All Transistor Data Sheet

[Edelmira Bendorf]

Thedatasheet gives the gain for Ic=5mA and Vce=2V as 25, and the remainder of the results seem to indicate something of a bell-curve of the gain plotted against Ic, which with some rough interpolation would allow you to find a very rough and approximate gain for any Ic value in between 5mA and 500mA; although this is all for a Vce=2.

The graph gives a shape expected of the results, but the tabular results don't seem to match the graph (i.e. 150 mA should be 40, but graph value seems closer to 160 - i.e. the maximum). It seems the graph is plotting the maximum gain rather than minimum - doesn't make the graph useless?

The tabular data, while useful is strictly for the stated values of Ic=50-500mA and Vce=2. Obviously, 2V is quite low, so if this was part of a circuit running at Vce=5 and the Ic pulling say 100mA, then how could this datasheet ever be useful, considering those values can't even be found in the datasheet?

As they've not labelled the different curves it looks pretty dubious to me. Normally the graph would apply to typical values so you would not necessarily find them in the tabulated values (this one does not show typical values).

I might guess it's showing you either temperature (highest trace would be highest temperature, lowest trace lowest temperature) or unit to unit distribution for the high gain version, with the middle trace the typical value. For what it's worth, the graph is unchanged from the Motorola BC635 datasheet. In any case, the relevant curve is probably the middle one.

hFE is not very dependent on Vce once Vce is high enough, so typically the gain would be around 150 at 100mA and 5V, but it might be as low as a bit less than 40 or as high as a bit more than 160 (assuming 25C, and assuming it could be any of those transistor types.

So you should design your circuit so it will function for hFE between (say) 35 and 180 and you'll be fine. If you need higher gain, or tighter beta specifications, other transistors have higher gain at 100mA and in some cases you can specify the beta bin to reduce the range to more like 2:1 than 4:1 (best avoided if you can).

Edit: To clarify where the numbers came from- what I get from the graph (taking the middle curve) is that the gain does not typically change much between 150mA and 100mA, so we can reasonably assume similar limits. Those limits (not the graph) are what is guaranteed for the transistor. Only those. It's dropping pretty fast above 150mA so I would not make the same assumption if you said 200mA. The 40 and 160 (from the tables) are hard guarantees of transistor performance (you can complain to the supplier if the transistor does not perform within those limits). The curves illustrate what usually happens under some conditions and if the transistor does something different, you have no cause to object.

Gain vs. Vce, as you get much above saturation (when Vce >> Vbe, say above a volt or so) the gain hardly changes. 2V is well above 1V, as is 5V, so we can expect the gain to be almost the same. To illustrate this, consider this set of 2N3904 curves:

If the gain stayed constant the lines would be horizontal with voltage- they are not quite horizontal at higher base currents, but they are reasonably horizontal, and (more importantly in your example) the gain is only higher at higher Vce, so we're fine on the low end, but had better add a bit to the maximum gain guarantee.

Sometimes you must just give up on a transistor if you can't trust the data sheet. For instance, there are three curves in the graph and they are unknown i.e. On-Semi have failed to give relevant information about what those curves mean or represent.

Unless there is something great about the transistor that is embodied elsewhere in this data sheet I'd give up and find a BJT with a data sheet I could trust. If you need it resolved because you have (say) just bought a kilo of these parts then start digging around other manufacturer's of this device to see what their data sheets say.

β (beta), the gain or amplification factor of a transistor, normally is given when solving a circuit equation. However, if it is not given, it can be calculated if the currents, Ib (the base current) and either Ie ( the emitter current) or Ic (the collector current) are known.

I was about to post a question asking for a schematic critique, although while checking things I've noticed a few things on the transistor datasheet that strike me as being a little strange. The datasheet in question is for an NPN, TO-92. The second page states the electrical characteristics, and they're making things a little harder to understand how transistors actually work.

The Vceo and Vces are marked down as minimum values, which would seem to imply that there must be a minimum voltage between the collector and emitter of 45V to 50V depending on the total current passing through the collector. Should these two values be marked as maximum rather than minimum?

Similarly, the Emitter Base Voltage, as I understand is the maximum voltage that can exist between the emitter and base, assuming that the emitter ended up with a positive voltage in reference to the base voltage. So again, should this be marked as maximum rather than minimum?

