Strange results measuring beta - Page 1
Forum section: electronics / projects

author: Analog Kid
author_rank: Super Contributor
timestamp: October 06, 2024, 01:10:55 am
content: [my first post here]I'm hoping someone here can show me what I'm doing wrong, because I'm getting some screwy results trying to do something really simple.This is an experiment, for my benefit, measuring the beta of a transistor. Should be simple, right? Drive the DUT (device under test) with a known base current, measure the collector current, divide ICby IBand voila. Except no voila here, just a bunch of values that make no sense.The rig I'm using is this:Pretty simple: I'm using a current mirror to supply the base current, adjusting it to a small known amount (using 20 μA). I'm using 2 meters, a DMM (cheap one but accurate) for the base current, an analog one for collector current. It's breadboarded on a board that seems to have good connections. Using a fresh 9-volt battery for the tests.I'll post a table of result values when I do some more systematic tests, but to give you an idea, I tested a 2N2222, applied 20 μA base current, got an ICof 25 mA. That would be a beta of 1,250, which seems ... unlikely.Another thing: some of the transistors being tested get really, really hot, even though the ICwould indicate that they're well within their current and power limits. That 100Ω resistor is an attempt to limit current enough to prevent heating; I don't like it being there, and it's probably interfering with accurate measurement.I tried using a 5-volt supply (well-regulated, using a 7805); the collector current barely registered at all (30 μA vs. 20 μA base current, beta basically zilch).My understanding of things transistor tells me that in order to make a beta test that's anywhere near accurate, the transistor needs to be within its active region. I'm not sure that's the case here.My intent here is to teach myself transistor basics through a hands-on experiment; I'd also like to come out of this with a simple, accurate-enough beta test rig to go through transistors and weed out bad ones. (After heating up some of them I'm pretty sure a lot of my 2222s and 3904s are fried!)So what am I doing wrong here?

author: mawyatt
author_rank: Super Contributor
timestamp: October 06, 2024, 02:14:39 am
content: Why bother with the 2N3906 current mirror, just use 100K pot across Vcc and 470K to DUT base from pot wiper.Check for DUT oscillations. A small cap (100pF) from DUT C to B should help, as does ferrite beads on the C and B DUT leads.Best

author: RoGeorge
author_rank: Super Contributor
timestamp: October 06, 2024, 02:30:01 am
content: Quote from: Analog Kid on October 06, 2024, 01:10:55 amIf the DUT heats at apparently low Ic, usually it means there are undetected RF oscillations.The measuring instruments might not see the oscillations. Sometimes the RF is so high that not even the oscilloscope can sense them. Usually the counter measure against oscillations is a ferrite bead inserted on the base terminal, but that usually happens for common collector, not for common emitter. Though breadboard might show enough parasitic L and/or parasitic C to make an unintended oscillator.I would add decoupling capacitors on the power supply, too, 10uF in parallel with 0.1uF placed near the circuit (the exact value is not critical, any values in that range should do it, decoupling the power supply might avoid undesired oscillations).Don't know if RF oscillations are the reason for heating your transistors.If you go near the max power specified in the datasheet you may see considerable heating. Often the max power in the datasheet is specified for an infinite radiator. To calculate the expected temperature for a given power, look in the datasheet for the thermal resistance between junction and capsule + thermal resistance between capsule and air.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 06, 2024, 03:12:00 am
content: Thanks for both those replies.I guess I was overthinking things with the current mirror. Using the pot as a voltage divider actually works better, give more range. And since the current source is adjustable, there's really no need for a constant current source.At this point I honestly don't think I have to worry about RF oscillations; it's a simple DC circuit with no feedback path. (There may have been one with the current mirror.) Now I'm getting believable measurements, in the 100-350 range.There was also a very weird thing--varying the base current had zero effect on collector current--until I learned (better late than never!) that a PN2222 isn't the same thing as a 2N2222.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 06, 2024, 03:27:42 am
content: My next question: parameters. I kind of randomly ended up using 20 μA as a base current, a nice low round number to give a fairly low collector current, but I'm not sure it's ideal. My guess™ is that a guy would want the highest possible value that wouldn't risk overdriving the DUT with excessive current. Can someone help me pick such a value? For now, this is only for small jellybean xistors, no power ones, no 3055s, TIP32s, etc., so I guess ICcan be limited to 50-100 mA max? (Hmm, maybe not that much: 100 mA = ~900 mW, way too much for some Qs.)How do the pro transistor testers do this?

author: RoGeorge
author_rank: Super Contributor
timestamp: October 06, 2024, 03:56:53 am
content: Quote from: Analog Kid on October 06, 2024, 03:27:42 amWhat your guy really needs is the beta at the specific Ic used in a given schematic.Beta varies with Ic (and varies with frequency, too, that's another story). So, a generic beta would be better than nothing, but what I would really want to know is the beta for the particular Ic I am using in my schematic.Often there are charts in the transistor's datasheet, chart(s) that represent typical beta vs Ic. In your case, you'll have to adjust the potentiometer until the Ic becomes the one you intend to use in your schematic, then read the Ib and make the ratio Ic/Ib.I don't know what is the typical Ib used by generic DMMs capable to measure beta, but your example with 20uA for Ib seems fine. Alternatively, you can search for a few DMMs schematics, and see what typical Ib they are using.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 06, 2024, 04:11:46 am
content: Easier to let the circuit set base current for you.Set up a base voltage divider, and use an emitter resistor. Measure Ib and Ic. This sets Vce (approximately) and Ic as well. Ib is then simply whatever's necessary to achieve that condition, usually a small fraction, as expected.You didn't mention what V, I you tested at, nor of what all devices; setting Ib on a Darlington for example could be disastrous, though how hot a part gets in relation to its size, and the load resistor, isn't clear.Beware also a MOSFET, if you accidentally test one, won't draw "Ib", and (unless it's an RF type, that is very old for one, but also likely ESD-zapped in the process of handling, unless you took considerable precautions?) draws more or less as much "Ic" as it can (the highest Rds(on) parts commonly available are 5 or 10 ohms still, quite low).Tim

author: jwet
author_rank: Frequent Contributor
timestamp: October 06, 2024, 04:19:04 am
content: Beta is a messy and kind of non-sense parameter. It useful to give you some feel for what's happening but the actual operation of a bipolar is much different. Biploar transistor are actually "transconductive" meaning that the collector current depends logarithmically on the base emitter voltage. Google "Ebers-Moll" model, this is a reasonable low frequency model that will illuminate a lot of the operation of transistors. A proper model for a bipolar should be a transconductance (gm) controlled by Vbe. You can find resources online that can explain this way better than I can.I applaud your quest for first principles knowledge. I would recommend a couple of books- Art of Electronics is one of the very finest. (oider editions are available to download online for free- the bipolar transistor hasn't changed since 1980 so you won't miss much). Another good intro is Electronics Circuits- Discrete and Integrated by Shilling and Belove, a relatively low theoretical content (minor calculus) but solid into to transistor and fets. Ironically, it covers a lot of topics but doesn't cover current mirrors. There are others that people may recommend here. The realization that transistor beta is mostly a fantasy driven by incomplete understanding of the real physics is a real eye opener for anyone in the field. Few people actually appreciate the real operation of these devices.

author: Smokey
author_rank: Super Contributor
timestamp: October 06, 2024, 04:50:53 am
content: LTSpice if you aren't already playing with that. It's significantly easier and quicker to "experiment" with theory in the simulator than it is on the bench.

author: magic
author_rank: Super Contributor
timestamp: October 06, 2024, 05:49:15 am
content: Yeah, you will quickly learn theory and then have no idea why it doesn't apply in real circuits.In this case, my bet on the transistor being reversed (collector and emitter swapped). Or maybe some other pinout oopsie. This would easily explain impossibly high β at 9V and very poor at 5V. And, by the way, SPICE doesn't simulate the 9V case correctly.Note that 2N2222 is a metal can device and there are various TO92 equivalents with subtly different part numbers and two common pinouts (EBC/CBE). And when it comes to "made in China", all bets are off.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 06, 2024, 07:00:45 am
content: Quote from: Analog Kid on October 06, 2024, 01:10:55 amIt is unlikely..Did you confirm your meters are properly in spec ?If you have a multimeter with a HFE testing socket, you can sanity check your results.Self heating can affect the numbers as HFE also varies with temperature, so I'd suggest simple fixed high value resistors for base feed, with momentary push buttons.eg Base current tests of 1uA 10uA 100uA would be useful for most small signal transistors.An alternative approach to HFE is to feedback regulate for a given emitter or collector current, at a defined VCE and then measure the base current that requires.A series base resistor can give a simple means to check base current, as usually multimeters can measure voltages over more ranges, easier than current.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 06, 2024, 08:35:28 am
content: Thanks for all the replies. I'm impressed just by the amount of traffic here!Few notes:o I haveThe Art of Electronics. Great book, although a little heavy on the math for my taste (I took calculus but it makes my head hurt). The guy who gave me his old copy is actually a good friend of Paul Horowitz.o i know from that book and others that beta (or HFE) is a parameter to be avoided when designing circuits, as it varies from part to part and in circuit operation. I'm mainly interested here in learning principles firsthand, and also in building a tester to separate decent working parts from junk.o No MOSFETs here, at least not yet. I'll burn that bridge when I come to it.

