How to REALLY measure hFE of NPN transistor? - Page 1
Forum section: electronics / beginners

author: dusan
author_rank: Regular Contributor
timestamp: June 08, 2023, 06:43:27 pm
content: So I googled how to measure hFE using two ammeters and this thread came up:https://www.eevblog.com/forum/testgear/measuring-hfe-of-a-transistor-by-multimeter/- OK, seems simple enough:hFE=154, seams reasonable for BC547. Then I got briliant idea to replace the transistor with short and the result:hFE=144, that's really good for piece of wire. Before I head to the patent office, are there any better methods at measuring hFE?

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 08, 2023, 07:21:52 pm
content: You can invalidate the 2nd scenario with an additional step in your test:Remove A2 (to break circuit) and ensure current reading on A1 drops to near zero. If A1 current stays where it was, this means a faulty transistor, and that the previous hFE reading was invalid.Note: You don't need R3 for these tests.If you want more detailed hFE measurements use a curve tracer.

author: wasedadoc
author_rank: Super Contributor
timestamp: June 08, 2023, 07:29:03 pm
content: Hint: What is 2756 Ohms multiplied by 3.24 mA?

author: IanB
author_rank: Super Contributor
timestamp: June 08, 2023, 07:40:22 pm
content: Quote from: dusan on June 08, 2023, 06:43:27 pmProbably any method is better than that oneThat measurement circuit is basically useless, as you showed by removing the transistor (good experiment, by the way).For a proper measurement of hFE, the value of VCEshould remain constant during the test. With the way R1 is arranged in that circuit, VCEis never going to remain constant. Always be suspicious of what you find on the internet, and verify with other sources.

author: IanB
author_rank: Super Contributor
timestamp: June 08, 2023, 07:46:36 pm
content: Here is a university electronics lab exercise that may give you some ideas:https://personal.utdallas.edu/~mtinker/EE3110/ElectronicDevicesLaboratory/LabManuals/7.%20Bipolar%20Junction%20Transistor%20Characteristics.pdfIf you don't have sophisticated source measure units to hand you will have to improvise that part, but you can follow the general procedure.

author: iMo
author_rank: Super Contributor
timestamp: June 08, 2023, 08:11:52 pm
content: My first beta (or aprox hfe or h21) meter had a switch with a couple of R_base resistors (like 3M9/390k) and a 4.5V battery.See below - with 390k base resistor the Vbe is always around 0.7V thus the base current is always aprox 10uA with 4.5V battery, npn transistor type regardless.The ammeter current is aprox hfe*I(R_base), as the base current is negligible compared to the collector current..In the below example hfe = 2.9mA/10uA=290PS: added more examples showing you cannot replace the transistor with a piece of wire while getting the same hFE

author: barshatriplee
author_rank: Regular Contributor
timestamp: June 08, 2023, 08:24:34 pm
content: Here is also an old thread about how to measure the hFE of NPN transistors. You may wanna take a look:https://www.eevblog.com/forum/testgear/measuring-hfe-of-a-transistor-by-multimeter/

author: Sredni
author_rank: Frequent Contributor
timestamp: June 09, 2023, 03:23:01 am
content: It doesn't work because you are saturating the transistor, as@wasedadochinted.What you are measuring is beta_forced, i.e. you are no longer on the flat 'horizontal' part of the VCE-IC characteristic, but you are on the slanted leg, instead.When the transistor is saturated, it appears as almost a short on the collector mesh, and that is why you get the same value for IC: its value is determined by Vcc and Rc alone.Change the base resistor to 2.2meg and your op will be in the linear region. I get a beta of 450, well in line with the parameter specified by the simulator (Microcap).

author: Picuino
author_rank: Super Contributor
timestamp: June 09, 2023, 01:37:40 pm
content: Here my prefered circuit.Beware of transistor heating!!!

