Is transistor matching really necessary for sound amplifiers? - Page 1
Forum section: electronics / projects

author: exe
author_rank: Unknown
timestamp: December 26, 2019, 09:14:02 am
content: I see a lot of DIY audio enthusiasts are really concerned to have matched BJTs / FETs in their power output stages.The logic behind is it should somehow reduce THD. Could someone explain why?To me this is not obvious. Imagine there are two different BJTs on the output: one with higher gain, another one with smaller (=worse). I can understand why one would want to replace the worse transistor with something better. But I don't understand why one would match the worse device for the sake of having a matched pair. In my opinion this will create more distortion.What do you think?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 26, 2019, 09:40:51 am
content: It depends on the details of the circuit. In some older circuits transistor matching can have a slight advantage in THD. Newer circuit tends to be build in a way that the exact hfe and matching does not matter much. There is not always a simple better or worse with parameters. So two matched low hfe transistors may be better than one at the high and low end each. More important than transistor matching is often a stable and well chosen standing current.Another point can be load sharing - if UBE is too different the emitter resistors may not be sufficient for good load sharing between paralleled transistors. This can be even worse with MOSFETs in parallel as parameter scattering is larger there.In the input stage matched transistors can help to get an operating point as intended without adjustments and this way better THD and especially less DC offset.Anyway most reasonable build audio amplifiers have low enough THD values. The speakers tend to have much higher THD. The weak point of higher power amplifiers is more with noise, if used with high efficiency speakers in a quiet environment (and thus low power). Even if the amplifier offers 120 dB SNR at full power, this usually goes down to some 60 dB if used at 0.1% of the rated power. Modern rare earth magnets alow for better efficiency. Another weak point of classical 80s style audio can be the passive cross over networks in the speakers. With active cross over and thus more lower power channels the tendency is to use IC amplifiers.

author: magic
author_rank: Super Contributor
timestamp: December 26, 2019, 11:42:37 am
content: If the output devices have different beta then current draw from the VAS on positive and negative excursions will be different. This may introduce even order distortion due to VAS output impedance. It will be attenuated by feedback so it's probably more of a problem in "low feedback", "inherently linear" amplifiers.Off the top of my head I'm not sure if matching to the lower beta transistor will increase or decrease overall THD, and it probably depends on operating conditions in many ways. You may play with it in SPICE or on breadboard and see what happens.Vbe mismatch will cause output stage offset voltage, again corrected by feedback. Difference in the transfer curves (Vbe vs Ic) will cause distortion if it's not a static offset but something dependent on Ic.

author: David Hess
author_rank: Super Contributor
timestamp: December 27, 2019, 12:23:08 am
content: The only matching I bother with is Vbe or Vgs for current sharing between paralleled devices.Quote from: magic on December 26, 2019, 11:42:37 amI would treat that as a design mistake in that the load on the VAS is too high. It would also mean that the speaker impedance is making it through the output stage back into the VAS.Some designs include buffering to unload the VAS from the output stage beyond the output stage drivers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2019, 07:03:14 am
content: No, not in any worthwhile circuit that uses feedback to account for device variation.If you're talking "hi-fi" designs, it might be necessary. Woe unto you, should the devices ever heat up mismatchedly too.Tim

author: floobydust
author_rank: Super Contributor
timestamp: December 27, 2019, 07:47:46 am
content: Matching is needed for an amplifier's front end, the diff amp. In circuit theory, a diff amp always has perfectly matched transistors.Otherwise you get DC offset and higher distortion. Douglas Self shows mismatch above 2% for the collector currents in the front end increases that stage's distortion.Output transistors don't need to be matched as they have local feed back (emitter resistors) for current-sharing too and ultimately they are in the overall feedback loop so mismatch is corrected for.

author: David Hess
author_rank: Super Contributor
timestamp: December 27, 2019, 04:12:00 pm
content: Quote from: floobydust on December 27, 2019, 07:47:46 amDouglas Self also points out that a collector load of a current mirror solves this problem by forcing the collector currents to be equal. Of course now the current mirror needs matched transistors but that is what emitter degeneration is for.Coarsely matching the input stage transistors is not difficult but Self concluded that most of the distortion came from the output stage anyway so with a good design, the improvement is slight. For a very simple design, transistor matching is more important.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: December 27, 2019, 04:28:42 pm
content: You going to spend hours to build something, why not measure those parts.