author: mawyatt
author_rank: Super Contributor
timestamp: October 06, 2024, 03:48:32 pm
content: Quote from: Analog Kid on October 06, 2024, 08:35:28 amPlease note post by jwet, learning the bipolar transistor is a formidable task that very few have mastered.Quality analog circuit designs (and designers) don't rely on Beta but on the semiconductor physics relationship between junction voltage and current which is fundamental in analog IC design involving bipolar transistors. The genius of Bob Widlar, George Erdi, and Dave Fullagar is shown in their brilliant analog IC designs and understanding of the bipolar transistor.Studying the details of transistor behavior from the inside out rather than outside reveals the remarkable characteristics that gets "covered up" with parameters like Beta and HFE (hfe) which BTW are not the same parameters.The bipolar transistor hasn't changed much over the years being displaced with the incredible shrinking CMOS, with only the introduction of Ge in the Si process for the SiGe device, and the use of InP rather than Si, both of which were to increase transistor speed.We've been fortunate enough to design in Si, SiGe and InP over our career and altho they are different when viewed from the outside their internal bipolar behavior isn't that much different when observed from a semiconductor physics standpoint inside.You are venturing into a fascinating trip wrt the bipolar transistor, please don't stop at the outside details, dive deep into the physics of the wonderful devices, they are absolutely amazingBest

author: PGPG
author_rank: Frequent Contributor
timestamp: October 07, 2024, 04:02:49 pm
content: Quote from: Analog Kid on October 06, 2024, 08:35:28 amIn my opinion...1. You don't need to measure Ib.Vbe is around 0.6 to 0.7V so if you power base by resistor from 9V you can simply count Ib=(9-0.7)/R and it is more precise than beta measurement precision needed.2. I think measuring beta for collector current about 1..5mA for signal transistor is good enough.3. Learn to understand Θja and Θjc parameters. If you dissipate 300mW in 2N2222 and its Θja (junction-ambient) is 350°C/W then junction is 0.3*350=105°C higher than ambient temperature. If air temperature near transistor is 25°C than junction temperature is 130°C. If Θjc is 97°C/W then case is 0.3*97=27°C lower than junction so case is 130-27=103°C. Certainly too high to be touched even it is only 300mW.Quote from: jwet on October 06, 2024, 04:19:04 amSmall signal transistor analyse is next step in transistor understanding than its DC polarization. DC polarization should also be made such a way that beta 'don't cares' but for DC working point calculations beta is fully sense parameter to be used to count what about is base current when collector current is as you decided you want to be in your circuit.Many transistor circuits can be designed and enough correctly counted not using small signal analyses. In oscilloscope I made when was young vertical deflection plates required 30V/cm. Designing whole its Y amplifier can be done without thinking about small signal transistor model at all.

author: TimFox
author_rank: Super Contributor
timestamp: October 07, 2024, 04:32:21 pm
content: Here's an elementary circuit, which I originally used for measuring grid current for vacuum tubes.You probably don't need all the meters, depending on what two power supplies you used, but you need to measure base current and emitter current (or collector current).I originally used a constant current supply for the emitter (or cathode), but that is not necessary.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 07, 2024, 08:51:21 pm
content: Yes. Here's the current test rig after removing the current mirror:Measuring emitter current would make it be off by the amount of base current, yes? Although IBwould be pretty much swamped by IC...

author: TimFox
author_rank: Super Contributor
timestamp: October 07, 2024, 08:58:48 pm
content: The reason for my circuit is to measureIbaseas a function ofIemitter, which is safer. It’s also useful sometimes to look at differentVcevalues.For critical applications, you can correctIcfor the base current. The negative power supply allows you to set the emitter current.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 07, 2024, 09:45:02 pm
content: Quote from: TimFox on October 07, 2024, 08:58:48 pmSafer? How?Not challenging you, just want to know.

author: TimFox
author_rank: Super Contributor
timestamp: October 07, 2024, 09:48:38 pm
content: The base current is a much slower function of the emitter or collector current than vice-versa, so it is safer to control the emitter current and measure the base current.If you apply enough base current to saturate the transistor, the collector current from a “stiff” voltage source can be too high for the device

author: Analog Kid
author_rank: Super Contributor
timestamp: October 07, 2024, 09:54:44 pm
content: So we can assume, for the sake of close-enough testing, that IC≈ IE, right?

author: Someone
author_rank: Super Contributor
timestamp: October 07, 2024, 09:57:15 pm
content: Quote from: Analog Kid on October 07, 2024, 09:45:02 pmThis is safer by directly adjusting the parameter that is most likely to result in failure, limiting the emitter current directly. If you only adjust the base current and leave the emitter current unbounded/unlimited then it could end up (unexpectedly) far above "safe" conditions.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 07, 2024, 09:59:06 pm
content: Quote from: Analog Kid on October 07, 2024, 08:51:21 pmI would swap the 470k and meter.Multimeters plus leads can pick up a lot of hum and general noise, which can affect sensitive measurements like this, and the base-emitter junction is a diode.Avoiding the effect of long leads is another reason for the suggestion of fixed switched base resistors.

author: Someone
author_rank: Super Contributor
timestamp: October 07, 2024, 10:00:05 pm
content: Quote from: Analog Kid on October 07, 2024, 09:54:44 pmThat is one approximation that could be valid. It depends on the accuracy of the measurements being taken. That approximation error might be less than the error of the meters being used, or in reverse; some accurate meters could measure 2 of the parameters and still have an accurate result of the 3rd parameter.

author: TimFox
author_rank: Super Contributor
timestamp: October 07, 2024, 10:06:00 pm
content: Algebra: if you measureIbandIe, then the ratioIc/Ib= 1 +Ie/Ibis accurate.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 07, 2024, 10:16:10 pm
content: Ah, yes, because ... alpha.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 07, 2024, 10:52:07 pm
content: So do you like this arrangement better?This assumes totally manual operation (start pot at zero, carefully bring it up to measure beta).Current limiting needed to protect transistor against excessive base current?Another question if you don't mind: we haven't even touched on whether the D.U.T. would be in the correct part of the active region to make an accurate test. How should this be handled? (Again, not aiming to design a super-accurate beta meter, just one good enough to weed out defective parts--and teach me a few things.)

author: TimFox
author_rank: Super Contributor
timestamp: October 07, 2024, 11:06:07 pm
content: Quote from: Analog Kid on October 07, 2024, 10:52:07 pmThe problem with that revised circuit is that the pot changes both the voltage and current on the collector.My circuit avoids that, but needs a second power supply.You can adjust the collector voltage independent of the emitter current, ensuring that you are in the active region (not saturated).For a go/no-go test, you can use fixed resistors to set a reasonably accurate emitter current and apply go/no-go to the base current.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 08, 2024, 12:46:33 am
content: Pardon me if this seems a bit dense, but I don't get how to vary the collector current independent of voltage. Do I need to construct a variable current source (like the current mirror I originally had)?Is it really important to vary emitter current as you suggest rather than collector current?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 08, 2024, 12:52:26 am
content: Quote from: Analog Kid on October 07, 2024, 10:52:07 pmIt really depends on what you are trying to measure.If you just want 'not defective' then try to avoid too many variables and apply KISS.If you get some cheap DPDT push buttons, you can use 3 to give you 4 curve intersect test points, with a simple scaled VU type meter.The two poles select BASE and Collector meter shunt resistor, so the meter mid point is always HFE (say) 200You can then quickly check at (eg) 100uA 1mA 10mA and one other (perhaps for opto couplers ? ).No fiddling or maths, and gives a direct analog meter result in seconds.You could add a simple current limited BJT as protection against E-C shorted (dead) parts.If you supply the base resistor from a 50% 4.5V you can test either NPN or PNP in the same fixture. One measures collector current, the other emitter current, but with HFE's of ~ 200 midscale that' a tiny error.

author: TimFox
author_rank: Super Contributor
timestamp: October 08, 2024, 02:58:04 am
content: Quote from: Analog Kid on October 08, 2024, 12:46:33 amIt's easy to vary the emitter current if you have the negative power supply, as in my diagram.You can set the collector-base voltage directly, and the collector-emitter voltage will be 0.7 V higher.You can set the emitter current either through an actual current source, or with a reasonable resistor and a variable voltage.It's difficult (not impossible) to vary the collector current directly at a given emitter voltage, using a more complicated op amp circuit.Using my basic circuit, probably the simplest go/no-go test would use two 9 V batteries and a 2.0 k emitter resistor to set the emitter current to approximately 4.2 mA at a collector-emitter voltage of 9.7 V, and measure the base current with a single microammeter (using a DMM on 200 uA scale).Collector current meter (20 mA scale) useful, but not required.You can change that resistor value to get a different emitter current.