author: dusan
author_rank: Regular Contributor
timestamp: June 09, 2023, 03:41:10 pm
content: The method with only base resistor works fine but only for ideal transistor:Code:* first line is ignored.model QMY_NPN NPN(bf=200)V1 1 0 9VA1 1 c 0VA2 2 b 0Q1 c b 0 QMY_NPNRB 1 2 47k.controltran 100u 20m 19mmeas tran ic max i(VA1)meas tran ib max i(VA2)let hfe = ic / ibecho "hfe = $&ic / $&ib = $&hfe".endc.endThe example above will print:Code:hfe = 0.0346132 / 0.000173066 = 200But when I use realistic transistor model:Code:* first line is ignored.model QMY_NPN NPN(bf=200 is=5e-14 ikf=0.3 ne=2 var=24 vaf=155 ise=1.4e-11)V1 1 0 9VA1 1 c 0VA2 2 b 0Q1 c b 0 QMY_NPNRB 1 2 96k.controltran 100u 20m 19mmeas tran ic max i(VA1)meas tran ib max i(VA2)let hfe = ic / ibecho "hfe = $&ic / $&ib = $&hfe".endc.endIt prints only 177 (the highest I ever get is with 96k base resistor):Code:hfe = 0.0153783 / 8.66167E-05 = 177.545All methods mentioned in this thread including that PDF shows lower than real value.

author: TimFox
author_rank: Super Contributor
timestamp: June 09, 2023, 03:54:06 pm
content: I have a Keithley constant-current lab supply that works at these current levels.If I supply the emitter through that supply (other end of supply grounded), apply the desired collector voltage from a power supply (other end grounded), and measure the base current through a microammeter (other end grounded), the ratio of emitter current divided by base current ishFE.If you don't own a constant-current supply, you can use a regular lab voltage supply and resistor, measuring the emitter current with a milliammeter in series with the resistor.Of course, both of these meters can be normal 3.5-digit DMMs.(I use that setup to measure the grid current in vacuum tubes, where I need to add a resistor in series with the cathode since the cathode-grid voltage will be positive and the output voltage of the current supply must be negative for my supply.)

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 09, 2023, 03:58:40 pm
content: With a single resistor such as picuino shows, it is quite easy to overload your transistor if the poteniometer is in the wrong setting, but if you add the "protection" resistors such as in the first circuit, then you have to verify the transistor is not in saturation.Also note that that Hfe is not a single number. It varies with operating parameters such as base current Emitter voltage and temperature.There can also be quite big changes between the Hfe's of different transistors of the same type, even when they are from the same batch. When designing a circuit you should always allow for changes in Hfe. Adding an emitter resistor is a common technique. In this case, when there is a lagre emitter current, the B-E voltage is reduced and this reduces the base current and stabilises the currents.If you have two SMU's then you can simply hook one up to generate a base current (for example 0 to 100uA) and use the other as an collector / emitter voltage source with a current limit.

author: mawyatt
author_rank: Super Contributor
timestamp: June 09, 2023, 04:15:28 pm
content: Quote from: TimFox on June 09, 2023, 03:54:06 pmHfe,hfe, or Beta is defined as Ic/Ib with Vce held constant, you are measuring Ie/Ib, which is close to Ic/Ib, since Ic ~ Ie for high Beta, but not so for low Beta.Best,

author: TimFox
author_rank: Super Contributor
timestamp: June 09, 2023, 04:28:27 pm
content: To be accurate,IE=IC+IBTherefore, calculatehFE=IC/IB= (IE-IB) /IB= (IE/IB) - 1 from my measurement.For both beta and my vacuum-tube grid measurements, if the plate-cathode or collector-emitter voltage be critical to the measurement, one can add a high-impedance voltmeter at the cathode or emitter to ground to correct the supply voltage measurement.Sometimes, you also need to know the base-emitter or grid-cathode voltage at a given current.Just as in Dante, about the 1% error in the Julian calendar:Ma prima che gennaio tutto si sverniper la centesma ch'è là giù neglettaParadiso, canto XXVII, line 143

author: Zero999
author_rank: Super Contributor
timestamp: June 09, 2023, 05:41:19 pm
content: Just connect the base to the collector via the ammeter and select a collector resistor to set the collector current.

