Transistor beta repeatability
Transistor beta repeatability
(OP)
The recent post on collector/emitter reversability has prompted me to ask a long-time question I've had...
In general terms, is there a family of inexpensive, small-signal transistors that has decent repeatability of gain from batch to batch?
For example, I commonly use 2N2222-style NPNs in a lot of my circuits (SOT-23 package). The gain is decent for my typical current and frequency range (<50mA, <1kHz) in the area of 120-150, and they're just so darn inexpensive (<<$0.25/pc in quantity, if memory serves).
Difference in part-to-part Beta is usually counted on one hand when comparing devices on a reel (and I would therefore assume from the same batch). However, comparing reel to reel the Beta can vary 20-30 or more, and that can cause issues with circuit repeatability.
Would there be a more appropriate line of transistors to use that are equally inexpensive, but have a tighter tolerance from batch to batch (never bothered comparing a large batch of 3906s, etc.)? Maybe a more recent process? This is more of a curiosity to me than a pressing design issue...
In general terms, is there a family of inexpensive, small-signal transistors that has decent repeatability of gain from batch to batch?
For example, I commonly use 2N2222-style NPNs in a lot of my circuits (SOT-23 package). The gain is decent for my typical current and frequency range (<50mA, <1kHz) in the area of 120-150, and they're just so darn inexpensive (<<$0.25/pc in quantity, if memory serves).
Difference in part-to-part Beta is usually counted on one hand when comparing devices on a reel (and I would therefore assume from the same batch). However, comparing reel to reel the Beta can vary 20-30 or more, and that can cause issues with circuit repeatability.
Would there be a more appropriate line of transistors to use that are equally inexpensive, but have a tighter tolerance from batch to batch (never bothered comparing a large batch of 3906s, etc.)? Maybe a more recent process? This is more of a curiosity to me than a pressing design issue...
Dan - Owner
http://www.Hi-TecDesigns.com





RE: Transistor beta repeatability
I have no idea. But since everything else in silicon is fabricated to narrow tolerances and processes are better controlled, I would think that transistors with their "gigantic" dimensions (compared to memories and such) should be very repeatable nowadays.
I haven't used transistors in applications where beta variations would be noticed for several decades. It was only *Before 709* that you had to do things like that.
Is there any application where a tight beta is needed anymore?
Gunnar Englund
www.gke.org
RE: Transistor beta repeatability
We used to send devices to a company called Solecon that did a measurement of doping profiles in the transistor. Solecon used to graph the device profiles with a series of "P" and "N" to denote p-type or n-type. Using our device thick nesses, each letter represented about 150 nm of depth change. We used to joke about our "one N" bases, e.g., a base that was about 150 nm thick.
This is formed as a result a double counter-dope, i.e., starting with a n-type collector, counterdoping to get the p-type base, counterdoping that to get the n-type emitter. So doping concentration variances, coupled with processing variances almost always resulted in a larger variation of beta.
The only time you can get closely matched beta is when the devices are processed on the same wafer and essentially next to each other. There's usually enough variance across a wafer to cause some amount of mismatch in performance.
TTFN
RE: Transistor beta repeatability
The transistors you are talking about are horrendously expensive. BC847/BC547 are much nicer. But they have always been banded in current gain because it is all over the place (factor of 4). If you need matched gains your circuit design is wrong. I just did a design for a PRT amplifier. The matched pair transistors were ok but expensive. I ended up using a pair of Darlingtons as the base current is then so low it really doesn't matter that it is not identical to the other pair.
RE: Transistor beta repeatability
TTFN
RE: Transistor beta repeatability
Contrary to popular belief the BJT is a voltage controlled device, whilst more base current will generaly increase the collector current it is the applied base emitter voltage which determines the actual collector current that flows.
BJT collector current is a logarithmic function of the voltage applied to the base emmiter junction.
It would be a gross error to design a system in which Hfe was an important or critical factor for functionality.
If you need to match pairs they should match well for Vbe at the operating collector voltage and current as well as a reasonable match for Hfe.
Try having a look here:- http://www.dself.dsl.pipex.com/index.htm
I trust this man, he seems to know what's what.
RE: Transistor beta repeatability
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Transistor beta repeatability
I(c) = I(sat)*[e^[V(be)/V(t)] - 1]
V(t) = k*T/q
q =~ 1.6022e-19 coulombs
k =~ 1.381e-23 joules / K
T = Absoluteo temperature
I(sat) is a function of the transistor geometry, doping, etc.
This describes the exponential response of a transistor which holds over 9 decades of collector current or more. Simple, basic.
The current that goes in when driving a BJT is just a nuisance. An important one, and one that can be exploited for many cool applications, but not a fundamental property.
The exponential relationship can be exploited for even cooler applications, transconductance amplifiers, multi-quadrant multipliers and such.
MacGyvers, if you carefully measure that base voltage, which is "pretty darned constant", you'll find it has an excellent exponential (or logarithmic, depending on your point of view) relationship with the collector current. A much simpler relationship that the collector current divided by the base current has.