author: G0HZU
author_rank: Super Contributor
timestamp: October 08, 2024, 07:48:59 pm
content: It's also possible to measure ac current gain vs frequency using a VNA. See below for the hfe vs frequency for a 2N3904 biased at 10mA Ic and 10Vce.You can see that the Ft (unity current gain) is at about 300MHz at marker 8. Usually, Ft is predicted from measurements at lower frequencies.For example:At 10MHz FT is predicted at marker 3 as 10MHz x 33.3 = 333MHz.At 20MHz FT is predicted at marker 4 as 20.19MHz x 16.7 = 337MHz.These measurements were made using a modern lab VNA with bias tees and this allows the BJT to be measured directly at its pins when biased at 10mA Ic and 10Vce.The other graph shows several curves that show how hfe varies with collector current Ic.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 10, 2024, 01:15:33 am
content: VNA? As in vector network analyzer? (I had to look that up, had no idea what that particular TLA meant.)That is so, so far away from anything I intended with this thread so as to be totally irrelevant.It's like suggesting someone invest in an electron microscope when they asked about obtaining a good magnifying glass.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2024, 01:39:22 am
content: Not really. It's a fancy instrument, to be sure, but you can perform precisely the same measurement, as point tests, with a signal generator, oscilloscope and bias tees.Since transistors are well-behaved, it suffices to take a few points along the curve. Datasheets don't even take a series, they just plot two points -- h_fe is typically measured at some low frequency (~kHz?), and fT at some (fixed) frequency; they don't actually measure *at* fT (for h_fe = 1).(h_FE is the DC, average (Ic/Ib), or large-signal, parameter; h_FE doesn't necessarily match h_fe (small signal, delta Ic / delta Ib, low frequency) at any given operating point, though they are typically close.)Tim

author: Analog Kid
author_rank: Super Contributor
timestamp: October 10, 2024, 02:50:41 am
content: OK, ready for another go-round here. Here's the latest test rig:One thing that works nicely is the current source; I tested it independently of the beta test function and it works well. (Stole it from Horowitz & Hill.) Gives a slightly adjustable current of ~10mA, which should be fine for testing li'l jellybean xistors.I took Tim's suggestion of varying the collector current (in lieu of varying IE) instead of varying IB.What doesn't work is the beta-test function. Using a PN2222 as a victim, I see a (slightly) variable ICwhen I twiddle the potentiometer. But the other meter (the one measuring IB) hardly budges at all. In fact, if I twist the pot to increase IC, IBgoesdown. WTF?(For the 2222, I get ~10mA collector current and ~1.6mA base current, which of course cannot be corrrect.)I'm guessing I screwed up somewhere with my biasing scheme for the D.U.T. The idea was to use those 4 diodes to put the base of it at ~2.8 volts, which should assure that the transistor is in its active region (turned on).If I could indulge those who have offered helpful suggestions here (mostly looking at you, Tim Fox), could you tell me what I'm doing wrong here, without necessarily offering other, different circuit suggestions? I'm trying to learn from my mistakes here, whatever they may be.One thing, Tim, I don't like about your suggested circuit is the added complexity of a second power supply. Unless there's some easy way to derive that second voltage from one primary supply.Anyhow, have at it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2024, 02:57:37 am
content: You're saturating the DUT -- measure Vce. Base bias diodes aren't doing anything either (measure Vbe).You need an emitter resistor to set current, and that obviates the current source; or swap it around to a current sink and put that under [the emitter] making a cascode.Tim

author: Analog Kid
author_rank: Super Contributor
timestamp: October 10, 2024, 03:06:17 am
content: Quote from: T3sl4co1l on October 10, 2024, 02:57:37 amOK; is that because the emitter (voltage) needs to be elevated above ground?Is the D.U.T. in saturation because base voltage is too high?QuoteHmm, let me scratch my head a bit more and see if I can manage that.Thanks.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 10, 2024, 03:08:18 am
content: Chances are the above schematic will saturate the transistor. This is not good for measuring Beta. Beta makes sense when the transistor works in its active region, not for when the transistor is in saturation.In theory saturation is when the BC junction becomes direct polarized.In that schematic it is possible to check for saturation either by looking at the Vbc (unusual) or by measuring the Vce. You want Vce to be of a few volts, if it's less than one volt you are too close to saturation. For less than 0.7V between CE you are certainly in saturation.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 10, 2024, 04:03:35 am
content: So many questions. Let me just ask one:Let's say a guy wanted to do as T3sl4co1l suggested and put the current source below the D.U.T. emitter. (Of course, technically this would then be a current sink, not a source, but whatever). How would he do that?Copying one of the examples out of Horowitz & Hill:How would a guy specify those resistors? And could it attach to V+ (the 9 volt battery in this case)?And since I want the current to be adjustable, which of those resistors do I replace with a pot? (I'm guessing the bottom base resistor.)Target current is in the range ~5-20 mA.

author: Kurets
author_rank: Regular Contributor
timestamp: October 10, 2024, 05:03:27 am
content: Quote from: Someone on October 07, 2024, 09:57:15 pmI would add that testing at a selected emitter current allows you to see beta at your operating current. Beta is not a fixed parameter, but depends on the full operating point of the transistor. So your selected bias current, voltage, and ambient temp (actually junction temp) will all affect your result.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2024, 05:05:18 am
content: You'd typically use a diode in series with the B-GND resistor to offset Vbe, and then make that resistor adjustable. The top resistor should be much larger value, approximating a current source, to minimize tempco. (In practice, tempco might be a span of 20% over commercial temp range, which is quite reasonable for a simple basic current source.)Vbe is ~fixed (actual, logarithmic with Ic), so by setting Vb, Ve is also fixed, and a pull-down resistor defines Ie. And thus Ic, because alpha = Ic/Ie = (hFE / (hFE + 1)) ~ 1.If DUT has low hFE, you might use the CCS, but you can also just as well use it directly in this circuit.Tim

author: Analog Kid
author_rank: Super Contributor
timestamp: October 10, 2024, 05:29:38 am
content: So something like this?(resistor values are guesses)Some things you wrote I don't quite get:Quote from: T3sl4co1l on October 10, 2024, 05:05:18 amWhat does that mean exactly, "approximating a current source"? How does a resistor approximate such a thing? (And you mean "larger than the other resistor", right?)Don't mean to ding you, as you're being helpful, I know, but I read stuff like this and it sounds like hand-waving to me. (Even in that great Horowitz-Hill text I find stuff like that, where you kind of have to be a mind-reader to understand what they're getting at.)And:QuoteWhat is a "CCS"?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2024, 05:55:07 am
content: Yes, like that. Probably with more like 10k and 1k (pot).As you read more H&H, or I believe they make this point, anyway-- you can use a resistor to approximate a CCS (Constant Current Source/Sink) by simply raising the voltage on one end relative to the other end (or more particularly, relative to the range of voltage on the other end).The fact that Vb will be 0.5-2V say, while +V is 12V or more, means the current through the upper resistor simply doesn't vary much.To see this exactly, set up the node equations and solve directly. I assume you at least have the beginnings of DC/steady state circuit analysis?We use a CCS circuit when we need a wide voltage ("compliance") range over which the current remains constant; whereas we might need a supply of 100V or 1000V or whatever to do the same with a resistor, wasting far more power in the process.And yes, indeed -- there is some hand-waving involved; analog design is rarely precise, as transcendental equations show up frequently (i.e., an equation of the form x = A e^x), for example even just solving for the current and voltage in a R+D divider. We use approximations instead: Vbe or Vf is typically about 0.7V (maybe down to 0.4, 0.3V at very low currents, and high temperatures, or even lower for Ge or schottky junctions); up to maybe 0.8, 0.9V at high currents and low temperatures, or even more for large and high-speed rectifiers). This lets us use simpler linear solutions to bound the result.Tim

author: magic
author_rank: Super Contributor
timestamp: October 10, 2024, 06:52:36 am
content: Do you know how β testers in cheap DMMs work?They have an internal +3V supply rail. The transistor gets 220kΩ pullup from the base to 3V, resulting in ~10μA base current, resulting in ~2mA collector/emitter current for a typical transistor with β=200. This current flows through a 10Ω resistor and the voltage drop is measured by an ADC with sensitivity of 0.1mV per digit.Don't overcomplicate it. The most likely explanation of your problems is that you simply installed the 2N2222 in reverse (swapped C and E).

author: Analog Kid
author_rank: Super Contributor
timestamp: October 10, 2024, 07:02:45 am
content: Quote from: magic on October 10, 2024, 06:52:36 amWellll ... ackshooly, I have a bunch of PN2222s that I'm using as test subjects. And stupid me, I didn't realize that PN2222 ≠ 2N2222 (I guess the latter is an old "top hat" type?). Different pinouts. (Why the hell would they use the same part #?) So that's not the problem.I'll try to emulate that DMM feature (but with a variable current sink). Thanks for that explanation of how it works. A bit ironic that I have a DMM with that capability (an el cheapo Harbor Freight one, but works fine), except that the socket is so munged up that the beta test feature is unusable.

author: magic
author_rank: Super Contributor
timestamp: October 10, 2024, 07:12:22 am
content: Ackshooly I do think it's the problemIt's impossible for a properly operating NPN to have Ic/Ib = 1250, unless:0. none of it happened, your measurements were wrong1. it's a "superbeta" transistor, which 2222 is not supposed to be2. it's a Darlington pair, which 2222 is not supposed to be3. the base-collector junction broke down, because it really was the base-emitter junctionThat's not a long list and most points are easy to check.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 10, 2024, 07:53:35 am
content: Now we're getting somewhere. Thanks to "magic", here's the latest iteration:And it appears to work! Gives a believable result (β = 180 for a PN2222; IB= 10μA, IC= 1.8mA). I'm still playing around with this.One thing I should mention, given some recent comments: I'm intentionally ignoring temperature effects at this point. (Horowitz & Hill have an interesting current-source example with temperature compensation, maybe I'll look into that later.) And I'm not interested in developing a super-accurate piece of test equipment. On the other hand I do want something better than just a go/no-go tester, something that will give reasonably usable quantitative results. Let's call it a decent first-order test.Comments on this version welcome, keeping that in mind. Thanks to all who helped me overcome my ignorance here.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 10, 2024, 08:10:45 am
content: One obvious deficiency here: can't test PNPs. (Yep, I tried putting one in "upside down" ...)So I guess if I want a CCS here I'd need to put itabovethe PNP, rather than below the NPN, correct? Now I've got to figure out how to do that without an umpteen-pole switch ...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2024, 08:29:12 am
content: The pinout is normal -- you're thinking of P2N2222, which sadly/shockingly, existsTo test PNP, just turn everything upside down. Swap P for N, reverse diodes (anode-cathode is P-N), turn +9V to -9V, reverse meters, etc.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: October 10, 2024, 09:43:53 am
content: Quote from: Analog Kid on October 06, 2024, 03:12:00 amWrong.For one example, seehttps://entertaininghacks.wordpress.com/2024/03/16/practical-traps-with-a-one-transistor-audio-amplifier-solderless-breadboards-and-oscilloscopes/

author: tggzzz
author_rank: Super Contributor
timestamp: October 10, 2024, 09:50:21 am
content: Quote from: Smokey on October 06, 2024, 04:50:53 amAgreed, but "all models are wrong but some are useful".Having the necessary and sufficient components in an LTSpice circuit is something that requires experience. One example of insufficient components is illustrated inhttps://entertaininghacks.wordpress.com/2024/03/16/practical-traps-with-a-one-transistor-audio-amplifier-solderless-breadboards-and-oscilloscopes/

