temperature compensated BJT current source/sink

That's all interesting and instructive, but for a really good current source, it's a lot easier to add an opamp and close a feedback loop on emitter current.

I could post a sketch if that sounds interecting.

John

You might want to look up "delta Vbe" or "bandgap". The basic idea uses two junctions with currents that are a known ratio.

Best regards,

Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis

Actually, I would like to see it. But I don't think seeing it will help me understand this approach better. I already knew that a negative feedback loop wrapped around a circuit, with appropriate gain, would make this easier. I'm looking to do this like it might be done __inside__ an opamp when creating current source/sinks, though.

How was temperature stability added to early transistor designs, where there were no ICs and the transistors weren't anything to write home about in terms of consistency between each other? I don't think this stopped anyone from designing and calibrating them, when they really needed such an element. So I'm curious about the approach used and I'd like to understand various good and better strategies one might use in those cases.

This needs to start in steps. In other words, I need to first know what the larger effects are and to correct for those. BJT current sources and sinks come in several varieties I'm already aware of, three of them at least being:

(1) the emitter follower with a resistor in the emitter, dragging the base up and down to set the current, (2) the 'I don't know what it is called' thing that I show in the above circuit with Q1, where the base and emitter develop a current across a resistor based upon Vbe, and (3) the current mirror form with incarnations including no emitter resistors at all to having similar or differing emitter resistors and a few or many BJTs each able to source or sink some current relative to a set one.

There may be others, too. Perhaps some that are more or less immune to temperature effects. I suspect, for example, that the current mirror in (3) shows somewhat better temperature stability than the version in (2), because the Vbe's are paralleled and then you are more worried about the side that sets the current and how that gets set for temperature drift. But again, I've not thought a lot about it, yet.

Anyway, large effects first. Understand those, fix them by good inherent starting design and not by adding a negative feedback loop. Then dig a little deeper and see what the next order of temperature effect is that remains. Ask if there is yet another means of improving the starting design to improve its inherent behavior, before taking on the idea of adding a feedback loop. If you are at the point of diminishing returns on the inherent features of the design, _then_ see about adding feedback to get that last bit you can.

This is about understanding. And I don't want to jump ahead with an opamp as part of a feedback loop until I've learned what I can about getting the design as inherently close as I can, first.

Does that make sense?

Thanks, Jon

Done, already. In fact, I think AofE lays out an general schematic approach that starts with a current mirror (which I think is probably better than the two subsections shown in the schematic I provided [a Vbe-set current and an emitter resistor set current]), and where the current set by one leg is then handled by some additional BJTs well strategized for additional temperature compensation. However, I'm struggling first to understand this pair of transistors and what can be achieved with them by careful design before proceeding to trying to do my own work understanding more about that one.

They develop a very nice parabola, temperature wise, and appear (from simulation only, so far) to do a remarkably good job for just two BJTs. However, of course, building a real arrangement from this topology would require a calibration procedure that sets the resistor values, to nail down well. I'm curious about understanding the details that develop the parabolic shape with respect to temperature, though.

In short, I want to take this in steps.

Jon

In case I wasn't clear, yes I am interested in other topologies as I said in my original post. So please, if you want to. I'd like that.

To be clear, though, when I wrote that I was thinking about "other BJT topologies." Of course, one would set out with chains of differential pairs and call it an "other BJT toplogy" but then... we'd be up to quite a few transistors by then. And I'd like to start small and work my way up to dozens of BJTs later on. I want to understand, at each step of additional complexity, what I am buying and why I am buying it.

Jon

Here's a typical circuit:

There's a small error from base current, generally not enough to worry about. A BCX71 has a beta in the hundreds, and it doesn't change much with temperature, so stability depends mostly on the quality of the bandgap and the source resistor. Of course, using a fet would eliminate base current error but would typically add more output capacitance.

There's no attempt to stabilize anything before adding the feedback; it wouldn't matter here.

Older IC datasheets often included the entire transistor-level schematic.

John

Okay. That one is dead easy to understand. Maybe. I gather the cap,

Yes. And that will be itself temperature varying, yes? (I'm thinking about 20uA, 50uA, and 100uA in particular, by the way. Not 10mA. But we can set that aside for now, I suppose.)

Ah, okay. So that answers the above question that it isn't "much" varying with temperature. I should look at the model.