author: TimFox
author_rank: Super Contributor
timestamp: June 09, 2023, 05:57:16 pm
content: A2 will short the collector to the base, and therefore forceVCE=VBE.edit: corrected typo for BE

author: wasedadoc
author_rank: Super Contributor
timestamp: June 09, 2023, 06:08:27 pm
content: Quote from: TimFox on June 09, 2023, 05:57:16 pmDid you meanVCE=VBE

author: TimFox
author_rank: Super Contributor
timestamp: June 09, 2023, 06:17:41 pm
content: Quote from: wasedadoc on June 09, 2023, 06:08:27 pmOops: I edited in a correction.

author: dusan
author_rank: Regular Contributor
timestamp: June 09, 2023, 06:28:13 pm
content: I used Zero999 method and here is the result:Real hFE is 200 but no matter what value of R1 is chosen it doesn't get close to 200, barely 160.

author: IanB
author_rank: Super Contributor
timestamp: June 09, 2023, 06:53:54 pm
content: Quote from: dusan on June 09, 2023, 03:41:10 pmThe real value is what you measure. If you do not measure it, it is not real.Consider: you think the "real" value is 200, but you try to design an amplifier circuit around that transistor that assumes the gain is at least 200. Your circuit will not work as designed, because how can your amplifier circuit behave any differently than your measurement circuit?

author: wasedadoc
author_rank: Super Contributor
timestamp: June 09, 2023, 07:00:24 pm
content: As has already been pointed out, Zero999's method has the transistor operating with almost no voltage difference between collector and base which means that the collector to emitter voltage is only around 0.7 Volts. The transistor's current gain in that condition is significantly less than when it is operated under the typical conditions encountered in a linear circuit.

author: Zero999
author_rank: Super Contributor
timestamp: June 09, 2023, 07:03:17 pm
content: Quote from: wasedadoc on June 09, 2023, 07:00:24 pmSo what?

author: TimFox
author_rank: Super Contributor
timestamp: June 09, 2023, 07:05:17 pm
content: The good practice I learned for designing circuits with discrete BJTs was to use aminimumvalue for beta for the biasing, then check with maximum values.Also, beta does depend onVCEas well as onIC.At higher collector voltages, the depletion region between collector and base narrows, reducing base current for a given collector current, increasing beta.This called the "Early effect", and is important to BJT distortion, since it makes the input current (into the base) a function of the collector voltage.

author: Zero999
author_rank: Super Contributor
timestamp: June 09, 2023, 07:13:29 pm
content: Quote from: TimFox on June 09, 2023, 07:05:17 pmThanks for explaining it. The idea is to get a low hFE, so once can be sure the transistor is suitable for the application.

author: floobydust
author_rank: Super Contributor
timestamp: June 09, 2023, 08:18:49 pm
content: I find hFEis best measured at your circuit's operating point wrt VCEif you need it tight. Also, always put a ceramic cap say 10nF across E-B to shunt RF pickup as well. Graph from the AoE showing some of their results.

author: wasedadoc
author_rank: Super Contributor
timestamp: June 09, 2023, 08:27:04 pm
content: One only needs to look at the schematics of transistor AM radios and see how AGC is implemented to confirm that gain varies with operating current.

author: IanB
author_rank: Super Contributor
timestamp: June 09, 2023, 08:40:29 pm
content: The most important fact is that the "real" HFEis the one that occurs under the circuit conditions of interest, when using the actual device under consideration. Any other number is simply an illusion.

author: Zero999
author_rank: Super Contributor
timestamp: June 09, 2023, 09:36:15 pm
content: An op-amp can be used to set the voltage and current independently.Here's an LTSpice simulation.And a practical circuit.I = V2/R1For the purposes of the test, ICcan be considered to be equal to IE, since the base current is small.Obviously V1 and V2 need to add up to the operating voltage specification of the op-amp.