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 10, 2024, 01:13:40 pm
content: Quote from: jwet on October 06, 2024, 04:19:04 amBeta works perfectly good for me, while Ebers-moll is completely incomprehensible. It does not make sense to attempt to apply a Base-Emitter voltage to set a collector current. It's temperature dependency is very much worse then changes in beta. It's the most extreme case of thermal runaway.You can also say there are the same. Base - Emitter junction is a diode, and therefore it has a predictable relation between voltage and current. So why make it difficult to work with the exponential voltage to current in the base, and then through the lograrithmic relation to get to a collector current?In practice you will always need a way to limit base voltage and current. And because typically this is done with resistors (which have a predictable current to voltage ratio) all things are related to each other and influence each other.In any practical circuit, when a BJT has collector current, it's Base - Emitter voltage will always be around 600mV. (for silicon transistors that is).In the last few days I have been doing some measurements on BJT's I bought from Aliexpress, and I am also using a similar setup. I have two adjustable power supplies. One to set the Collector - Emitter voltage, and the other has a series resistor to the base to set the Base - Emitter current. (I'm swapping between 100 Ohm to 10kOhm depending on the base current I need)In the last few days I have tested around 10 power transistors to failure. Some exploded, others just stopped working properly. One enlightening experience is how Beta changes with different working points. These are power transistors, and once the collector current gets above approximately 2A Beta starts dropping dramatically, even though the transistors are rated for 6A. I do mount them on a heat sink, but I have not managed to get them to dissipate more then 40Watt, while according to the datasheet 65Watt should be possible. At very low collector voltages (i.e. 5V) you can't get to the 65Watt, while at higher voltages (I tried 10V, 15V and 30V) the transistors tend to pop at lower currents then expected. One factor here is the secondary breakdown.These tests have a quite big educational value for me. Simulators are no substitute for hands-on experience. And with a simulator you also can't test whether the trannies you bought from aliexpress are real or fake. And even with the tests I did I have difficulty to figure that out. I'd like to compare it with some transistors from other sources, but shipping costs for small orders are prohibitively expensive.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2024, 01:25:21 pm
content: Quote from: Doctorandus_P on October 10, 2024, 01:13:40 pmHorses for courses. If all you're doing is switches, or basic amplifiers, a linear-offset voltage model, and hFE bounded, is sufficient for most any such work.You will never solve the deeper mysteries that BJTs present us, using such tools.We can compensate much of the tempco and manufacturing error, and craft for example the OTA (operational transconductance amplifier). How pray tell, would you explain its operation, other than a model such as Ebers-Moll or more advanced?Indeed, we don't have to give up such conveniences as the offset-linear model. We can derive it from the more advanced model by taking its derivative (a fairly trivial operation on the exponential function: bring the argument down as a coefficient). This gives the familiar r_e = Vth/Ic relation.The unreliability of hFE becomes more apparent when one realizes that the I_s used in the equation, and the equivalent circuit of base and emitter, are in general different: there is internal base resistance, as well as common-emitter resistance (which contributes an Ic feedback term to Vbe), and all of these together give subtly different slopes and thus the ratio between them changes. Okay well the voltages don't affect the currents directly, but they affect the V-I relation which is the important part there.Tim

author: G0HZU
author_rank: Super Contributor
timestamp: October 10, 2024, 04:35:26 pm
content: Quote from: Analog Kid on October 10, 2024, 01:15:33 amIf it helps, you can buy new NanoVNA for about $45 delivered and this can measure hfe up into UHF.

author: TimFox
author_rank: Super Contributor
timestamp: October 10, 2024, 05:29:52 pm
content: hfe(beta) vs.gmfor simple applications, DC biasing:1: At a givenVce,Icand temperature, for a given device,Ibis given byIc/hfeto a reasonable approximation.2: Also,Vbeis given by the usual diode equation as a function ofIc, which is a weak function here, and for many purposes is almost a constant value, independent of the individual part with a given 2N number.3: However,hfevaries considerably from part to part with that 2N number at a givenIc, e.g. 100 to 300. Since beta varies greatly from part to part, the difference betweenhFEandhfeis not important.Therefore, for DC bias design, I recommend starting with desiredIcandVce, which definesVbealmost exactly, and design a bias network (choice of resistor circuits from the textbooks) that works for the manufacturer’s minimum beta spec. Then verify reasonable performance at the maximum spec.The collector current determines the base voltage in this procedure, rather than the base current or voltage determining the collector current. Of course, they are all related.By the way, my suggested circuit with two batteries, one resistor, and one microammeter is much simpler than other circuits shown above to test individual transistors.

author: TimFox
author_rank: Super Contributor
timestamp: October 10, 2024, 06:19:47 pm
content: Continuing my discussion of elementary examples, consider the voltage gain of a BJT circuit at lowish (e.g., audio) frequencies.Consider the transconductance of a BJT for small variations around a quiescent value ofIc: the transconductancegm= dIc/dVbeis approximatelyIcq/(26 mV), whereIcqis the quiescent (DC) collector current, independent of beta, at 25oC.Since your bias network determinesIcq, this parameter (along with the circuit components, load, and output impedance of the BJT) will determine the small-signal voltage gain.The not-well-determined (from part to part)hfeserves to calculate the input resistance (along with circuit components, Miller effect, and other details).Even for audio, however,hfecan fall off at higher frequencies: ifft(where beta falls to unity) = 1.0 MHz and DC beta = 100, thenhfewill start falling at around 10 kHz, for a numerical example).For a 2N3904, the manufacturer quotes a minimumftof 300 MHz, but power transistors such as a 2N3055 have a minimum of only 2.5 MHz.This discussion is for elementary purposes only: for better approximations, one can use Spice with a proper model for the transistor, to avoid explicit invocation of Ebers and Moll, letting Spice worry about them.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 10, 2024, 07:06:19 pm
content: Quote from: G0HZU on October 10, 2024, 04:35:26 pm$45? My little beta tester is going to cost me less than $5 in parts (ones I don't already have). Making your suggestion more than irrelevant. No, it doesn't help.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 10, 2024, 07:15:05 pm
content: Quote from: TimFox on October 10, 2024, 05:29:52 pmYou mean the one inthis post? Well, it is and it isn't; all those meters. Not sure how exactly you'd get it down to one meter; presumably the one measuring IC(or IE)? How would you know what IBis to make the beta calculation?Any chance you could post a simplified version with only those 4 parts? Thanks.

author: TimFox
author_rank: Super Contributor
timestamp: October 10, 2024, 08:35:59 pm
content: Quote from: Analog Kid on October 10, 2024, 07:15:05 pmIn text posts, after posting that circuit with lots of meters, I suggested a fixed resistor and a base current meter, see replies 26 and 29.Here's a diagram: in my post, I discussed the value of the resistor and if you need a collector current meter.You will change the collector current by a negligible amount when you change DUT, so you only need to measure that once.Using two batteries allows you to determine the collector-emitter voltage (technically the collector-base) and the emitter current (thereby the collector current) independently and measure the required base current.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 10, 2024, 10:36:29 pm
content: Quote from: Analog Kid on October 10, 2024, 01:15:33 amI thought it was a great suggestion.A use I had never contemplated, and it generated a wry smile for the 'out of left field' curve ball.Gives you both HFE and FT and some snazzy graphs !

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 10, 2024, 11:13:17 pm
content: Quote from: Analog Kid on October 10, 2024, 07:15:05 pmYou can measure Ib once, or calculate it. With large enough drop across the resistor, the Vbe changes become non significant.You can do this even simpler withonly one battery, andmanage either PNP or NPNwith no changes, and do it maths-free.This simple circuit give 10uA into either a PNP or NPN base, (both test cases shown for clarity) and gives mV across the loads directly as ~HFE.Having identical small droppers in collector and emitter is somewhat optional.It keeps the circuit very symmetric, so currents match, and you can probe Ie and Ic if you really want to.If you are unsure if you have NPN or PNP, check the base voltage : Just under 1V is NPN and just over 8V is PNP.Multimeters measure mV easier than they measure mA. Best to avoid mA, unless you like chasing the fine fuses.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 10, 2024, 11:28:43 pm
content: That's pretty kewl.So how do we know that IBis (almost exactly, according to your sim) 10μA? I mean without measuring it. The voltage divider puts the bases at V/2, but how does this get you that exact current? I've tried a few permutations of Ohm's Law using 710K, 9V (or 8.3V) and 10μA but none of them seem to yield that value.I sure like the direct-measurement part.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 10, 2024, 11:42:42 pm
content: Quote from: Analog Kid on October 10, 2024, 11:28:43 pmThis gets you pretty close :eg (9-0.927)/710k-0.927/710k = 10.0647uAmost of the current goes into the base, but a small portion goes into the other resistor, with about 10% of the voltage across it.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 10, 2024, 11:51:57 pm
content: OK, but where does0.927come from? I thought the B-E drop was more like 0.7 volts. Is the difference (~0.2 + change) from the bottom 710K resistor?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 11, 2024, 12:04:15 am
content: Quote from: Analog Kid on October 10, 2024, 11:51:57 pmThe difference is due to the small drop across the 100R resistor. With this HFE it is just under 300mV.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 11, 2024, 12:12:53 am
content: See, this is where my head starts to explode trying to do the math. (Not "maths"! Why plural?)OK: so what we have is 710K ohms in series with the base, and then another 100 ohms after the emitter. That's the path for the base current.We want to calculate that current.We know that I = E / R.The total voltage from top to bottom is 9 volts.Using 710,100 ohms (right? the top divider R + the emitter R), I get 12.6 μA.And isn't there some circular logic here? I mean, if we want to know the voltage drop across the 100 ohms we need to know the current, right? But ... we're trying to calculate the goddamn current in the first place!Aaaaaargh!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2024, 12:17:12 am
content: Yes, there are two branches and thus two equations, to be solved simultaneously -- they depend on each other. That nasty old highschool math has an application after all!Tim

