance can be giga ohms if you wish. With a dual op-amp you still may be at the same price and board size as a single op-amp design.Bob