author: dusan
author_rank: Regular Contributor
timestamp: June 10, 2023, 09:54:00 am
content: Ok, so here is conclusion. I was using model of BC547 which had BF=325. No matter what method I used and what values I used, I could never get with hFE above 280. Zero999 used BC547B which has BF=294.3 and his charts showing hFE well above 300. I used his model and now I am able to get same result (315) which means I can now find a setting where hfe is exactly same as the model. I don't know which settings yet, but at least it is now theoretically possible, which is improvement.

author: MrAl
author_rank: Super Contributor
timestamp: June 10, 2023, 10:18:49 am
content: Quote from: dusan on June 08, 2023, 06:43:27 pmHi,Not sure if anyone mentioned this yet, but the circuit with resistors means almost nothing except that if you had a transistor that acted more like resistors then you would get a false reading. Another example is when you measure the resistance of a diode. If the diode measures 100 Ohms, does that mean it's now a resistor. Certainly not, it only looks like that for one particular test current. If we vary the current we find that it does not work like a regular, fixed resistor.In short, a transistor is not a set of resistors it is much more dynamic than that. When you replace the transistor with resistors you are no longer measuring the Beta you are measuring the currents into a three resistor network.What this means is when a transistor goes bad its Beta is usually going to go off the charts. For example, a transistor that normally measures 100 will measure 10 or even lower when bad. That's one of the assumptions of this test.In reality the transistor needs to be tested at several operating currents anyway. That means the input current has to be stepped from say maybe 100ua to 1ma to 10ma, and read the output current. The point where it enters saturation is also a good test, as well as the saturation voltage with different collector current levels.The best test is to vary the input current from some low value like 10ua up to maybe 10ma (depending on the transistor) and measure the Beta spread. The Beta will change wildly with different input currents. A transistor that measures 100 will often measure just 10 when near saturation because the collector base diode becomes a little forward biased and therefore 'steals' some of the current from the base, meaning the external base current has to be higher than usual.When we use simple, basic tests like this, we always have to live with some assumptions. If we are not willing to do that, then we have to move on to a much more sophisticated test or set of tests. The simpler tests work in most cases and are designed to be simple and fast.If you really want to test a transistor completely, you've got a very long way to go. Even an automatic curve tracer isn't going to tell you everything. There are many, many more specifications that have to be tested.

author: Sredni
author_rank: Frequent Contributor
timestamp: June 10, 2023, 01:53:05 pm
content: Quote from: dusan on June 10, 2023, 09:54:00 amWhat values of current did you get in your original circuit (the one in the first post) when R2 is 2.2meg (and all other components are unchanged)?Because your problem was definitely that you drove the BJT into saturation, and not some obscure model shenanigan.

author: dusan
author_rank: Regular Contributor
timestamp: June 11, 2023, 05:53:13 am
content: BC547 - Ic 667uA, Ib 3.8u, hfe=175 (model's BF=325 )BC547B - Ic 1150uA, Ib 3.77u, hfe=305 (model's BF=294.3)

author: Unknown
author_rank: Unknown
timestamp: June 11, 2023, 08:27:25 pm
content: it'd be great if this conversation produced a cool method to measure hFE in the end...most people, like myself, don't know much about transistors, actually...but we would like to know that magic number...so we could build a fuzz face based on some calculations... for example...the problem is that different DMM-s will throw different numbers...so trusting those numbers is like flying blind on some occasions...MrAI's suggestion, however, to sweep over a large range of current values seems to open up some perspectives, doesn't it?even if people could add the current value to the hFE value they measured would help... say hFE 178 @ 18microA... than that could be a reference for anyone else reading the report on a good sounding effectMeasuring hFE at multiple current values would be even better...one person's report could be used by others..also, fuzz face calculators could be used more intelligently...etc, etc..