author: Analog Kid
author_rank: Super Contributor
timestamp: October 11, 2024, 12:23:30 am
content: So what are the two equations? If it's just a matter of algebra I can handle that OK. Differential calculus, not so much.I guess what I really want to know, @PCB.Wiz, is how you got those component values. We know that 10μA isn't just some random value: it's a magic number, your target IB. So where did you plug that number in before turning the crank and out pops 710K?I went ahead and simulated that circuit (IRL on breadboard): got an IBof 11μA, not too shabby (5% resistors, had to use 2 for each 710K, which conveniently is 560K + 150K). With a PN2222 got 185mV across the emitter resistor, a believable number.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 11, 2024, 01:13:30 am
content: Quote from: Analog Kid on October 11, 2024, 12:23:30 amThat comes from experience, and a couple of iterative runs in Spice.Knowing the finer details interact a bit, we take a first guess, and then converge from there.The circuit above is very simple, but those interactions do give a slight error on very high gain transistors. ie It measures HFE's above 1000, slightly low.If that bothers a purist, you can add a more active circuit, that gives a regulated PNP and NPN base current - via a bidirectional current limiter.We still want this automatic for PNP or NPN parts.This time there is more interaction, so we converge on some more magic numbers

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 11, 2024, 01:22:11 am
content: Quote from: Analog Kid on October 11, 2024, 12:12:53 amOutside the USA, speakers of the original English use Maths as the shortened version ofmathematicsQuote from: Analog Kid on October 11, 2024, 12:12:53 amYou've slipped up a little pulling in the 100 ohms - that does not carry the base current, it carries the emitter current.You also cannot forget the small error from the 'redundant' 710k - the one that is not doing anything useful for NPN or PNP cases, but is there to make the circuit auto-selecting.Spice let's you be lazy with the finer calculations.

author: TimFox
author_rank: Super Contributor
timestamp: October 11, 2024, 01:25:52 am
content: How is this easier than my two batteries, one resistor, and one microammeter (DMM on 200 uA full scale) circuit?Even the arithmetic is simple.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 11, 2024, 01:34:28 am
content: Well, since you asked: only one battery (I really don't like multiple supplies), more Rs, yes, but measuring V instead of A, and--best of all!--direct reading! No arithmetic!Although I still like my adjustable current rig. For now.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 11, 2024, 01:37:51 am
content: Quote from: TimFox on October 11, 2024, 01:25:52 amI'm too lazy to find and connect and house a second battery (ugh), and too lazy to do any maths. I'm also never sure which of my meters have 200uA working....

author: Analog Kid
author_rank: Super Contributor
timestamp: October 11, 2024, 01:38:24 am
content: Quote from: PCB.Wiz on October 11, 2024, 01:22:11 amButbutbut ... and this is a nitpick, but the 100 ohmsdoescarry the base current (in the NPN case). Plus the emitter current. Right? Where else is the base current going to go to get to ground?

author: TimFox
author_rank: Super Contributor
timestamp: October 11, 2024, 01:43:25 am
content: Incidentally, although “maths” is the normal abbreviation for “mathematics” in some countries, the noun “mathematics” is always singular and takes a singular verb, as in “mathematics is difficult”.I am not aware of “econs” as an abbreviation for “economics”, but “statistics” is a plural noun.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 11, 2024, 01:46:55 am
content: Quote from: Analog Kid on October 11, 2024, 01:38:24 amWell, yes, but the emitter currentisthe sum of collector and base currents, with the base being some small fraction of %.You can 'reflect' the emitter resistor back to be base side, by doing this multiply of the 100 ohms by HFE(9-0.65)/(710k+(100*298)) = 11.286uAThat's closer, but it has not included the second 710k current subtraction path yet, which is why it is still a tad high.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2024, 02:05:26 am
content: Quote from: Analog Kid on October 11, 2024, 12:23:30 amWell, write them out:Label each node in the circuit. The sum of currents into each node must equal zero. Consider the components connecting to each node: a resistor for example might connect from node 1 to node 2, and flows a current proportional to its voltage drop: \$I_{R_1} = \frac{V_1 - V_2}{R_1}\$.We need a model for the BJT, but assuming it's "on", using a basic (e.g. offset-linear) model for Vbe, and assigning the terminal currents appropriately (Ie = Ic + Ib), connects the C and E loops to the B node and we can solve the system.Note that a voltage source creates a supernode, i.e. it defines the voltage exactly between two given nodes. These nodes are trivially substituted by this offset, and that voltage drop is not affected by any other variables in the circuit so we eliminate that equation from the system (or, equivalently: the equation involving the voltage source is trivial).We also define one node as reference, "ground". All voltages are understood as with respect to that node, so we don't have to forever write differences of voltages, we can just call out a single variable as a voltage and let that be that.Putting all these relations into a pile and solving for results, we get voltages and currents in the circuit. This might be formalized by arranging the equations in the form,\begin{eqnarray}V_1 \left( \frac{1}{R_1} + \frac{1}{R_3} + \ldots \right) &+& V_2 \left( \ldots \right) &+& V_3 \left( \ldots \right) + \ldots &=& 0 \\V_1 \left( \frac{1}{R_2} + \ldots \right) &+& V_2 \left( \ldots \right) &+& V_3 \left( \ldots \right) + \ldots &=& 0 \\\ldots\end{eqnarray}the coefficients for the node voltages, can be packed into a matrix, and then we have a linear algebraic equation of the form \$\mathbf{A}\vec{x} = \vec{b}\$ which can be solved by inverting matrix \$\mathbf{A}\$ and getting the vector of solutions \$\vec{x}\$ (where \$\vec{b}\$ might be an array of independent current sources, or equivalent description of supernodes as needed). Linear algebra is just a way to manipulate arrays of equations all at once, which happen to be linear as the name suggests, and which follow an algebra.Further reading is found under DC / steady-state circuit analysis, nodal and loop analysis, Kirchoff's laws, etc.Tim

author: TimFox
author_rank: Super Contributor
timestamp: October 11, 2024, 02:16:19 am
content: Once again, the equations are simpler with the two-battery circuit, assuming you know the battery voltages:Ie= [Vneg- (0.7 V) ] /RMeasureIb.Ic=Ie-Ibor measure it with the optional collector meter.Althoughhfeis not really constant, it is usually given byIc/Ib, at the collector-emitter voltage (Vpos+ 0.7 V ); use a calculator or sliderule if you must, but all you need do for a test is to calculate (once) the maximumIbyou can accept.The inaccuracy of the approximationVb= 0.7 V is negligible compared with the assumptions abouthfe.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 11, 2024, 05:48:24 am
content: @PCB.Wiz: I made a little improvement to your ultra-simple beta tester:The pot allows you to dial in 10μA exactly (at least on my setup: might have to reduce the 710K otherwise). Then you can just jumper that connection and test away, maybe recalibrating occasionally.I think I'll build this into a li'l case (Altoids-type tin is my fave) for my V.1 project.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 11, 2024, 07:38:41 am
content: Quote from: Analog Kid on October 11, 2024, 05:48:24 amThat's ok, but because this can also work with PNP transistors, you might want to add a similar pot on the lower 710k for PNP trim.

author: TimFox
author_rank: Super Contributor
timestamp: October 11, 2024, 01:53:40 pm
content: If you are trying to evaluate measured beta with respect to the data sheet, note that beta is specified at given values ofIcandVce.Never specified at given values ofIb.https://www.onsemi.com/download/data-sheet/pdf/2n3903-d.pdf

author: TimFox
author_rank: Super Contributor
timestamp: October 11, 2024, 04:16:41 pm
content: OK, here's my next suggestion.This adds complications to my simple circuit to allow a single power supply, to setVceandIcaccurately to compare with the data sheet, and to test multiple units of the same part number after setup.Yes, you will need to do simple arithmetic: there is algebra in the real world, and it's all story problems.As drawn, the resistor values will setVce= 3.0 V andIeto 5 mA.You can change R5 to change the current and R2 to changeVce.For tweaking voltages, you can use trimpotsin series with suitable fixed resistors at R2 and R4, but that is more than required for a simple beta test.With those values of PS1 and R1, the maximum current available is about 10 mA.Note that the upper shunt regulator is the low-voltage version of the TL431, since the normal TL431 is only good down to 2.5 V.The normal tolerance on the regulator voltages and 1% resistors should be good enough for the test.Modification for PNP devices is left as an exercise for the reader.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 11, 2024, 06:49:07 pm
content: Quote from: TimFox on October 11, 2024, 01:53:40 pmFor some values of 'never'The table values usually choose some Vce,Ic, but HFE follows a well behaved curve and the graphs often looks like this : Voila,Ib specified.

author: TimFox
author_rank: Super Contributor
timestamp: October 11, 2024, 06:56:11 pm
content: Yes, but that family of curves varies dramatically from unit to unit for a given part number.At least 3 to 1 for common transistors.That graph is “typical”, not a spec.Again, if you force base current on a given device, you will obtain different collector current for different units.When designing an actual amplifier, you normally start from the collector current.(Don’t mind me, I have done such things since 1968.)

