Transistor ratings - Page 1
Forum section: electronics / general

author: akis
author_rank: Frequent Contributor
timestamp: June 22, 2014, 06:00:57 pm
content: On another thread I asked about transistor voltage ratings - same package but some can tolerate higher voltages than others of the same type.I was told that the main criterion is the leakage current, and thus transistors are placed in binned accordingly.In addition I just ran some tests on 30-odd PNP and NPN complementary power transistors and found that the PNPs have much higher beta than the NPNs and also even between same type they have differences of 200-400.If we are given a bucket of those transistors, is it best to use the lowest leakage ones (highest voltage rating) ? Also, is it best to select the highest beta ones? Are we then getting something for nothing?

author: Matje
author_rank: Regular Contributor
timestamp: June 22, 2014, 10:25:57 pm
content: Quote from: akis on June 22, 2014, 06:00:57 pmHow are you getting something for nothing if you throw away a bunch of devices (that you paid for, or how do you get them?) and on top of that expended the effort to characterize/measure them all?Usually you design with the leakage range and beta range given in the datasheet in mind, so that you don't need to select devices.

author: akis
author_rank: Frequent Contributor
timestamp: June 22, 2014, 10:50:42 pm
content: If someone offered you to pick transistors out of a bucket for free, all the same manufacturer and type, would you pick the ones with the highest beta because they are inherently superior to the others? Or pick the ones with the lowest leakage, because again, they are superior? Or does a higher beta mean we are sacrificing something else, maybe linearity, or something?I am not counting here how we got to have a bunch of transistors or how long it takes to measure them each one by one.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 23, 2014, 12:38:58 am
content: Well, in the sense that an ideal BJT has zero leakage, infinite breakdown voltage, constant Vbe, and either constant or infinite hFE, you could select on that basis.But that raises another question: why do you have applications that require such stringent testing of the parts? What good is it actually doing you?Also, I don't see how leakage at an undefined voltage is related to breakdown. Usually leakage scales with junction size as well as width, so a 1000V 10A transistor will probably leak about as much as a 100V 1A device (give or take; it's probably not a linear tradeoff). Best way really is to test at what voltage the leakage rises sharply (like 1mA).Tim

author: Bored@Work
author_rank: Super Contributor
timestamp: June 23, 2014, 05:26:32 am
content: Quote from: akis on June 22, 2014, 10:50:42 pmNo, I would take a handful at random and say thank you. If I recognize the part number I would make it a good handful.

author: mzzj
author_rank: Super Contributor
timestamp: June 23, 2014, 06:39:31 am
content: Quote from: akis on June 22, 2014, 10:50:42 pmI would choose the ones with high beta(or preferably beta matching close to each another)Usually it's not important but there are some situations where beta-matched transistors are nice.

author: akis
author_rank: Frequent Contributor
timestamp: June 23, 2014, 09:16:32 am
content: I should have even phrased my question like this: if between two transistors of same maker and type, one has a higher beta, does it mean it has sacrificed something else, equally important, to get to that higher beta than his little brother? If not, then it is something for nothing.*****************************Why is a higher beta better?A high beta allows you to use less driving current and hence smaller driver transistors or other devices. Sometimes you can get away without a driving transistor at all due to the beta being high. A smaller, or none, driving transistor means less energy spent as heat, fewer losses.Why is beta important?Outside of linearity, you may also want to do matching, in my most recent project I have a push-pull stage and the PNPs have a beta over 100-150 higher than the NPNs as measured at uA and at 1A.Because the beta is relatively high at 100-150, I can get away with small, TO-92 drivers, in other circumstances I would need TO-225 drivers!

author: Dave Turner
author_rank: Frequent Contributor
timestamp: June 23, 2014, 07:26:24 pm
content: In my opinion it's only worth spending the time to match components if you are designing for low volume production. If you're designing for high volume then it makes more sense to design well within the published parameters.

author: Matje
author_rank: Regular Contributor
timestamp: June 23, 2014, 10:30:44 pm
content: Quote from: akis on June 23, 2014, 09:16:32 amSometimes, maybe. If we are talking about analog drivers however power dissipation may rule...Hmm, a higher beta may actually mean it will be more likely to oscillate in some uncontrolled fashion, depending on the actual circuitry used.Don't misunderstand me, higher beta is nice, but it is no magical thing and the actual difference between a high beta device having a beta of 300 or 500 is, umm, largely not that interesting.Quote from: akis on June 23, 2014, 09:16:32 amBeware. Compare the allowed power dissipation of a TO-92 and a TO-225 (with the option of a heatsink too), unless you drive digital signals (with "good enough" input signals, yada yada yada etc. ;-) a higher beta may not buy you that much. That magic smoke is rather easy to release.