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 11, 2023, 08:36:06 pm
content: Quote from: jeepe on June 11, 2023, 08:27:25 pmWell, that's the kind of info you'll also find in many datasheets likethis one for the 2n2222.On page two, you will find "DC Current Gain" under the heading "ON CHARACTERISTICS" where it gives you a table of minimum hFE for different collector currents.Then on page 3, there's a nice graph of the same sort of data but with more detail such as how temperature affects hFE.

author: bdunham7
author_rank: Super Contributor
timestamp: June 11, 2023, 08:41:41 pm
content: Quote from: jeepe on June 11, 2023, 08:27:25 pmIf you understand that hFEisn't a single number and that it naturally varies according to conditions, then the methods of measurement will be fairly obvious. If you don't understand that, then I'd suggest you learn as it isn't all that complicated to attain at least a functional understanding of the transistor. You don't need calculus, quantum physics or Ebers-Moll (although the latter is worth a read even if you don't fully understand it) to get it well enough to make stuff.https://en.wikipedia.org/wiki/Bipolar_junction_transistor

author: Zero999
author_rank: Super Contributor
timestamp: June 11, 2023, 09:06:14 pm
content: Quote from: dusan on June 10, 2023, 09:54:00 amWait a minute: you've not actually tested it, only simulated it.Do you know hFEis also temperature dependant? This means it will change, as your circuit warms up. The good thing it has a positive temperature coefficient, but it's important to bear in mind, for low temperatures.The bottom line is your circuit should work with the lowest and highest possible figures. The circuit I posted with base shorted to the collector, will give the lowest hFE, for any given current, at the temperature under test of course.

author: TimFox
author_rank: Super Contributor
timestamp: June 11, 2023, 09:20:53 pm
content: In Spice, "BF" is the maximum value of beta for the device, not the beta at any other operating point.There are other parameters that determine how beta varies with the operating point.

author: wasedadoc
author_rank: Super Contributor
timestamp: June 11, 2023, 09:50:43 pm
content: The actual hFE of an individual specimen of the transistor type number is of limited importance. Proper circuit design will ensure that the effects of variations between different specimens and of an individual specimen with temperature, current etc are not major factors impacting circuit performance.Expressed another way. If you need to know the actual hFE that a transistor will have in your design, the design is suspect.

author: MrAl
author_rank: Super Contributor
timestamp: June 11, 2023, 10:06:50 pm
content: Quote from: jeepe on June 11, 2023, 08:27:25 pmHello there,Yes that's an interesting idea. If everyone did this and contributed to the data base, we would have a list of actual test results for various transistors, which would not depend on the data sheet. Without that though we have to check out the data sheet and look for test values for the test we are doing.I have to recommend at least 3 different base current levels are to be used, and they should be spread out like my example of 100ua, 1ma, and 10ma. We have to be careful though because if the transistor collector current goes too high and the voltage stays high, the transistor could blow out while testing it, which would be bad.In many cases you can use a potentiometer to vary the base current. This makes it easier to set the current. When dealing with higher power transistors though that would not work unless the pot was rated for very high wattage, because we might have to input 100ma on some power transistors, maybe even more like 1 amp. That would mean we would need three power resistors for setting the base current.It's not too hard to get an idea how the transistor is working though by just doing the test and not looking at any other data. If you see a gain of 100 with a normal transistor it's probably OK. Darlington transistors will of course have higher gain so maybe look for a gain of 500 or something.There are some transistors that dont have much gain so we would have to know that ahead of time. To add to that, when in saturation (with higher base current) the gain of a transistor with a normal gain of 100 could go all the way down to 10. That's a special case though.The best thing to do is try this. Pick a common transistor like the 2N2222 or something and try testing it with three different base currents. It's not hard to do really. Experiment a little and see what you can come up with.

