The Opamp Nobel Darwin Award of The Week Goes To... - Page 1
Forum section: electronics / general

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 07, 2012, 07:10:28 am
content: joint award goes to NJM4558LD, NJM4580L and 4558D!clap! clap! clap!achievement: not following +input in "the simple" voltage follower configuration. ie when +input = -rail, output = +rail. novel! better (read "worst") than classic 741 and 324.so here they are.

author: amspire
author_rank: Super Contributor
timestamp: February 07, 2012, 07:44:35 am
content: So when you take the inputs outside the common mode range, the opamp does not work properly?Anyway, the only way I can see the output can go high is if you manage to cause the inverting input transistor base emitter junction to go into avalanche breakdown. How did you do that? I guess you were running it at +/- 18V supply and took the input from the posive rail down to the negative rail very fast (like shorting it with a wire), and the output doesn't have enough slew rate to follow, then perhaps it is possible. What you describe should not be possible with a supply less then +/- 15V, unless there is something you are not telling us. Like an input went above the positive rail or below the negative rail, or you got the inputs reversed.Richard

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 07, 2012, 07:59:12 am
content: my setup is a potentiometer connected to both rails at both end pot's pins (tried both 1K and 10K pots), the middle pin goes to the +input. i've tried many ±V supply, ±3, ±5 up to ±12 (with my newly "handmade" adjustable dual rail tracking linear psu). playing with potentiometer knob and look at the scope showing the pot's middle pin potential (+input) and the opamp output. no avalanche, no excessive Vsupply, everything under spec, even i've tried to pull the output using 10Kohm to -rail, the behaviour is persistence, the said "darwin" opamps do their job other than +input near or equal -rail though. also tried on 741 and 324 with the same setup, no pull down needed no darwin/murphy effects, just jumpers, everything work as expected (except non rail to rail output which is acceptable). though the said darwin's opamps are salvaged from differents old boards, if the argument is damaged parts, i'm not sure, they all behaving the same. i will avoid in my next order.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 07, 2012, 08:08:33 am
content: maybe the argument is my not so "lab quality psu"? just tried it on LA batteries, same. i looked closely, the effect is happening when +input comes close at a distance a bit less and lesser than a volt to -rail (-12V from second LA battery, edit: lesser than 0.75V to be exact to the best of my eyes and scope can see).

author: JuKu
author_rank: Frequent Contributor
timestamp: February 07, 2012, 08:49:36 am
content: The op-amps clip to reverse polarity? Not at all uncommon, welcome to real world, learn to read to small print of the data sheet.(Yeah, I've been hit by this, too...)

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 07, 2012, 10:01:47 am
content: yes reverse polarity output only happens when +input -> -ve rail. for +input -> +ve rail is fine. not in the datasheet that i have, small prints say different things. maybe this can be a standard test for a newly obtained opamp? not a good idea if this is happening and we expect possibility of +/- input going hard to -/+ rail in a circuit.

author: amspire
author_rank: Super Contributor
timestamp: February 07, 2012, 12:54:34 pm
content: Go ahead and present the award!After a second look at the op-amp circuit, I realized I missed the fact that the second stage was a darlington, so if the input to the second stage falls below about 1.4 volts, the second stage turns off, sending the output high.The second stage will fall below 1.4 volts in when the non-inverting input gets to within about 0.7 volts of the negative rail.They definitely deserve a Darwin award for not mentioning a minimum voltage for the inputs. They mention that the typical working limit for inputs is within 1 volt of the rails, but there definitely should have been a clear statement not to let the non-inverting input to be closer then 1V to the negative rail over all supply voltages.Richard.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 07, 2012, 02:00:07 pm
content: Quotei wish i can see the way you see. when i look at any opamp circuit, all i see are girls and chicks. i only care is how they perform. there's nothing worst than making a circuit, feeding a clean sine input (yellow trace below) and get spurious output (blue trace), and then try to figure out what, taking in and out, replacing other components, try to put in the best filter available on earth and still not solving the problem, in the end, just changing this not performing opamp is the simple solution, i maybe lucky today, but who knows, Murphy (and Darwin) will always be there...