author: David Hess
author_rank: Super Contributor
timestamp: December 27, 2019, 04:58:36 pm
content: Quote from: Jan Audio on December 27, 2019, 04:28:42 pmLike I said, coarse matching by say Vbe at the selected operating current is not difficult to do. It only takes a few minutes to get several matched pairs or quads.I usually find the Vbe matching is good enough and that for a given lot of transistors, it correlates well with hfe. I have seen production done where the incoming transistors were simply all graded into 2 millivolt or less bins at which point you have to control for temperature also.Dual and quad matched transistors are available if you want to go that route.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: December 28, 2019, 01:10:54 pm
content: You can not go place them in a bin, and measure tomorrow some more, it has to be the exact temprature, possible at the same moment.Your measurements will be off tomorrow.First you get some isolation and place all transistors in there, let it get on temprature, then you can measure while the isolation is still on there.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: December 28, 2019, 05:29:10 pm
content: True, most precise measurement would be at midnight, during full moon period.

author: floobydust
author_rank: Super Contributor
timestamp: December 28, 2019, 07:41:33 pm
content: Voodoo would prefer every transistor closely matched but that's just being a bit unrealistic. Some amplifier architectures even tolerate mismatch, others do not.Again, for the diff amp pairs you want them matched and care less about overall hFE. So I would put a 160 and 158 part together, and 320 and 305 parts together. I use a paper egg carton and sort them into measured hFE (at voltage) bins. Handling small parts does heat them up so I've never tried VBE matching.I have seen output transistors house-numbered (grouped) in old 1970-80's gear and believe they just wanted the high hFE parts, as back then power transistors performed poorly you could get hFE<25 worse at high currents so stability would suffer.

author: David Hess
author_rank: Super Contributor
timestamp: December 29, 2019, 06:06:05 pm
content: Quote from: Jan Audio on December 28, 2019, 01:10:54 pmAdjusting for the ambient temperature is not difficult. Or a constant temperature fixture can be used like you suggest.For production, a testing circuit can be made which measures the junction temperature using the delta Vbe method to automatically calibrate the absolute Vbe measurement.I just match transistors pairs and quads as I need them but I am not doing production.

author: Finderbinder
author_rank: Regular Contributor
timestamp: April 16, 2023, 04:56:05 pm
content: While replacing a failed big cap in active subwoofer I also checked big output transistors with transistor tester. To my amazement I see a very mismatched ones (especially hFE)Should I worry? The subwoofer is of well known manufacturer, I believe made in 2004.There are some photos:

author: magic
author_rank: Super Contributor
timestamp: April 16, 2023, 05:54:35 pm
content: The datasheet shows lower typical β for the NPN and it decreases towards low currents (note that the plot only goes down to 100mA).34 may seem a bit low, but considering your very low test current this transistor is probably OK. The only guaranteed spec is ≥25 at 5A test current.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 16, 2023, 06:05:21 pm
content: It usually matters where you have multiple paralleled outputs. There are also several ways to match. I think Crown had a test procedure that matched by Vbe. Beta, hfe and HFE are all slightly different. Makes my head hurt. I only match if the manufacturer says to, using whatever method they (hopefully) recommend.

author: bson
author_rank: Unknown
timestamp: April 16, 2023, 06:13:48 pm
content: In a feedback amplifier they only have to meet a minimum gain so the clip circuit can function and you get a full output swing.As pointed out above, the long-tailed pair at the input isn't controlled by the feedback and should be both matched and thermally coupled.

author: 2N3055
author_rank: Super Contributor
timestamp: April 16, 2023, 06:45:00 pm
content: One thing nobody mentioned is that today's transistors have less crazy spread in parameters that they had in 60-ies..Many "rules of thumb" date in "olden" days where it was different...Tighter specs for devices (better process control) "audio" transistors (specifically made for that kind of application) and proper schematic (as was pointed out) means no manual matching is really needed today.....

author: strawberry
author_rank: Super Contributor
timestamp: April 17, 2023, 07:42:45 am
content: you need tranzistors from same batch to match close enough.in some radiotehnika amplifiers they use parallel mismatched TO-220. if mismatch is significant(>50%) then higher gain transistor will fail

author: magic
author_rank: Super Contributor
timestamp: April 17, 2023, 07:46:38 am
content: Are you sure it's about β and not Vbe?What's the output stage like?