author: bdunham7
author_rank: Super Contributor
timestamp: June 12, 2023, 01:33:38 am
content: Quote from: MrAl on June 11, 2023, 10:06:50 pmIf you have a few things on your bench--a current-limited power supply and 2 DMMs, at least one of which uses fixed test currents for all or most of the resistance ranges and another that has a reasonably low current range, this is very straightforward. Set it up so the power supply goes through the current jacks of one DMM and then from C to E of the transistor. Then use the other DMM set to resistance to supply the base current from B to E.I use a Fluke 8842A which can give me fairly accurate test currents of 1ma, 100µA, 10µA, 5µa and 500nA which is probably adequate for almost all but the largest of transistors. I'll know if it is in the compliance range if I see a valid resistance indicated. Using this setup you can vary VCEdirectly and easily calculate the gain for each step. A random 2N2904 gave me ~370 in the linear range, about 450 after I accidentally 'warmed it up' a bit and interestingly enough, a VCEof only 0.11V with a 10mA current limit and 1mA base current (a 'forced hFE' of 10). VBEwas 0.71V in this case, so the collector is a full 0.6V below the base. With this setup you can easily tell when you depart the linear range because the PSU voltage, which is also VCEsince there is no dropping resistor, will start to drop.Quote from: jeepe on June 11, 2023, 08:27:25 pmPlugging the same 2N3904 into a Harbor Freight 'free' meter gives me 369 and it varies with temperature (goes up with my finger on it) so perhaps this is the simpler answer to your question. The 'cool method' that can become yourde factostandard is the DT830 hFEfunction at 72F and with a fresh battery installed.

author: Sredni
author_rank: Frequent Contributor
timestamp: June 12, 2023, 02:21:42 am
content: Quote from: dusan on June 11, 2023, 05:53:13 amIn MicrocapBC547 - model BF=438 I get IB=3,75u, IC=1696u, betaDC=452.3BC547A - model BF=191 I get IB=3.78u, IC=656u, betaDC=173.3BC547B - model BF=294 I get IB=3.77u, IC=1055u, betaDC=279.3BC547C - model BF=630 I get IB=3.75u, IC=1697u, betaDC=452.5None of the above is saturated, they are all shown to be in LIN mode.Your original circuit was at fault because it used too low a value for R2 and saturated the BJT. The values you get now are in line with whatever the actual beta for the currents should be according to the given model BF, it seems. (Each simulator has its own quirks on how to compute that, I guess, so there is no point in trying to get the same exact values between simulators).

author: dusan
author_rank: Regular Contributor
timestamp: June 12, 2023, 05:22:00 am
content: Quote from: jeepe on June 11, 2023, 08:27:25 pmI've tested another method (in simulator). I checked all the equations for common collector and found that it's input impedance have beta in it's formula, so I measured the input impedance and derived the beta from it but it also didn't produced exact value as the model. But this is what I try next, take common emitter, common base and common collector, and find any formula that contains beta in it, and then try to use it to measure beta.

author: Unknown
author_rank: Unknown
timestamp: June 14, 2023, 09:00:23 pm
content: Quote from: bdunham7 on June 12, 2023, 01:33:38 amWhat's missing from your comment is whether your fluke will give you the same value (at any setting)... as the plastic DMM?I remember arranging BJTs on a sheet of paper, drawing circles for heaps and writing down hFE values...and then, when using another plastic DMM correcting all those numbers...and taking some with me to a friend who had a more expensive DMM just to get still some different numbers...

author: IanB
author_rank: Super Contributor
timestamp: June 14, 2023, 09:21:09 pm
content: Quote from: jeepe on June 14, 2023, 09:00:23 pmBut truly, the exact value is not supposed to matter. If a circuit design depends on the precise value of the transistor gain, the design is bad.What is supposed to matter is the general magnitude of the gain, for example is it greater than 50, greater than 100, greater than 300? Almost any tester, applied appropriately, should be able to give you this value.

