Multiple collectors/emitter per transistor - Page 1
Forum section: electronics / beginners

author: Moriambar
author_rank: Unknown
timestamp: March 24, 2020, 12:26:49 pm
content: Hello, I just found this schematic for an op ampImagethe tranny in the top left corner (Q6) seems to have multiple collectors. Is this the same as connecting multiple outputs to the same collector or is there any difference?Are these available (curiosity, I don't have a specific application since I don't know what it its) as discrete components or are these just done on ICs?Thanks

author: Zero999
author_rank: Super Contributor
timestamp: March 24, 2020, 12:42:38 pm
content: I can't see the image. Please upload it as an attachement on this site.It's literally a BJT with multiple collector junctions. It's roughly equivalent to more than one BJT with the bases and emitters connected together, but the collectors left separate. In reality the collector junctions might have different areas. This is often done in ICs. I've not seen discrete transistors with multiple collectors, but some might have existed in the past.

author: Moriambar
author_rank: Unknown
timestamp: March 24, 2020, 12:44:11 pm
content: Quote from: Zero999 on March 24, 2020, 12:42:38 pmThanks, I fixed the image link.You explanation is also awesome thanks, now I understand it.

author: Zero999
author_rank: Super Contributor
timestamp: March 24, 2020, 12:53:51 pm
content: Quote from: Moriambar on March 24, 2020, 12:44:11 pmNo, I meant upload it here, not link it. The file size can also be reduced by converting to greyscale.It's an op-amp. Q6 is part of the input bias current cancelling circuit. It sources a tiny current to cancel the currents taken by the bases of Q1A and Q2A. The idea is to make an op-amp with BJTs, with a low bias current as well as a low offset voltage.

author: magic
author_rank: Super Contributor
timestamp: March 24, 2020, 01:29:40 pm
content: What opamp is it?Looks a bit like LT1028 but it isn't the known LT1028 schematiceditAs for emitters, this also exists (see NE5532) and works like two NPNs with bases and collectors tied together.

author: Kirill V.
author_rank: Regular Contributor
timestamp: March 24, 2020, 01:32:20 pm
content: There are discrete transistors with two emitters, and they can be used as efficient switches.Transistors with multiple collectors are unique to integrated circuits. In principle, integrated technologies allow you to create arbitrarily complex transistor structures, if it makes sense.

author: Moriambar
author_rank: Unknown
timestamp: March 24, 2020, 01:37:42 pm
content: Quote from: magic on March 24, 2020, 01:29:40 pmOp 27 - Op37 fromhere

author: vk6zgo
author_rank: Super Contributor
timestamp: March 24, 2020, 01:42:01 pm
content: Some RF output transistors have multiple elements like that.Back in the day, the received wisdom was :-"Transistors don't just lose gain & die, like tubes-- they either work or they don't"We had an epidemic of transistor PA stages that just "dropped in power gain" over time.They would test perfectly using "small signal" tests on a transistor tester.After much research (no Internet back then), we found out the truth about them.They were multiple element devices, & as sections died, the gain fell.

author: Renate
author_rank: Super Contributor
timestamp: March 24, 2020, 10:45:02 pm
content: I'm surprised nobody brought this one up.The most conspicuous example of multiple emitters is the TTL logic family.All inputs go to an emitter.