author: Analog Kid
author_rank: Super Contributor
timestamp: October 11, 2024, 07:31:11 pm
content: Quote from: PCB.Wiz on October 11, 2024, 07:38:41 amNo need. Here's your circuit, all packaged up to make a real-world beta tester for both NPN & PNP:Calling this V1.0. The only slight weirdness is that in PNP mode the HFEvoltage will be negative, but no biggie, just ignore the sign. (BTW, I need to check whether I'm using the right symbol there; hfe? Hfe? HFE? hFE? I know they're different, some being small-signal, others being DC ...)Now to look at @TimFox's latest. Tim, any chance that your power supply could be changed to 9 volts? 15 volts is just not convenient, at least not here at AnalogKid Laboratories, GmbH.

author: TimFox
author_rank: Super Contributor
timestamp: October 11, 2024, 07:40:42 pm
content: If you reduce the single supply to 9 V, then you should change the U2 regulator to 4.7 or 5.7 V, and change the emitter resistor R5 accordingly (for 4 or 5 V). You need to keep the total power supply current high enough for your max device current plus at least 2.5 mA for the shunt regulator string. This is determined by the power supply, the total regulator voltage, and the series resistor R1.With the lower U2 voltage, there is some loss of accuracy, but no big deal.You also need U1 voltage high enough to set the collector voltage.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 11, 2024, 08:26:35 pm
content: In this thread there's been a lot of attention paid to using emitter current (IE) instead of ICas a way of measuring beta.I'm having trouble wrapping my head around what seems to me like something of a distinction without a difference. Here's my reasoning:One can visualize how a transistor works using a plumbing metaphor:(the thing in the middle is a valve)Seems to me that, to a first approximation, this should be accurate enough for the purposes of measuring beta (IC/IB). As the "pipe" goes straight through, IC≈ IE(more precisely, IE= IC+ IB).In fact, the esteemed Horowitz & Hill even use a similar metaphor in their book ("Transistor Man"):So why can't we just use IC/IBas our beta value? To be clear, not as the end-all and be-all accurate measured value such as one would get with a professional analyzer.

author: G0HZU
author_rank: Super Contributor
timestamp: October 11, 2024, 08:28:57 pm
content: Quote from: Analog Kid on October 10, 2024, 07:53:35 amYour circuit (shown below) doesn't look good to me as drawn as the operating point will vary from device to device depending on the base current.Also, although it appears to be a dc circuit, it will almost certainly be a negative resistance generator up at RF frequencies. So, depending on layout it could go unstable up at VHF for example. One way to minimise the risk is to put some series resistance as close as possible to the base leg of the DUT. This is known as a base stopper resistor. You could also swap the order of the ammeter and the 220k resistor to put the resistance at the base leg and achieve the same thing. However, I would recommend a physically small resistor of maybe 10k at the base as it will have a very low parasitic capacitance.Note that a 2N3904 biased at a few mA can go unstable and self oscillate up as high in frequency as about 1GHz although I think your circuit is more likely to go unstable up at a couple of hundred MHz or so. A lot depends on the wiring and the size and layout of the major components.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 11, 2024, 08:36:10 pm
content: I'm having a really hard time imagining any kind of RF oscillations occurring with such a small, simple circuit, assembled reasonably well on a board with short leads, soldered together. How likely, actually, is this to occur? Where would the feedback path be? How possible is this with devices with typical FTs of <300MHz?But just to humor those who caution against this, I wonder if it would possible to pick up such oscillations with a scope. I have an older Tektronix (475): would this be able to pick up any such oscillations, with the sensitivity turned up?

author: G0HZU
author_rank: Super Contributor
timestamp: October 11, 2024, 08:46:09 pm
content: If you look closely at your circuit, the DUT BJT resembles an emitter follower circuit. If you attach an emitter load circuit that has a medium to high resistance and small amount of capacitance to ground (your other BJT current source and the emitter ammeter provides this) then the necessary feedback path involves this capacitance and the internal base-emitter capacitance of the DUT (emitter follower) BJT.If you draw it out with these capacitance added to your circuit, you will see the feedback circuit of a typical Colpitts oscillator. This will generate negative resistance over a huge frequency range at the base pin of the BJT and this can extend up into UHF for something like a 2N3904.If your ammeter at the base connection has a reasonable length of wire and it also provides some shunt capacitance to ground then this can complete the oscillator circuit.In reality, I think you be very unfortunate if it actually did oscillate because it would require a certain wire length and also some shunt capacitance to ground. The risk should be negated if you put a stopper resistor at the base of the DUT

author: G0HZU
author_rank: Super Contributor
timestamp: October 11, 2024, 08:53:41 pm
content: QuoteThe Ft frequency is the unity current gain frequency and this can be in the region of 300MHz for a 2N3904 at 10mA Ic. However, the 2N3904 can still provide power gain up to about 1GHz if biased at (say) 10mA. So in theory at least, it can oscillate up as high as about 1GHz.It may depend on the manufacturer though. I'm referring to a genuine 2N3904 made by OnSemi for example and not something bought on ebay or ali-express.

author: G0HZU
author_rank: Super Contributor
timestamp: October 11, 2024, 09:02:20 pm
content: Many years a go I made a beta tester for BJTs and I used an opamp as an error amplifier to define the collector current at several (switch selected) currents. IIRC this let me check beta at 0.5mA, 1mA, 2mA, 5mA and 10mA Ic. It had a switched sense resistor at the collector and the opamp (error amplifier) provided the base current drive to hold the collector current at exactly 1mA for example. It was a feedback circuit in other words.I've tried to find it but I can't. it only worked for NPN devices anyway. It included an accurate 5V regulator and it was probably able to define the collector current to within about 1% and I used a H/H DMM to measure the base current. I mainly used it to match BJTs at a chosen collector current.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 11, 2024, 09:09:33 pm
content: So you're saying my circuit might be susceptible to RF oscillations because it's an emitter follower? I thought one of the requirements of an oscillator is that it has a gain > 1.

author: TimFox
author_rank: Super Contributor
timestamp: October 11, 2024, 09:11:29 pm
content: MeasuringIcorIe: No practical difference if you also measureIb, since the emitter current is the sum of base and collector currents. Beta, to the extent that it is meaningful, is usually defined as the ratio of collector current to base current. I’m sure that if you re-read my comments above you will find mention of that and the accuracy thereof.Since the emitter presents a low resistance to the circuit, it is easy to force a current into it.Emitter-follower oscillation: one of my undergraduate professor’s comments became a catch-phrase: “An emitter follower, what can go wrong?”. G0HZU’s comments above are very true for BJT, FET, and vacuum-tube followers. Sometimes, this is exploited to make an LC oscillator.A small stopper resistor at base and another at the collector are a good idea. The 475 should see oscillations somewhat above 200 MHz, probably good enough forft< 300 MHz.Old rules of thumb: an amplifier will oscillate unless forced not to, but self-starting oscillators won’t.I originally used my two-supply circuit to measure grid current in triode tubes. Grid current was measured on a sensitive Keithley “electrometer” with coaxial input, forcing the grid voltage to ground. My “Institute for Radical Macroelectronics” has several precision voltage supplies and a Keithley precision DC current source.At low cathode current, the cathode is quite positive with respect to the grounded grid, so a relatively large resistor from current source to cathode is required to keep the output voltage of the current source negative (one-quadrant only).To avoid oscillation, I bypassed each pin of the tube socket directly to the ground plane of the test jig with an appropriate disc ceramic capacitor.To measure the grid-cathode voltage, I connected a very high input resistance (100 megohm) from cathode to ground.RF pickup usually doesn’t screw up a DC measurement, but with oscillation the DC (mean) values can change drastically.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 11, 2024, 09:12:25 pm
content: Quote from: G0HZU on October 11, 2024, 09:02:20 pmSounds nice. A bit overkill for my purposes, but I'm sure there are others here who might appreciate it if you could find that circuit (or just approximate it).

author: Analog Kid
author_rank: Super Contributor
timestamp: October 11, 2024, 09:17:21 pm
content: Quote from: TimFox on October 11, 2024, 09:11:29 pmAh, tubes; I loves 'em. It's a bit late for me now, but if I had it to do over again I'd devote myself to studying, experimenting and playing around with them.

author: G0HZU
author_rank: Super Contributor
timestamp: October 11, 2024, 09:20:40 pm
content: Quote from: Analog Kid on October 11, 2024, 09:09:33 pmProbably the best way to answer this is to alert your attention to a classic Colpitts oscillator. It is often based on an emitter follower. There will be a power gain of >1 when starting up and this is due to the external feedback capacitance. This generates negative resistance at the base. If you then add a resonator at the base, the circuit will oscillate if the negative resistance generated by the follower (at its base) can offset the passive loss resistance of the resonator.Google for a Colpitts crystal oscillator circuit and you will see plenty of examples that are based on an emitter followerIn your case, the feedback capacitances are hidden from your emitter follower circuit. But they really are there, and they are the internal base-emitter capacitance and the shunt capacitance of the ammeter and the BJT current source at the emitter of the DUT BJT. There should be enough capacitance in each case to generate negative resistance right up into UHF.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 11, 2024, 09:46:49 pm
content: Quote from: Analog Kid on October 11, 2024, 07:31:11 pm? You've added aDPSTDPDT battery reversal switch where none was actually needed before - just plug in a PNP transistor.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 11, 2024, 09:53:03 pm
content: Quote from: TimFox on October 11, 2024, 06:56:11 pmErr, yes, that's precisely the point of a HFE tester. They change in the Y-axis, they do not change much in the X-axis.Notice how flat those curves are, it does not really matter if you test at 1.5mA or 3mA, temperature will have a far larger effect than moving left or right on that curve.The OP just wants to check for 'dud's so simplicity is important, and another common use is HFE matching, or HFE binning.