Also, I imagine that there is offset drift vs temperature in the opamp, as well. Another factor?

I'm just focused on mastering BJTs, for now.

How many transistors are present in the opamp and the bandgap ref? How much intelligent design time went into those? How much went into whatever offset drift over temp the LM7301 exhibits?

What I'm looking for is the knowledge involved to _doing_ that. In other words, not in using advanced designs to make a simple current source/sink. But in what crafted thinking is involved in making those parts, themselves. As I said earlier, it's the knowledge not the part that I'm looking for.

Yes, but they don't teach well if you don't already know exactly what each section is there to achieve, beforehand. I'm hoping someone who already has been there, done that, can express some of what they know about these things.

Of course, I need to set down and actually do the experiments myself. But I like to first have a theoretical basis upon which I make predictions, which I can then test to see how well that worked in something built up. Poking around empirically to "get something on accident" might work at times, but it's not a good way in general. I'm trying to "see" better, right now.

I sincerely appreciate the thoughts so far. Don't get me wrong. It's just that the circuit you provided doesn't enlighten me that much about temperature stability. However, let's talk about what I am a little less sure of:

I think I have a feel for the 1nF -- I use exactly that kind of approach to stabilize a power supply feedback loop, in fact. In my own mind it helps stabilize a lot against ripple in the 12V supply rail, for one thing. And that plus the 1k filters things to around the microsecond area in the feedback. The 47 ohm, I think, is to deal with oscillations that sometimes take place with BJTs when there is too low of an impedance driving their base hard. But I'm open to correction on all of that.

Jon

Yes, but microvolts (ie, ppm's of the current) per degree C, totally swamped by the bandgap and resistor temperature drifts.

Well, nowadays electronic design consists of connecting complex boxes, with hundreds or millions of transistors inside, that somebody else designed.

Agreed, it's best if you can understand what's happening all the way down the abstraction stack, clear to the device physics.

All correct. The 1 nF was a guess, but with 1 nF and 1K, the opamp's feedback becomes "local" at around 150 KHz, plenty good enough to keep a 4 MHz opamp happy.

John

And therein lies a serious problem for me. I want to understand the details so that I can _be_ the somebody else, if I want to.

Bingo. I can actually solve eigen vectors and get eigen values and find observables in simple quantum mechanical problems. (So I can do some small things at the very tiny level, if I've a mind to struggle there.) And I don't really want to go that that level, .... yet. Right now, I'm on a __slightly__ more practical bend. I'd like to start with a simple EM1, hybrid-pi approach to this essentially DC problem and "get there" on my own. Well... somewhere, anyway.

Thanks for the confirmation. Then it all makes sense to me and I can say that I probably didn't learn a lot from the schematic -- except a confirmation of what I could already mostly imagine there.

Jon

Two possible paths are designing the ICs themselves, or designing discrete circuits that are not suitable to integration, for any one of a number of reasons.

It's surprising how little really rigorous theory is involved in most circuit design. In fact, circuit design is an emotional, qualitative sport that real academic, analytical braniacs aren't necessarily any good at. That is an occasional source of amusement.

It gets more interesting if you want to maintain a high output impedance, and a constant current, out into the GHz. Or if you want the current to be constant to a few PPM precision at, say, 100 amps.

John

ICs have design capabilities I cannot access well. So discrete is all I'm about, here.

I know it is. I do a lot of construction in practice of things I design in theory -- even quite complex optical systems, for example. And those things work quite well according to theory, so I suppose that's one reason I am looking for something like that here. If it isn't readily available, that's fine. I can hack things out like the next guy. I'd just rather have as much theory as is available.

If you are telling me that it isn't available, I'll take that on advisement.

That's beyond me, of course. I need to take the early steps. But I grant you that when reality impinges on a variety of fronts, each important in some fashion and at the same time, then yes this is what makes it more interesting once you've a solid handle on the basics.

Jon

Okay. Here's another approach:

In this, D1 is there to provide just the right amount of compensation for the Vb2 of Q2 and it simulates well, though again I still need to try it out in practice with several kinds of diodes and BJTs.

Actually looks like something I might consider using as one side of a differential pair being used as a voltage comparator, in fact -- thinking here of R4 as the shared emitter resistor, since the current to be split would then be held fairly constant (constant current in the emitter leg is that "good thing" in diff pairs.)

Jon