author: David Hess
author_rank: Super Contributor
timestamp: April 18, 2023, 05:09:32 am
content: Quote from: Conrad Hoffman on April 16, 2023, 06:05:21 pmJust rough matching for parallel transistors helps with current sharing by a lot allowing for lower values of emitter series resistance. My own experience is that Vbe matching (at higher operating current) is sufficient because the hfe correlates well with Vbe at least within the same transistor production lot.

author: Finderbinder
author_rank: Regular Contributor
timestamp: April 18, 2023, 07:37:49 am
content: And what if like in my situation? No paralleling, just one pnp and one npn.PNP hFE=191, Vbe=546mVNPN hFE=34, Vbe=632mVDoes such mismatch matters something?

author: magic
author_rank: Super Contributor
timestamp: April 18, 2023, 07:41:10 am
content: It's a meaningless measurement because these transistors will not operate at 6mA bias in circuit

author: David Hess
author_rank: Super Contributor
timestamp: April 18, 2023, 04:08:34 pm
content: Vbe matching is also meaningless for complementary pairs except in unusual situations. Idle current adjustment would be in a separate part of the circuit like a Vbe multiplier. Complementary pairs might be matched for hfe at high current to lower distortion, but I would not expect Vbe to correlate with hfe for complementary pairs.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 18, 2023, 04:36:23 pm
content: IMO, the first myth is that symmetrical appearing circuits using NPN and PNP devices are symmetrical. Finding a match between N and P devices can be near impossible. To play devil's advocate, I will say I accomplished that with an old SWTPC Tiger amp and it did reduce the THD by a small amount. Then I blew it up and never found another matched pair of N and P devices, so it didn't really matter.

author: exe
author_rank: Unknown
timestamp: April 18, 2023, 07:03:24 pm
content: Quote from: Finderbinder on April 18, 2023, 07:37:49 amWhere did you get NPN with such a low hFE?? I'd say such a low gain will load the previous stage, increasing THD. But I don't know how much, and if it really matters. I would get a better transistor, but that's my OCD.

author: magic
author_rank: Super Contributor
timestamp: April 18, 2023, 07:16:02 pm
content: Study the datasheet carefully, maybe read my earlier response for a hint.

author: james_s
author_rank: Super Contributor
timestamp: April 18, 2023, 08:04:48 pm
content: I thought the matching was to encourage paralleled output transistors to share current equally? I do recall matching the transistors when I built an amp years ago but I didn't do any comparisons.

author: Xena E
author_rank: Frequent Contributor
timestamp: April 18, 2023, 09:46:09 pm
content: Matching of the push pull output transistors gain is a vestige of early push pull common emitter output stages, notably those that had transformer coupling, and little or no negative feedback: these provided voltage gain and were usually specified to need matched transistors to prevent excessive nonlinearity/harmonic distortion. The Fi wasn't ever very Hi. This in itself was a hangover from Tube amplifier design practice, where in most cases transformer coupling at least at the output is almost unavoidable, and cathode coupled and split load output stages to circumvent some of the drawbacks of more conventional designs were rare and considered exotic.Compare with that more recent designs and you often find that the output stages are in the form of a voltage follower, (classically, a complimentary pair or quasi complementary arrangement with emitters feeding the output). This kind of topology really doesn't benefit from any kind of matching, those who insist it does are just hanging on to the myth based on incomplete understanding of the history of development in design...In any case, Matching a transistor at one collector current guarantees squat, it's a ball park match at best... an amplifier design that needs to have its output stage transistors matched to guarantee reasonable fidelity is complete shit and needs to be binned.More attention given to the amplitude linearity of the driver and voltage amplifier stages is far more worthwhile IMHO.The only caveat is the matching of multiple parallel connected transistors that inhabit each side of the complimentary output, this as has been stated is to ensure current hogging doesn't occur.

author: exe
author_rank: Unknown
timestamp: April 19, 2023, 07:36:26 pm
content: What I don't understand is the following. In push-pull output, if we have one bjt with high gain, and another one with low gain, would it be making more distortion comparing to two bjts with low gain?I'm no expert in THD, but in my head if the amplifier works well at least for half of the wave, it means less distortion (don't know about spectral density of the distortion).

author: Kleinstein
author_rank: Super Contributor
timestamp: April 19, 2023, 08:04:32 pm
content: Having the same gain for both divder sides can help a little with the linearity of the output stage. How relevant this is depends on the details - quite often the loading due to the base current is relatively small anyway, there are possibly other nonlinearities (e.g. the gain changing with current or voltage) in the loop . Usually feedback can correct quite some nonlinerity in the loop - the cross over from PNP to NPN side is somewhat nonlinear anyway. The VAS usually also has some inherent nonlinearity. So the circuit should be able to correct nonlinerity in the output stage anyway and gain matching is one one factor.Most modern designs don't need matched transistors to get good enough THD, so that the actual speakers are the more relevant part.Depending on the circuit details the input stage may want matching for absolute lowest distortion. This may not be matching for gain, bit matching of the early voltage - which often correlates with gain.

