Tantalum or electrrolytic? Can I use either here? - Page 1
Forum section: electronics / projects

author: Chris Wilson
author_rank: Super Contributor
timestamp: January 03, 2015, 04:45:52 pm
content: I am about to build the section of the circuit below that forms the RF output amp. I have no need for anything before that, I am just wanting a simple amp to get from 0dBm at 136kHz, to about 17dBm.So I am building from the 1uF Tant feeding U7A and to the right.The circuit, and the photo of the completed unit shows the designer has used Tantalums in the RF amp stage. Do I need to order some in, or can I use electrolytics instead, which I have? I am never sure just where Tants need to be used, and where plain electrolytics are OK. Thanks.

author: SeanB
author_rank: Super Contributor
timestamp: January 03, 2015, 05:00:00 pm
content: Use low ESR electrolytics there, at 136kHz they will still be a capacitor. At 10MHz that might be different, but here it is still an audio amplifier with somewhat extended frequency response. For the 1uF you can use any value from 1uF to 4.7uF with little difference. Same with the 47uF, any value from 47uF to 220uF will work, just has to be a very low ESR unit. The small capacitors will most likely be easiest to get as 50 or 63V units, while the larger would be best as 35V units.

author: nctnico
author_rank: Super Contributor
timestamp: January 04, 2015, 01:02:21 am
content: Never use tantalums; they are troublesome parts to use and need materials mined in valuable rain forrests. Use MLCC or electrolytics. At 1uf there is no reason not to use an MLCC capacitor but do check the voltage dependant capacitance loss in the datasheet.

author: ovnr
author_rank: Frequent Contributor
timestamp: January 04, 2015, 01:29:26 am
content: Quote from: nctnico on January 04, 2015, 01:02:21 amAnd by "troublesome", he means "they explode rather violently". I think it's a rather fetching attribute; but then the last dozen times I used tants it was to intentionally blow them up.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 04, 2015, 02:09:35 am
content: Note, ceramics in the signal path may result in undesirable microphonics. Probably not a big deal at line level, or for something like amateur radio, but something to watch out for.Electrolytics in the signal path are okay as long as the leakage and ESR aren't a big deal. Usually not, but, say, out of a 50 ohm signal path, the insertion loss will be noticeable for electrolytics that size (at least at some frequencies).Films of course are great, but huge and pricey. So..Tantalums are really quite ideal for coupling applications; it's a shame about their drawbacks (such as cost, flammability in power supply applications, and political sources).Aluminum polymers (even better than tantalum!) that range do exist, but at a glance, it looks like availability and pricing still stink. So, deal with ceramic if you can, I guess.As for the supply bypasses, the location of those will not matter at all. Place them in relatively distant locations, and add a few 0.1 ceramics along the way, if you like.Tim

author: Kalvin
author_rank: Super Contributor
timestamp: January 04, 2015, 01:24:06 pm
content: Quote from: T3sl4co1l on January 04, 2015, 02:09:35 amCeramic capacitors with X5R and X7R dielectrics have also a slight voltage dependency ie. the capacitance changes as the voltage changes across the capacitor. This may create unwanted distortion in sensitive circuits and applications. C0G or NP0 dielectrics is a better choice if available. However, the distortion resulting from X5R and X7R dielectrics can be ignored in most practical circuits, but it is a good thing to keep in mind if dealing with a high precision circuits and applications."Signal distortion from high-K ceramic capacitors"http://www.edn.com/design/analog/4416466/1/Signal-distortion-from-high-K-ceramic-capacitors"Choose capacitor types to optimize PC sound quality"http://m.eet.com/media/1134061/14392-pdf.pdf

author: wraper
author_rank: Unknown
timestamp: January 04, 2015, 01:37:46 pm
content: Better to use tantalum, electrolytic is no good here. As of those saying that they explode violently, obviously not where it is used in this circuit.

author: penfold
author_rank: Frequent Contributor
timestamp: January 04, 2015, 02:09:44 pm
content: If a tantalum is intentionally specified (perhaps there was some addition writing that may not otherwise have been there), there is normally a good reason. If used properly, modern tantalum caps just don't explode sporadically, solid tantalum is frequently the choice in enhanced reliability designs (where cost is still an important factor, otherwise liquid tantalum is often desired).The parasitic resistance in a tantalum is often the reason they are used as it damps out oscillation that can occur due to the ESL that would otherwise cause problems in electrolytics. So when used with additional HF decoupling they provide a nice setup, however, they are really not recommended where there is any significant current that will flow in them.

author: Chris Wilson
author_rank: Super Contributor
timestamp: January 04, 2015, 11:35:02 pm
content: Thanks for all the info guys, despite some of it being contradictoryI decided to invest in some Tantalums as it would be good to have a choice, and most of my electrolytics are off Ebay, and probably not the best branded stuff. For HV power supplies and the like I buy branded from Farnell or RS, but for breadboarding I tend to use cheaper stuff.I have (yet another) capacitor question, but I'll start another thread. Thanks again everyone.