author: SeanB
author_rank: Super Contributor
timestamp: June 24, 2014, 12:05:29 am
content: You select the device for the application, and the only time you would actually want to select the device is if you need a matched pair. Otherwise the spread of gain with so many other variables makes the point moot. Hfe at a single current will vary as the collector current changes, the voltage across the device changes and the device temperature changes. That is why your datasheet has a limit set of minimum and maximum values, and for a lot of devices you can get them in a gain group that was selected at manufacturing test to fall into a bin.

author: tautech
author_rank: Super Contributor
timestamp: June 24, 2014, 12:21:29 am
content: As you are probably aware transistors are often grouped in a family of similar characteristics with different device numbers for the various voltage ratings in that family.Also there can be sub groups of the different ranges of gain, often identified by A, B, or C.Very old transistors could be color coded for the spread of gain in that transistor. I have seen up to 5 for the one transistor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 24, 2014, 10:07:05 am
content: Speed is a good reason: typically, higher hFE is achieved with a thinner base layer, which has more resistance, so an external signal takes longer to reach the most distant parts of the base. Compare 2N5088 to 2N3904, for example (note: I have no idea if they are otherwise similar in geometry; this may actually be a terrible example). Higher gain doesn't mean more bandwidth, because the fT (frequency at which gain drops to unity) is what's typically listed for bandwidth.RF transistors also further trade off breakdown voltage (of both C and E junctions), so that the B-E junction is typically rated for only two or three volts, and C-E, as little as useful (e.g., 5-15V is common for small signal devices, up to 60-100V for power transistors -- intended for operation from 24V DC supplies).Superbeta transistors (hFE over 500 or so, without resorting to a Darlington structure) are handy from time to time. Examples include low input bias amplifiers (many precision op-amps use them internally), certain optimizations in audio or other amplifiers (e.g., not requiring an additional driver stage before the output transistors), or low-power or low-noise power supplies (e.g., a capacitor multiplier with minimal voltage drop; ideally, infinite hFE allows for infinite base-supply resistance, so that very little capacitance is actually required to set a very long filter time constant).Tim

author: akis
author_rank: Frequent Contributor
timestamp: June 24, 2014, 10:20:05 am
content: Trying to express it better, my original question regards the manufacturing process. How is the beta determined, and why do we get large variations amongst transistors of the same type, created in the same factory by the same manufacturing process? Is it just a random artefact of the manufacturing process which does not affect any other parameter, or are there other, necessary changes for that beta to be different? It seems the answer is no, there are no other, necessary changes (emphasis on necessary).Very good information and thanks to all.

author: tautech
author_rank: Super Contributor
timestamp: June 24, 2014, 11:03:11 am
content: Quote from: akis on June 24, 2014, 10:20:05 amThe Beta is determined by post manufacture testing and transistors are then grouped/graded into a beta ranges.Manufacture for a specific transistor is based around a specific die and that die can be the same used for other transistors. There are many die configurations and many case/die combinations.The case/die combination often controls the power handling capabilityAs for the doping of the die and layers, other users might have a greater understanding than I

author: dannyf
author_rank: Super Contributor
timestamp: June 24, 2014, 11:22:46 am
content: Quote1. higher beta doesn't make it "inherently superior".2. many times, you don't care about beta; or even desire lower beta.3. for some applications, the point measurement of beta is far less useful than the flatness of beta over a current range.

author: SeanB
author_rank: Super Contributor
timestamp: June 24, 2014, 06:20:11 pm
content: You have a spread of values across the die, generally as a result of variations in processing, exactly how much of a particular dopant went into a particular device and how hot it was in processing. The variation is small across a wafer, but this is compounded by each processing step so the overall variation is larger. Thus with IC manufacture you find that tolerances for the exact value are large, but the design is such that they largely are either cancelled out or are changing such that they cancel out. Transistors have no such luxury, you get what is there, and see the variation in the process that a less than 1% difference causes.