author: ciccio
author_rank: Frequent Contributor
timestamp: February 07, 2012, 02:53:32 pm
content: I've noticed a similar effect with TL072.In non inverting operation if the input goes higher than +V or lower than -V you have a "frequency doubler"...A NE5532 works fine.When I have some spare time I'll test a NJM4580.Best regards

author: ejeffrey
author_rank: Super Contributor
timestamp: February 07, 2012, 03:29:32 pm
content: This is quite common in JFET opamps. It is called "phase inversion" and the moral of the story is: keep the device within the specified operating range. You cannot assume that going slightly outside of the specified common mode range will cause gradual performance degredation. You end up forward biasing junctions that are supposed to be reverse biased and the resulting behavior is not what you would expect at all.CMOS opamps, and in general any rail-to-rail device should be immune to this. Another solution is to use inverting amplifiers. With an inverting amplifier both inputs stay near the middle of the common mode range at all times, and when driven into clipping they just clip the way you would like them to.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 03, 2012, 02:25:37 pm
content: Quote from: ciccio on February 07, 2012, 02:53:32 pmwhy i didnt notice this warning!while searching for recommended general purpose opamp, one audio guru guy came out with this name, googling and 'loupe'ing the datasheet, the spec seems good! 3MHz GBW, pA offset/bias current etc. now i'm stuck with 70 effective TL071 opamp (TL072 and TL074) in my stock. argh!now again... opamp darwin award for this week...TL071 and the bros (TL072, TL074)for the discovery that it has no mentioning/clue in the datasheet about this polarity reversal effect! (or i havent figure out yet).Quote from: ejjefreythe specified common mode in the datasheet is ±11V and input range is ±15V. the behaviour is still the same with less than that figure for ±Vs. i tried say, ±6V supply, reversal occured at around -5V at non-inverting input. i havent exceeded the common mode or input range spec or supply rails yet. it should be mentioning input range in term/relative to supply rails. i saw some other opamp specifying input range = ±Vs (full Vs range) or input range = ±Vs - 3 for eg, so we will know we should not exceed Vs minus 3V at the input.though i missed your point about JFET opamp (vs BJT and rail-rail). i hope the fancier TL071 spec (higher GBW lower bias/offset) will do me some good (compared to my slower 741 and high distortion 324), albeit with some extra biasing or external circuit etc effort if i have to do it in non-inverting config. thanks for advice using inverting config, can be a good idea encrypting some section in a circuit.

author: Unknown
author_rank: Unknown
timestamp: March 03, 2012, 03:38:38 pm
content: Quote from: Mechatrommer on March 03, 2012, 02:25:37 pmThe marketing blurb on the first page of the TI TL071 datasheet mentions that the common-mode input voltage range includes VCC+, this alone should be a hint that the negative rail is not included. Common mode input range is specified as +/- 11 V minimum at 25°C, and -12 to +15 V typical. This is at a +/- 15 V supply voltage. How you would conclude that the same range would apply at lower supply is beyond me. They don't specify operation at lower supply voltages, probably because it's an old part not aimed at low power operation. Assuming that the offset to the rails is constant would probably be a fair approximation, see the input common-mode voltage vs. supply voltage graph in thePhilips uA741 datasheetfor an example.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 03, 2012, 04:22:43 pm
content: Instead of just guessing that the TL071 would work better, you could actually buy an opamp specified as rail-to-rail input, or one that says 'input common mode range includes the supply rail', or one that is 'designed for single supply operation'. Note that 'single supply' opamps may not work properly at the + rail. All the major manufacturers let you filter their parametric lists by whether they are rail-to-rail.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 03, 2012, 05:15:20 pm
content: Quoteok thanks. now its clear. mission accomplished!@jeff: i have this feeling that rail-rail opamp is more expensive than regular opamp. i got what i pay.