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2020, 01:47:44 am
content: Quote from: Zero999 on March 24, 2020, 12:53:51 pmIn this particular case, multiple collector Q6 is configured as a current mirror. Draw out a PNP current mirror which has one input and two outputs and you will see that the emitters are tied together, the bases are tied together, and one of the collectors is tied to the base just like the symbol for Q6.Sometimes the multiple collector transistor shown as a current mirror will have groups of collector outputs tied together to indicate the current mirror ratios of different outputs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 26, 2020, 05:12:46 am
content: Basically, it's a bunch of ordinary BJTs wired in parallel, the common terminals being the single connections, and the independent connections being, well, independent.So you'd construct an equivalent by wiring three PNPs with the bases and emitters together, and the collectors as shown.Real multi-electrode BJTs are more complex than that, actually -- in the same way that emitter and collector are equivalent to some extent (both P diffusions into an N base), the multiple collectors/emitters can share current between each other, under appropriate conditions, serving as emitters or collectors respectively.Take the lateral PNP for example. It was invented to solve a process shortcoming -- at the time, additional diffusion steps to construct real, useful PNPs weren't available, or sucked (poor performance or yield/quality), or were simply more expensive (making a complementary device more expensive than an economized LM324 or what have you). What they did is simply make an NPN with two collectors and oops forgot about the emitter -- whichever collector is higher voltage, acts as the emitter, and the result is a symmetrical device with low Vce(sat), high breakdown voltage (Vebo ~ 30V, because it's actually Vceo!), and the downside, pitiful hFE (typically 3-10). (Hrm, that doesn't make sense, that describes a "PNN" transistor, which is just NPN again. Well, it's not the greatest description then...)So, when you see current mirrors with multiple collectors, they probably are constructed that way, and there can be subtle interplay between the collectors, which aren't actually independent in the real device -- this isn't often used though, so for the most part, it's merely something interesting to keep in mind.Interestingly enough, they show the diff pair as independent transistors wired in parallel -- this is a common motif in precision op-amps, where the four transistors are arranged in a square, with parallel pairs in opposite corners. This way, a temperature gradient across the die cancels out for both pairs; only higher order (gradient of gradient) temperature patterns can cause offset. The transistors also tend to be quite large, physically, which reduces parameter spread errors by symmetrically covering more die area. (Higher order patterns have been used as well, e.g. a 4x4 grid.)Finally, the huge transistors are typically biased generously -- up to several mA -- which demands a lot of base bias current. The triple-collector transistor in question is supplying exactly this current, so as to reduce input bias demands. Interesting note: because the collector currents are correlated (they're from the same current mirror), so are the noise currents. You want to supply equal resistances to the two input pins, so that the correlated noise currents develop equal voltages; mismatched input resistance will significantly increase input noise!(At least, I recall most of this from the LT1115, a notably low noise amp employing these techniques. It looks like most of this applies to the OP27, but I'm not actually all that familiar with the OP27 itself so YMMV.)Incidentally, when you see multiple collectors or emitters, they're often of different active areas. This leads to current mirrors of useful gain ratios; or, emitters of different current densities, which therefore have different tempcos, which can be subtracted from each other, making what's known as a bandgap voltage reference.Tim

author: greenpossum
author_rank: Frequent Contributor
timestamp: March 26, 2020, 05:25:28 am
content: There was a logic family based on integrated multiple collector transistors:https://en.wikipedia.org/wiki/Integrated_injection_logicalsohttps://www.computer.org/csdl/magazine/co/1976/02/01647276/13rRUwInvns. It's easy to do on a die. Somebody has played with a discrete version of that:https://hackaday.io/project/159059-adventures-in-time-integrated-injection-logicMOS overtook I2L so this family is mostly forgotten but designers know about it. There are still some old chips made these days that use I2L, e.g. SN76489 sound generator chip.

author: magic
author_rank: Super Contributor
timestamp: March 26, 2020, 07:21:15 am
content: Quote from: T3sl4co1l on March 26, 2020, 05:12:46 amNot really. It's like an NPN with two bases (P) within one collector region (N) and no emitters, hence PNPIt's not symmetrical either, except for illustrations in electronics textbooksThe emitter is a small disk and the collector is a ring which completely surrounds the emitter. The ring of N silicon between them is the base. Connection to the base is made outside the collector ring to increase chances that holes injected into the base by the emitter end up running into the collector rather than exiting through the base terminal. Still, plenty of holes are emitted downwards and pass below the collector ring to the base terminal, resulting in low beta. Swapping C and E reduces beta further, because half of the holes are emitted outwards.I have heard that typical beta for such transistors is on the order of a few tens. Lateral PNPs are also slow, only good for a few MHz. They are the reason why most oldschool chips have rather limited bandwidth. Some opamps like LM318 and NE5534 achieve higher BW by using PNPs only in one stage and bypassing them capacitively above 1MHz or so.A picture of a real world lateral PNP is attached. This is from Noopy's photograph of some LM723 regulator. Green is P, yellow is N. I chose this picture because the base is not covered by metal and visible. Normally, the metal disk which contacts the emitter is made larger so that it covers the whole base. This is supposed to slightly improve performance, I don't remember why.There are two PNPs here which happen to have their bases shorted. The right one has two metal connections made to its collector, but it's not a split collector PNP. It would be a split PNP if the collector ring were divided into two disjoint sections, either semicircular or in any other arbitrary proportion.