author: G0HZU
author_rank: Super Contributor
timestamp: October 11, 2024, 09:57:50 pm
content: Quote from: Analog Kid on October 11, 2024, 09:12:25 pmThe component count was quite low, not much more than some of your circuits. It was really just a lashup circuit to let me match some NPN BJTs that were used in the repair of a metering circuit inside an old piece of HP test gear. So it was really ugly and not boxed. It only took a few minutes to make it. I soldered in the current defining sense resistors rather than switched them.I could have a go at making it again, but it will look just as ugly as last time. I don't think I have a suitable opamp here though. It ideally needs to be a rail to rail device.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 11, 2024, 10:01:33 pm
content: Quote from: PCB.Wiz on October 11, 2024, 09:46:49 pmDPDT. Except that without that, the adjustable leg of the circuit will be at the wrong end and won't work for the alternate (PNP) polarity. I know, I tried that.You're right, though: it'd work that way without the pot (the way you originally posted it), since it's symmetrical.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 11, 2024, 10:06:55 pm
content: Quote from: TimFox on October 11, 2024, 06:56:11 pmButbutbut ... isn't that the definition of beta (IC/IB)? Which after all is a parameter unique to each device, not a spec?

author: TimFox
author_rank: Super Contributor
timestamp: October 11, 2024, 10:13:06 pm
content: Each example of a 2N3904 will have a different beta value at a given operating point (IcandVce).The manufacturer specifies the allowed range of this beta value at one or more operating points.That’s a spec.Again, it is unwise to use beta directly to define the performance of a transistor amplifier.Instead, determine what range of beta you can tolerate for your amplifier.(Beta is not so good a parameter as is the resistance of a metal-film resistor.)

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2024, 10:25:42 pm
content: Quote from: TimFox on October 11, 2024, 10:13:06 pmThat is doubly true because higher beta devices, for instance especially superbeta transistors, have worse Early voltage making them worse amplifiers.I must remind you that high-beta transistors (300 and up) still have disadvantages in terms of voltage gain or mu. When the beta gets too high, and because mu is inversely proportional to beta, the voltage gain is hurt. I remember a test that asked how much voltage gain a particular amplifier design has. The answer was supposed to be 20,000. But the gain was really 9000, as the betas were too high and the Early Effect was too strong. I passed the test after I explained my solution.https://www.electronicdesign.com/technologies/power/article/21759455/whats-all-this-vbe-stuff-anyhow-part-2

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 11, 2024, 10:31:49 pm
content: Quote from: Analog Kid on October 11, 2024, 10:01:33 pmIf youreallywant to trim the base current to a precise 10uA, just add a second trim pot for PNP. Less wiring and bulk than a DPDT switch.Given the existing variations of HFE with temperature and across devices, I'm not sure chasing fractions of a percent on base current is worthwhile, other than as education.Some vendors sell transistors HFEmatchedwithin 5% or 2%, but they do not commit toabsoluteHFE other than a broad bin of roughly 2:1 range.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 11, 2024, 10:35:19 pm
content: Quote from: PCB.Wiz on October 11, 2024, 10:31:49 pmSure, pretty much 6 of one, half-a-dozen of the other. Cheap and easy to add in either case.QuoteMainly satisfies my residual knob-twiddling impulses ...

author: TimFox
author_rank: Super Contributor
timestamp: October 11, 2024, 10:49:09 pm
content: Testing with forced base current or forced emitter current:Before retiring, I often had to write formal test specifications.I submit that for testing a transistor for use in an amplifier at 5 mA current, the test (1) below is not as relevant as is test (2) below.(1). Force 30 uA current into the base and measure collector current. Acceptable values are between 1.5 mA and 15 mA.(2). Force 5 mA current into the emitter and measure base current. Acceptable values are between 10 uA and 100 uA.Both cases correspond to beta between 50 and 500, a bit wider than normal to see the difference.Of course, the collector voltage and possibly the temperature need to be specified for both test cases.

author: G0HZU
author_rank: Super Contributor
timestamp: October 11, 2024, 11:45:59 pm
content: Quote from: Analog Kid on October 11, 2024, 09:09:33 pmI had a go at putting together a mockup of your test circuit on a sheet of bare copper PCB. I don't have an inline ammeter to put in the base connection so I just used a piece of blue wire to mimic the ammeter. This wire provides inductance and distributed capacitance along its length.If you look at the image below, I've marked up the DUT BJT and the other BJT is the current source and I've set it to 2mA. The only thing this current source is guilty of is that it provides a high resistance and a small shunt capacitance. This is enough to let the DUT BJT generate negative resistance at the base.The resistor between the two BJTs is there to mimic the ESR of the other ammeter.You can see on an old Agilent spectrum analyser that it oscillates at just under 300MHz. I had to move the wire around to find a position where it would oscillate. I have a lot of experience with RF circuits so I can just look at your original circuit and know it will be potentially unstable when built up.It isn't a neatly built circuit, but the connections are similar to those that would involve adding an ammeter inline with some wires.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 12, 2024, 12:02:53 am
content: Interesting.Now I want to build something like that andtryto make it oscillate.Heh; like @TimFox said up there:QuoteOf course, -32.69 dBm: not a whole lot of juice there, right?Oh-oh, I see a white van outside that says "FCC" on the side ...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2024, 12:18:55 am
content: Quote from: Analog Kid on October 11, 2024, 08:36:10 pmThe hazard is that wires are not ideal connections between points, but transmission lines upon which waves travel.A 2N3904 at the end of a meter-long cable, maybe with a bypass cap on the near end, is likely to oscillate with the wire as a 1/4 wave resonator -- one meter resonates at 50-60MHz, or maybe 1/2 wave (100-130MHz).See this "gimmick" here:https://www.eevblog.com/forum/projects/challenge-thread-the-fastest-breadboard-oscillator-on-the-mudball/msg3102355/#msg3102355(A "gimmick" is a piece of seemingly-nonfunctional wire or cable, added to a circuit, say to contribute capacitance, or extend a transmission line. Whereas strays are usually parasitic, the use is intentional instead. In this case, B-C have an extended twisted pair, as well as the contact strips in the breadboard itself.)This isn't normally a problem, but sometimes one encounters issues, for example, power amplifiers with BJTs on too-long leads, and the inductance of the B and C wires, resonating with Ccb, looks like a Hartley oscillator or something like that. Indeed this can be a repair issue, as original slow-ass hometaxial 2N3055s haven't been made since the 80s (late 70s even? not sure) and the epitaxial replacements that fill that part number since, perform much better (fT > 1MHz).The safe solution is to put a "base stopper" resistor very close to the transistor, so that the stub length to the base is small, and resistance high -- killing gain at high frequencies. Or use a ferrite bead on B or E (same idea, provides AC resistance).Tim

author: G0HZU
author_rank: Super Contributor
timestamp: October 12, 2024, 12:32:13 am
content: Quote from: Analog Kid on October 12, 2024, 12:02:53 amA 2N2369A BJT would also oscillate up at VHF with that test circuit. Probably more readily than the 2N3904. Note that I think that circuit would be quite wild if you tried to test an MPSH10 BJT, even with much shorter connections.If you want to see a 2N3904 oscillate up at UHF, I can show you some easy to build 2N3904 circuits that will reliably oscillate up at >600MHz and it is possible to get oscillations up to about 1GHz. However, this will have to wait until tomorrow as it's late here.

author: G0HZU
author_rank: Super Contributor
timestamp: October 12, 2024, 12:42:44 am
content: QuoteThat's just stray leakage power picked up with a nearby H field probe. The probe is quite deaf and it is the black circular device near the blue wire in the image. The oscillator is probably generating about 1mW at 295MHz.

author: TimFox
author_rank: Super Contributor
timestamp: October 12, 2024, 12:57:38 am
content: All three triode devices ( BJT, FET, and vacuum triode) connected as followers, due to their internal terminal to terminal capacitances, are capable of exhibiting negative conductance at their input.

author: David Hess
author_rank: Super Contributor
timestamp: October 12, 2024, 09:38:52 am
content: Quote from: TimFox on October 12, 2024, 12:57:38 amHere is an article which I have posted before which describes what is going on to produce the oscillation and how to prevent it.Curve tracer fixtures usually include lossy ferrite beads on all three terminals at the transistor connection.

author: tggzzz
author_rank: Super Contributor
timestamp: October 12, 2024, 11:32:06 am
content: Quote from: Analog Kid on October 11, 2024, 09:09:33 pmSigh, let's tryagain.I'vepreviouslygiven you this URL, but you seem to have avoided reading/understanding it.https://entertaininghacks.wordpress.com/2024/03/16/practical-traps-with-a-one-transistor-audio-amplifier-solderless-breadboards-and-oscilloscopes/

author: Analog Kid
author_rank: Super Contributor
timestamp: October 12, 2024, 11:13:07 pm
content: 1. Almost all of what was written there concerns the pitfalls of using solderless breadboards, which are well known (even I know about them: stray capacitance, contact resistance, etc.).2. I'm not going to stop using solderless breadboards; they're just too useful.3. When I actually build a circuit (soldered and all) I take care in component placement, lead length, bypassing, etc.And as someone else pointed out here, even if one of my cobbled-together experiments did result in some RF oscillation, it probably wasn't strong enough to significantly affect the results I was getting.

author: TimFox
author_rank: Super Contributor
timestamp: October 12, 2024, 11:36:10 pm
content: What I said is that a bit of RFpickupshould not interfere with DC measurements, but RFoscillationcan shift the DC values dramatically.(This is not from a book: it is from experience.)