Self has a good explanation at http://ww
As I recall, historcally, When Shockley, Bardeen and Brattain were working at Bell Labs, they were really trying to produce a solid state equivalent of a vacuum tube, more like a FET, (Which wouldn't have all that pesky base current) but they didn't have sophisticated enough equipment for processing that. Failing that, they figured they'd publish thir results on the bipolar junction device.
RE: Transistor beta repeatability
Both are right. For practical work, the BJT is mostly regarded as a current controlled device. To check if you got enough drive for a switch, you divide Ic by beta and many times a simple bias network is based on Ib and beta. But, of course, the Transistor Equation is correct (no Ib, there). And that is probably what academia thinks is valid in the engineering world, too.
Since this is Eng-Tips, not academia, I prefer to regard the BJT as being current controlled.
Gunnar Englund
www.gke.org
RE: Transistor beta repeatability
To wit, the reason behind the original question. When driving LEDs, it makes little difference to the consumer if they're lit up using 30mA or 25mA, they pretty much look the same in brightness. For small units, the effect is irrelevant. But when I place them into large arrays (thousands of LEDs), I would like to characterize the panel's power draw a bit more closely than +/-20%. If I can find a transistor family whos Beta remains within a few percent of nominal, so does my power draw, and that makes my life easier (not to mention changes in brightness not seen on small units are more easily discerned on larger units).
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Transistor beta repeatability
For your current source, by adding an emitter resistor to drop a volt or two the few tens of millivolts difference the exponential relationship will give over operating conditions generally won't matter. If they do, you can compensate with an appropriate diode drop in the base circuit which can also compensate for temperature variation, but guess what? That equation will describe what's going on.
Not even chip designers depend on I(sat); it's all in the geometry, matching, area scaling, minding temperature gradients, clever topology and such.
RE: Transistor beta repeatability
First off, I use a constant collector current... PWM (or similar), so it's on/off, but I do not change the drive current. LEDs, being what they are, prefer a specific small drive range for their specified color temperature, so dimming is performed by way on/off modulation.
Second, temperature changes are continually monitored, and the software modifies the modulation frequency to keep the LEDs within spec and drive current within tight tolerance. Temperature change is pretty consistent across an entire board, so deltas in Beta from one transistor to another remain quite small. If the Betas were fairly consistent to begin with, my regulation scheme becomes easier.
If I could place a large number of transistors on a panel that have relatively close Betas (regardless of what Beta may be), my job becomes easier. If I can make my job easier by simply changing the BOM, I declare that change worthwhile.
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Transistor beta repeatability
TTFN
RE: Transistor beta repeatability
Scan through till you find one that falls in range.
Otherwise I would suggest sample testing. The dropouts
can be used elsewhere.
Put those transistors in a differential pair config and
then you do worry about their voltage dependent properties.
(transconductance) as that impacts the gain of the pair.
RE: Transistor beta repeatability
RE: Transistor beta repeatability
As I said, it doesn't matter for the small boards, but for the large boards every bit of control you can get back in your posession is a goldmine.
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Transistor beta repeatability
Skip the base resistor and connect directly to your logic output.
Insert an emitter resistor instead, remove the collector resistor and place the LED directly between supply and collector.
You now have a nice constant-current drive for your LEDs, practically independent of Beta (if there's enough of it).
LED current spread is dependent on logic supply voltage and VBE only. VBE variances should be negligible, supply voltage you can make as precise as you want.
By the way, LED supply voltage needs to be higher than the logic supply, of course.
There! I just saved you a resistor per LED
Regards,
Benta.
RE: Transistor beta repeatability
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Transistor beta repeatability
Placing a resistor in the emitter forces the voltage drop from the transistor to ground to be my digital supply voltage (5V). These are for the automotive arena, so my main supply voltage can be as low as 9-10V (though typically closer to 11V), leaving me only 4-5V to work with for the LEDs. Two LEDs are in series, and their combined forward voltages can by in excess of this... that's a problem.
I could mitigate this somewhat by moving to a 3.3V processor, but that may cause issues elsewhere finding 3.3V tolerant parts (some components are still only made in economical 5V parts, unless you want to pay a premium).
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Transistor beta repeatability
You could certainly achieve something on the order of 0.5% matching using precision resistors, precision voltage references and wrapping the LED in the feedback loop of an op amp, so something similar:
http://www.4qdtec.com/csm.html
TTFN
RE: Transistor beta repeatability
Circuit 3 would be downright scary in a production circuit as the forward voltage of an LED can vary significantly from lot to lot (depending upon binning, even from unit to unit). For a one-off, it would be great.
Circuit 4 would probably be the extent to which I could modify, so long as I stick with SOT-23 transistors, but even then I don't know where I would get the extra space from. There will be a lot of experimenting going on, that's for sure...
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Transistor beta repeatability
What level of accuracy or matching are you trying to achieve?
TTFN
RE: Transistor beta repeatability
In that case, you could drive the transistor base from a voltage divider, giving perhaps 2.5 V base voltage? Even at 9 V supply, you should have 6 V available for the 2 LEDs.
Again, if you transistors have enough Beta, base current is so small that it should not be a problem to make the voltage divider with resonable resistance.
D.mn I just added a resistor per LED
Benta.