Finally, the DC Current Gain has a minimum of 100, but this appears for an Ic of 100mA. Am I able to assume that I'll always have a current gain of at least 100, irrelevant of Ic? If not, how am I to know the minimum current gain if I'm not passing in 100mA?

EDIT:After looking at a different transistor, I've found the 2N5551 from Fairchild Semiconductors, and the respective datasheet. It's far easier to understand, as the parameter names are far less ambiguous. For example Collector-Emitter Breakdown Voltage etc, the keyword being breakdown in the name. Also, they also provide a spice model and plenty of graphs. Seems there's a large difference in the quality of datasheets.

It's saying that the transistor can be operated up to 45V. This has to be specified as a minimum value because if it was specified as a maximum value then you wouldn't know the lower limit that might cause it to fail.

On the other hand if the car manufacturer specified acceleration from 0 to 60mph as 4 seconds, you'd want that to be a maximum value i.e. you can always guarantee to do 0 to 60 in 4 seconds (max). Think about it.

Regarding hFE, use a better transistor that has a full spec. The spec you linked doesn't even have a part number other than the generic (but suspicious) name "T0-92". Here's what the current gain for the BC847A transistor looks like in its data sheet: -

Usually the columns min/typical/max on datasheet refer to the spread of the mentioned parameter among a manufacturing batch. This means that, for example, Vceo is guaranteed to be at least 45V for any specimen in the batch (at the given conditions).

In other words, if you buy 100 such BJTs and determine their Vceo you'll discover that each one has a different value, but all the values will be at least 45V. Could they also list the maximum value? Sure! But that won't be useful in designing a product: who cares if one out of 100 BJTs has a Vceo of 60V? The designer need to know what's the guaranteed minimum value so that its design doesn't break when a random sample is placed on the PCB!

The confusion stems from the use of the words "minimum/maximum". Here you use it twice, with different meaning: Vceo as reported is the minimum maximum Vce value, i.e. the minimum value (among the batch) that you can get for the maximum allowable Vce voltage.

On the other hand, Icbo is reported as a maximum value, because that is the maximum guaranteed value among any sample in the batch. Why not reporting the minimum? Again, the relevant parameter for reliable design here is the maximum: Icbo is a leakage current, ideally you would want it to be zero! So when you design a product the worst case scenario is when Icbo is big, hence you need a guaranteed maximum value.

As for hFE, in most design it is useful to have a guaranteed minimum value, because most of the time you use negative feedback circuits which work well as long as the gain is at least a specified value. Here they report also the typical value, since this gives you an idea of the parameter spread among the batch. In the specific case you have min 100 and typical 400, so you could guess that for 100 samples of that BJT most of them will have an hFE around 400, with a minimum of 100 guaranteed and, by symmetry probably you won't have samples with hFE greater than 700=400+(400-100).

Imagine you (using a curve tracer) increase the collector voltage (with zero base current) until the collector current is 1mA. That voltage will be a minimum of 45V. Similarly for Vces, collector current of 0.1mA and emitter current of zero.

As far as current gain goes- the short answer is that you don't know for sure. You cannot assume the gain is >100 for other currents. You can look at the typical graph of gain vs. current and make some reasonable assumptions of what the minimum gain is for other currents, but there is no guarantee (so make conservative assumptions if you want a reliable design). If you are at a bit less than 100mA usually the gain will be similar. At 50nA or at 500mA you're likely to see quite different behavior.

That's pretty much spot on for the transistor I think - however you need to realize that when outputing 30mA the Arduino pin will be struggling and will be below 5V (see the ATmega328 datasheet). Its close to the limit for a single pin too (40mA).

So the easiest thing will be to try a 150ohm and measure what actually happens with your transistor (current gain specs are worst-case and there's usually a big spread (factor of 2 or more) in actual device gains.

This is a great example of where a simple constant current generator comes into it's own, a transistor, led, and two resistors. See my scribbled sketch :- ( I still haven't mastered the inserting pics here ! )

As long as the LED on the base is lit, it will have 2.1v on the base for a red LED, so the emitter will always have 0.7v less due to the base emitter forward bias, = 1.4 volts ( in our example with a 2v1 LED and a 0v7 base emitter bias voltage )

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