author: Unknown
author_rank: Unknown
timestamp: June 14, 2023, 09:23:17 pm
content: Quote from: bdunham7 on June 11, 2023, 08:41:41 pmEverybody understands that, that's the starting point. But this doesn't make the methods of measurement "obvious"...But since I've described my aspect, seeking a way to be able to get a hFE measurement that can be relied upon, in terms of how a fuzz face actually sounds,instead of dropping a wikipeda link, which is a funny thing, cause this is now how knowledge actually works, you could have told me:you've got a wrong approach... I mean, I...and that this conversation is not a DIY group for building fuzz faces whatsoever, but an engineers' corner...since I'm not en engineer -- because at one point I chose not to become that -- I see no point in reading your forum, not to mention asking questionsthe forum's look is what misled me...the same as the freestombpboxes.org's... (the same software)my bad,bye

author: TimFox
author_rank: Super Contributor
timestamp: June 14, 2023, 10:11:34 pm
content: What's wrong with my reply #10 above, refined by #13?They give a way to "REALLY measurehFE".

author: wasedadoc
author_rank: Super Contributor
timestamp: June 14, 2023, 10:23:34 pm
content: Quote from: jeepe on June 14, 2023, 09:23:17 pmSurely it has been made clear in the replies above thatanymethod which accurately measures base current and collector (or emitter) current can (by trivial arithmetic) provide an answer for hFE which istotally reliableas being correct for that transistor operating under the conditions at which the two measurements were made.It is the same as a car's miles per gallon (or litres per 100 km) figure.Youcan do the test on your car but you will find that a single figure is not sufficient. The mpg varies with several factors including, but not limited to, speed, wind conditions, temperature, humidity, air con on or off, lights on or off, payload (occupants and luggage etc), tyre pressures, fuel octane, engine mileage, engine oil, service history.Furthermore, if you repeat your test or tests on another car of exact same model under exact same conditions you will not get the same mpg figures. Might be close. But not exactly the same. Tolerances!To paraphrase what I wrote earlier. If a ballpark figure for a transistor's hFE is not "reliable" enough then the design of the circuit it is to be used in should be modified. Might be as simple as adding a preset type variable resistor. Might be a different ciruit configuration where some passive components with narrower tolerance, not the transistor hFE, dominate the circuit's performance.

author: bdunham7
author_rank: Super Contributor
timestamp: June 14, 2023, 10:35:21 pm
content: Quote from: jeepe on June 14, 2023, 09:23:17 pmThere are some things that simply can not be properly understood unless you take it upon yourself to acquire a certain amount of knowledge. The threshold isn't all that high here, but when you say this...Quote from: jeepe on June 14, 2023, 09:00:23 pm...then the 'basic knowledge' that you have to acquire includes the fact that it is entirely possible that each of those DMMs gave you a completely correct number even though they were all different. Characterizing parts for applications where within-spec component parameters matter a lot--as your application seems to--is quite tricky and requires you to figure out an define what you are looking for. It's not a generalizable topic, so if you want to know any more, post a schematic of a specific fuzz face and perhaps someone can give you an idea as to what conditions hFEshould be measured.

author: IanB
author_rank: Super Contributor
timestamp: June 15, 2023, 01:48:48 am
content: OK, this is weird, here is a link to a "modern" article about fuzz face circuits, published in 1998, and referring to the state of things 30 years before that (in 1968, when germanium transistors were still a thing):http://www.geofex.com/article_folders/fuzzface/fffram.htmThe article points out that leaky transistors can completely mess up gain measurements, and you need to check for leakage before you try to measure hFE.Apparently, the fuzz face circuit is deliberately designed to distort in an extreme manner, and depends heavily on the characteristics of the particular transistors used, to the extent that you have to carefully pick transistors from a batch to find the "sweet" ones (good distortion vs bad distortion).Which makes me think that measuring hFE for this application is a bit futile. It would be better to build the circuit on a breadboard and plug in different transistors until you find the ones that sound good.

author: MrAl
author_rank: Super Contributor
timestamp: June 15, 2023, 01:11:12 pm
content: Hello there,Measuring Beta is a common practice and gives you some idea about, well, the Beta.It's important to know if the transistor has enough current gain, that's the main thing.There are of course other tests, but the test for Beta is one of them for sure.Some people like to measure the gm and they are usually the ones that like to use gm instead of Beta for calculations. Spice models always seem to use Beta though.