0
\$\begingroup\$

In Class-B amplifiers complementary PNP-NPN transistors are used, e.g. TIP2955 and TIP3055. https://www.electronics-tutorials.ws/transistor/tran_3.html says that

They both have a DC current gain, Beta, ( Ic/Ib ) matched to within 10% (...)

Is this really matched only to within 10%? Is this so difficult, would be expensive, or for what other reason accuracy is just that? What are the reasons that it is matched only to within 10% in TIP2955/TIP3055.

\$\endgroup\$
  • \$\begingroup\$ The tighter they set the limits a part should adhere too, the less components will make the bin, raising the price of those that do. \$\endgroup\$ – Unimportant Dec 31 '18 at 13:35
  • 1
    \$\begingroup\$ Getting a tighter match usually requires both transistors to be on the same die and such parts can be very expensive (in the sense of electronics component costs). See this table for some matched pairs (not complementary, but the pricing is informative). analog.com/en/products/analog-functions/… \$\endgroup\$ – Peter Smith Dec 31 '18 at 13:43
  • \$\begingroup\$ I think you have a pretty good answer or two here. But are you thinking about doing a design of your own and asking about the impact of matching BJTs vs not matching them? Or just if the web site you reference is accurately writing about complementary BJTs? \$\endgroup\$ – jonk Dec 31 '18 at 23:43
  • \$\begingroup\$ @jonk I am just curious about the DC gain match accuracy of pair of NPN - PNP transistors, what are the reasons that it is matched only to within 10% in TIP2955/TIP3055. \$\endgroup\$ – 4pie0 Jan 1 at 13:18
1
\$\begingroup\$

In addition to peufeu's answer I would like to mention that if your circuit requires the beta to be matched within 10% then your circuit isn't a good design.

It is difficult and costly to get matched beta values but it is not difficult or costly to improve your circuit such that the value of beta becomes less relevant.

Experienced circuit designers usually dimension their circuits such that the circuit will work even for the lowest expected value of Beta and higher values. For example instead of using a single transistor you could make a Darlington configuration which increases Beta a lot. Then use (local) feedback to make the circuit behave in the way that you need it to.

I know this sounds easier said (written down) then done, what you can do to see how this is done is look at common designs of for example Audio Power amplifiers and see how the transistors are used.

Also, the TIP2955 and TIP3055 are ancient and also well known to have very small betas when Ic becomes large. That's why many old audio amplifier designs that did use these transistors use some type of Darlington configuration.

Also note that in these audio amplifiers, the value of Beta does not determine the output current. Even though a class B common collector output stage has a current amplification of Beta the audio signal is a voltage so it is not directly affected by beta. My point is that even with a different beta for NPN and PNP it is still possible to make a decent class B output stage.

\$\endgroup\$
3
\$\begingroup\$

Is this really matched only to within 10%?

Quick datasheet check:

enter image description here

Indeed, hFe seems well matched between the two, but keep in mind that these curves represent typical characteristics. Only values in the characteristics table are guaranteed by the manufacturer:

enter image description here

Under the specified conditions (Ic=4A Vce=4V) the minimum hFe is 20 and the maximum is 70. The hFe value around 50 from the curve above is right in the middle, so we should kinda expect a gaussian distribution, but they're not telling about its variance. Besidees, hFe varies a lot depending on temperature:

enter image description here

Curves are from another transistor but you get the idea. Since we're talking about power transistors, they will presumably be used in a power amp and thus they will heat, and not necessarily to the same temperature, also temperature will vary, and maybe one transistor is mounted on a cooler part of the heat sink, etc.

In other words, the truth is "between 20 and 70" under specified conditions, which is nothing like this "hFe matched to 10%" thing.

Having similar hFe for both transistors means that both will load the driver circuit (which provides their base current) in the same way and this should reduce some forms of distortion.

This is what you would normally expect from a "complimentary pair": two transistors that the manufacturer will try to make quite similar, but really not enough to be called "matched". Besides, it is not possible to manufacture a NPN and a PNP with identical characteristics.

If you want two transistors to be as matched and identical as possible, the only way is to manufacture them from the same silicon wafer, and have them next to each other so they are made in the exact same conditions. They will have to be of the same type (ie, 2x NPN for example).

Example: SSM2220 or much cheaper DMMT series. In this case the transistors are low power, in the same package to try to keep their temperature as close as possible, and they are either on the same silicon die or from the same silicon wafer with closely controlled matching, verified by testing. A very important bit about these matched transistors is Vbe matching, as this is what determines the input offset voltage if they are used as a long tailed pair.

PS: If you want to build an audio amp, 2955/3055 in class B is not really recommended.

EDIT:

It is easier to match NPN and NPN because they are the same fabrication process and you can make both transistors in the exact same conditions, same wafer, same fabrication run.

However, if they are not made next to each other on the same wafer, stuff like doping, vapor deposition etc is never perfectly uniform across your wafer, and it will change across wafers... so your two "identical" transistors will have lots of dispersion in their characteristics... it's a fact of silicon chip manufacturing.

NPN and PNP use different fabrication processes, with different steps executed in different order, which will depend on different parameters... so matching them is even more difficult.

Thus, 10% is actually not bad at all.

An analogy would be: a machine which makes screws will easily be able to make almost identical screws in the same fabrication lot as long as it uses the same tools and settings, no need to adjust it super accurately. But if you want to make nuts that match the screws exactly, then you have to adjust your nut-making machine to match your screw-making machine very precisely, which is much harder...

\$\endgroup\$
  • \$\begingroup\$ Thanks @peufeu. I saw the datasheet, my question was more about DC gain match accuracy of pair of NPN - PNP transistors, what are the reasons that it is matched only to within 10% in TIP2955/TIP3055. \$\endgroup\$ – 4pie0 Jan 1 at 13:24
  • \$\begingroup\$ @4pie0 Do you mean: why did the manufacturer match it, or why is it only matched to 10% and not better matched? \$\endgroup\$ – peufeu Jan 1 at 15:47
  • \$\begingroup\$ The second one, what are the reasons for only 10% accurate match. Your answer was very helpful, Bimpelrekkie's answer complementing your's and delivering the direct answer by accident. Sorry if I was not very clear about the question. \$\endgroup\$ – 4pie0 Jan 1 at 16:26
  • \$\begingroup\$ Alright, I edited the answer, added a bit at the end \$\endgroup\$ – peufeu Jan 1 at 16:46
  • \$\begingroup\$ Thank you very much. True - 10% doesn't look that bad at all if you think about all fabrication process' details. \$\endgroup\$ – 4pie0 Jan 1 at 17:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.