author: JoeyG
author_rank: Regular Contributor
timestamp: October 13, 2024, 10:52:40 am
content: Here's a voltage to current source in to R2_load simulated in LTspiceIt's taken from Wireless World MagazineYou could replace R2_load and insert in to the base of the transistor under test.LTspice Simulation attached.

author: tggzzz
author_rank: Super Contributor
timestamp: October 13, 2024, 11:05:40 am
content: Let's re-insert the context you snipped...Quote from: Analog Kid on October 11, 2024, 09:09:33 pmI responded with a URLdirectly addressingthatspecificsituation.Now you writeQuote from: Analog Kid on October 12, 2024, 11:13:07 pmThat clearly indicates that while you have readpartof the post, you haven't read all of it and/or you haven't understood it. The post indicates why your presumptions are incorrect, some visible and invisible consequences, and how to ameliorate the issue. Plus pointers for further understanding.Your point "3" is no more than what you would like to be the case. Physics takes no account of your desires. That's one reason I like physics (and chemistry)Your last paragraph is naive. Before making such assertions, you have to test and verify. Sorry about that.TL;DR: the electromagnetic fields don't care how you implement a circuit - they just do their thing. You have to adapt your understanding to fit in with that. section "The Problem’s Cause"

author: tggzzz
author_rank: Super Contributor
timestamp: October 13, 2024, 11:08:31 am
content: Quote from: TimFox on October 12, 2024, 11:36:10 pmIt is also in the booksSimpler books just note the "magical practice", but better ones show the theory that explain the practice.Butyouknow that

author: watchmaker
author_rank: Unknown
timestamp: October 13, 2024, 12:51:00 pm
content: As a fellow traveler, I applaud Analogkids effort to understand how transistors work.I agree it is essential to understand the physics that determine how devices behave; but experimenting as he has done, and with his determination, he is uncovering some of that. Hopefully it leads him to even more research. For example, not initially understanding RF oscillations. Analogkid: have you completed a syllabus of EE101 that covers EMF (inductance of leads), nodal analysis, semiconductor physics, etc? There are some excellent texts with available lab manuals that provide proven structure for learning.I think Analogkid is capable of finding his way down these rabbit holes "with a little help from his friends".This whole area is a puzzle that requires tenacity and a pinch of masochism. Especially for an independent learner. In forty years of internet groups, this is the most generous and authoritative. It is now the only one in which I participate.I have learned that fully qualified engineers who mastered the theory still have to make adjustments to designs based on empirical results. This is not due to "magic" but due to the inability to account on paper for all the interactions.Here are some youtube creators I have found useful for understanding the basic physics of the PN junction generally and transistors specifically.w2aew]https://www.youtube.com/@w2aewEEVblog]https://www.youtube.com/@EEVblog

author: tggzzz
author_rank: Super Contributor
timestamp: October 13, 2024, 01:11:27 pm
content: Quote from: watchmaker on October 13, 2024, 12:51:00 pmI wholeheartedly agree.Another necessary attribute is being humble, because there isalwaysmore to learn. FortunatelyQuoteSci.electronics.design had some extremely qualified participants and extremely subtle discussions. Contributors like Winfield Hill, John Larkin, Phil Hobbs, Bill Slowman were top notch; there are few as competent here. The cretinous trolls that materialised could be easily plonked using newsreader's "ignore subthreads by" filter. S.e.d. was also notable for taking no prisoners; if someone thought you were incorrect, they would tell you in no uncertain terms.That kitchen was hot, and the heat rapidly shrunk violetsQuoteOnce the interactions are accounted for and added back into the theoretical model, the results match much better. Knowing which interactions are significant/unimportant requires skill and experience.Even so, experienced people still get caught out; one example is the oscillating emitter follower, as outlined in TAoE x-chapters.

author: G0HZU
author_rank: Super Contributor
timestamp: October 13, 2024, 03:52:54 pm
content: Whoops, I didn't really intent the emitter follower stuff to cause this much discussion. All that was really needed was a suitable stopper resistor at the base.However, I think it might now be worthwhile to show that the 2N3904 (as an emitter follower) will generate negative resistance at the base even with a tight layout. You don't need a sloppy breadboard layout to still have problems. See below for a simulation and a measurement of a real 2N3904 and this shows negative resistance is generated up to about 600MHz in this case. I chose to bias it at 10Vce and 7mA Ic as this will make it more prone to generating negative resistance up into UHF.You can see that the s2p based model and the real circuit gave very similar results in terms of negative resistance. I measured it using a VNA and then added a resonator at the base (made from a piece of straight wire and a low loss dc blocking capacitor).The PCB layout is quite tight with short connections but when I add the resonator you can see that it oscillates at just under 600MHz. If I shorten the resonator, the oscillation frequency goes up but once it goes above about 600MHz the oscillation stops. This is because there isn't any negative resistance above about 600MHz.If I change the circuit around the 2N3904 (away from an emitter follower) and increase the bias point, the 2N3904 can generate negative resistance up to about 1GHz. So it is possible to make it oscillate at just over 1GHz, but this is probably the practical upper limit for the 2N3904.

author: TimFox
author_rank: Super Contributor
timestamp: October 13, 2024, 05:01:07 pm
content: In general, there are two approaches to analyzing an oscillator (or potential oscillator): negative conductance across the resonator, or feedback through the resonant circuit.In G0HZU’s example of a Colpitts oscillator, the two approaches agree and it’s the designer’s choice as to which is more convenient.I fully agree with the use of series “stopper” resistors. For a BJT, a belt-and-suspenders approach is to add one in series with the base and another (smaller) one in series with the collector, both as close to the transistor as practicable. Their values should be small at the operating frequency and DC bias.

author: Analog Kid
author_rank: Super Contributor
timestamp: October 20, 2024, 03:14:13 am
content: Sorry to divert the present conversation about parasitic oscillation and all (but of course we can always return to that).I just wanted to present my finished project here. What I've done is basically just package the simple circuit that PCB.Wiz cooked up. it's built, working and useful. Used one of my favorite form factors, the throat-lozenge tin (like Altoids, which actually have a reputation for this in the DIY electronics world).Here's the final circuit:Funny how things change: I had gotten it working perfectly on a solderless breadboard, but after building it I found I couldn't quite get IBup to 10μA with the trimmer all the way over; I had to add a 56K resistor across the 150K (one of the pair making up the top 710K). Probly just fell into a 5% tolerance hole or something. Anyhow, it works great.Next project!

author: Analog Kid
author_rank: Super Contributor
timestamp: December 18, 2024, 02:03:04 am
content: So AnalogKid Laboratories, GmbH is pleased to announce a new version of our beta tester. Ta da ...Certainly no lack of respect to@PCB.Wiz, whose contributions here led to my first design (see above).But the thing I didn't like about it was that I had to constantly adjust that trimmer to get exactly 10μA of base current. Kind of a pain in the ass.Anyhow, long story short: I decided, since I had a bunch of LM334s laying around, why not use them? After all, that's they live for, supplying constant current. So I cobbled up a beta tester similar to the one I already had using them.It took me a while to solve the puzzle of how to supply both NPN and PNP base current without some complicated switching scheme. Turned out to be simpler than I imagined; I ended up using 2 334s, one for NPN, the other for PNP.It looks funny, with the two sources (actually one source and one sink) connected together like that, but they work fine, and they're completely independent of each other, depending on the switch setting and the DUT being tested.Anyhoo, here's the schematic:I'm about to complete this by putting it into a case (a throat-lozenge tin, natch). Will post pictures when that happens.It works fine; agrees (within 5-10%) with my DMM hFEfunction. Close enough for my purposes.I had to fiddle around a lot with resistors to get 10μA for both transistor polarities, but the current is rock-solid.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 18, 2024, 02:15:18 am
content: Quote from: Analog Kid on December 18, 2024, 02:03:04 amA current regulator also makes this less supply dependent.You can do it with a single current regulator, by just switching the current path, if you do not mind moving the meter.It can be useful to also bring out the BASE terminal, to allow you to also measure VBE, as often that is of more interest when matching.That also allow you to measure diodes, LEDs and Zeners at 10uA test ( 50uA zeners are becoming more common these days)

author: Analog Kid
author_rank: Super Contributor
timestamp: December 18, 2024, 02:21:00 am
content: Quote from: PCB.Wiz on December 18, 2024, 02:15:18 amYes, but I was trying to avoid that.I figured out that I could eliminate the switch by using a different socket for PNP & NPN transistors, but I'd still need a switch, since the current-sense resistor needs to go to the collector (I'm measuring ICinstead of IE). So this way worked the best.QuoteI'm going to be building a more complex test rig that does measure VBEas well, as part of my continuing exploration into the fascinating world of the transistor.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 18, 2024, 02:29:15 am
content: I see the LM334 has a slight temperature variance.TI give a circuit that reduces that, at the cost of 1 diode drop added.

author: Analog Kid
author_rank: Super Contributor
timestamp: December 18, 2024, 02:42:12 am
content: You must mean this, yes? (from Nat'l Semicircular*)*Someone I knew who worked at National used to call it that.

author: TimFox
author_rank: Super Contributor
timestamp: December 18, 2024, 03:45:28 am
content: Note that that ap note uses a 1N457 diode, which is quite different from a more popular 1N4148.Pease had a discussion somewhere about the choice.

author: David Hess
author_rank: Super Contributor
timestamp: December 19, 2024, 12:28:55 am
content: Quote from: TimFox on December 18, 2024, 03:45:28 amThe significance of the 1N457 is that it is not gold doped, unlike the 1N4148. A high conductance low leakage diode is called for here, with modern examples being the 1N458 (still available) and FDH300ATR. A small signal transistor base-emitter or base-collector junction would also work well.