author: tmadness
author_rank: Regular Contributor
timestamp: April 20, 2023, 02:45:54 am
content: What are some of the strategies or topologies for reducing matching errors in the input stage?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 20, 2023, 06:46:16 am
content: For the input stage (long tailed pair) there can be a small contribution from the Early effect (gain and VBE changes with CE voltage). A similar effect also exists with JFETs. One can reduce it with a bootstrapped cascode stage to operate with an essentially fixed CE voltage for the input stange. I have not seen this with discrete audio amplifiers. It is used with high end DMM input stages to improve on the linearity (and to keep the input bias more stable). Some high end (and super beta types) OP-amps also seem to use this also. This would be an effect to worry about when you are after THD < 0.001 %, so deep in the nuttery range.Another point can be using an inverting amplifier, so that there is no common mode voltage at the input stage.

author: magic
author_rank: Super Contributor
timestamp: April 20, 2023, 09:12:46 am
content: Quote from: exe on April 19, 2023, 07:36:26 pmI presume you mean current gain, or β, or hFE.Yes, this is a possible outcome.As a reminder, conventional audio amplifiers are essentially oversized opamps wired for noninverting gain. Output voltage is proportional to the differential input voltage, which is the difference between the feedback voltage (a fraction of the output) and the input. This is an error, but as long as this difference is proportional to the signal, it is only a gain and phase error (feedback signal has slightly less amplitude than the input and a small delay).Distortion occurs when the output voltage is not exactly linearly proportional to the differential input voltage (and vice-versa). Feedback generally ensures that the output approximately tracks the ideal output, so it's the differential input voltage which gets weird. This means that the feedback voltage no longer accurately tracks an almost-unity fraction of the input, and distortion occurs. A simple brute force solution is to have more loop gain, so that the differential input voltage is smaller with respect to the signal - feedback is a hammer which turns every other problem into a stability problem.So now to the output stage. First of all, in audio power amps the output stage usually contains one or two driver stages in succession - a variety of Darlington/Sziklai configurations is possible, but what they all have in common is that the driver transistors provide extra current gain which attenuates variations in output device base currents. The drivers work with lower currents so their own base currents are less significant.Low current gain of the output stage places a load on the earlier stages, in audio there are typically two such stages. If the second stage has low current gain, it may similarly load the input stage, forcing a higher differential input voltage to develop. This means that open loop gain drops in response to output load, and it usually is a sign that the amplifier could use more current gain in some stages to drive this sort of load.If the early stages have good linearity, linear output stage base current results in linear differential input voltage and no distortion occurs. In this case, increasing current gain of one class B half reduces gain error a little but increases distortion. If these stages aren't linear enough, simply throwing more load at them may result in nonlinear differential input voltage and hell knows which output stage option is worse then.

author: David Hess
author_rank: Super Contributor
timestamp: April 20, 2023, 02:02:06 pm
content: Quote from: Kleinstein on April 20, 2023, 06:46:16 amDouglas Self says that there is no significant linearity improvement, but that it allows higher hfe and super-beta devices reducing distortion from input bias current variation.Bob Cordell shows cascode JFET stages simply because of JFET voltage limitations, so I would always expect cascodes to be found with JFET designs.While looking to restock my supply of high hfe transistors recently, I noticed that the 2N5087/2N5089 series seems to have been deprecated in favor of the KSA992/KSC1845. The later are 120 volts instead of 40 volts allowing them to be used without cascodes in power amplifiers, but I wonder if they are otherwise as good as the transistors they are replacing because of their much higher voltage rating which I would expect to compromise other parameters.QuoteHigh precision bipolar operational amplifiers are usually using super-beta transistors anyway, whether the datasheet says so or not, but at higher collector currents so they do not deliver the typical low input bias current of super-beta advertised parts. So they must use cascode input stages.

author: Finderbinder
author_rank: Regular Contributor
timestamp: April 26, 2023, 09:35:42 am
content: Quote from: exe on April 18, 2023, 07:03:24 pmThe subwoofer came from manufacturer with these transistors.