author: jmole
author_rank: Regular Contributor
timestamp: January 05, 2015, 05:10:15 am
content: A tantalum cap *is* an electrolytic cap. An option for low ESR, sans explosions, is aluminum polymer caps.ninja edit: the explosive reputation of tantalum is a bit undeserved, IMHO; most PC motherboards/graphics cards use them for decoupling nowadays, and if it were a serious problem you'd see a lot more laptops exploding. As far as use for a filter/dc blocking cap goes, I don't have the experience to offer an opinion there, perhaps others might.

author: c4757p
author_rank: Super Contributor
timestamp: January 05, 2015, 05:44:00 am
content: Quote from: jmole on January 05, 2015, 05:10:15 amPet peeve here - yes! Gladsomebodycould do my nitpicking for me

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 05, 2015, 07:33:25 pm
content: Well, they're all polarized, at least.As far aselectrolytic, the tantalum oxide I think is produced electrolytically (anodizing, as with aluminum and niobium types as well), but the electrolyte is cleaned out and replaced with a heavy-ish layer of MnO2 ("solid electrolyte") and a thinner coating of silver (and/or graphite?? I forget). I forget if polymer types are anodized in place (thus obtaining oxidation from the solid polymer electrolyte), or if they are formed before the polymer is applied.So, as far as what is "electrolytic" about them: it's probably the least questionable to call "wet", formed types (aluminum electrolytic, wet slug tantalum, and maybe I'm missing a few?) electrolytic, in that they involve an electrolyte, and their nature depends upon an electrolytic reaction, both during manufacture, and in maintenance of operation.As for  tantalums and polymers, it's somewhat more questionable to call them "electrolytic", as manufacture involves an electrolytic step, but they do not use an electrolyte, at least in the conventional sense. (They probably should still be, as electrolytic reaction is still possible -- tantalums can be "reformed" to a small extent, though it's my understanding that it's more of a self healing process (which should exhibit a minuscule decrease in capacitance?). So, does it look like a goose, or quack like a duck? I'm not sure. I'm not sure offhand if polymers exhibit similar reactions; it's my understanding they behave pretty much like film caps, and don't really reform or degrade.)Electric bilayer (supercaps) types, conversely, are specifically designednotto be electrolytic. That's why they all have such a consistently low voltage rating: it causes breakdown of the electrolyte. But, they are still "wet", in that an ionic liquid -- an electrolyte -- is involved.To extend the nit-picking even further... if the condition of "electrolytic" depends only on using an electrolyte in the design, or an electrolytic step in the manufacture, then shouldn't any capacitor which utilizes an electrolytic reaction in the active region, be labeled as such? If so: this would include the opposite process, that is, applying a metal contact to an insulating dielectric. An example would be [usually "electroless", but hey, redox is redox, right?..] silver or nickel plating, say for ceramic (the macroscopic kinds) or mica types. I doubt this is actually used in any current production capacitors (vacuum deposition seems much more likely, as I know is the case for film caps), but the possibility gives an interesting twist in exploring the scope of possible definitions in this conversation.So...TLDR: it probably is more correct to say all polarized caps (and "bipolar", which are just polarized pairs, connected back-to-back internally) are electrolytic. How correct it is is probably a matter of interpretation, with some being more likely than others. So, if you forget sometimes, that's OK. And you can never go wrong by being specific about the type, without having to worry about the definition (a tantalum cap is a tantalum cap, whether the oxide was grown by anodizing or by virgins warming billets of compressed tantalum between their soft, thermally enhanced thighs). (Wait, what were we talking about again?)Tim

author: rs20
author_rank: Super Contributor
timestamp: January 09, 2015, 09:45:18 pm
content: Quote from: Kalvin on January 04, 2015, 01:24:06 pmThe whole point of a coupling cap is for it to (nominally) maintain a constant voltage across itself; so the only time you get problematic distortion if if you have,1. A strong signal, AND2. A AC coupling cutoff frequency close to the signal frequency (or else the attenuation from signal --> voltage across cap is huge), AND3. An X5R/X7R/similer dielectric.Don't avoid item 3 unless you've established that 1 and 2 haven't taken care of the problem for you. It's not some mystery voodoo, these are quite easily calculable things.

author: G0HZU
author_rank: Super Contributor
timestamp: January 09, 2015, 10:52:12 pm
content: I would not recommend using 47uF tants on the 12V supply for bypass unless you include something to add some inline loss in your power supply feed to limit inrush (and discharge) current. Even then I don't think I would want to use tants there.I've been designing products with tant caps for >25 years and still use them as much as possible because they offer good performance over time and temperature and high reliability. I've designed very large and complicated boards that sometimes have dozens and dozens of SMD tants and these boards sold in qty of many tens of thousand with high reliabililty across wide temperature ranges. But only because I designed within the accepted design rules for tants. Break the design rules for tantalum caps and you run the risk of early failure. Sometimes the failures can be spectacular if you break the rules!