Pillaging the Wealth of Information in a Datasheet

It’s a fair assumption that the majority of Hackaday readers will be used to working with electronic components, they are the life blood of so many of the projects featured here. In a lot of cases those projects will feature very common components, those which have become commoditized through appearing across an enormous breadth of applications. We become familiar with those components through repeated use, and we build on that familiarity when we create our own circuits using them.

All manufacturers of electronic components will publish a datasheet for those components. A document containing all the pertinent information for a designer, including numerical parameters, graphs showing their characteristics, physical and thermal parameters, and some application information where needed. Back in the day they would be published as big thick books containing for example the sheets for all the components of a particular type from a manufacturer, but now they are available very conveniently online in PDF format from manufacturer or wholesaler websites.

A 2N3904 in a TO92 through-hole package
A 2N3904 in a TO92 through-hole package

Datasheets are a mine of information on the components they describe, but sometimes they can be rather impenetrable. There is a lot of information to be presented, indeed when the device in question is a highly integrated component such as a DSP or microprocessor the datasheet can resemble a medium-sized book. We’re sure that a lot of our readers will be completely at home in the pages of a datasheet, but equally it’s a concern that a section of the Hackaday audience will not be so familiar with them and will not receive their full benefit. Thus we’re going to examine and explain a datasheet in detail, and hopefully shed some light on what it contains.

The device whose datasheet we’ve chosen to put under the microscope is a transistor. The most basic building block of active semiconductor circuits, and the particular one we’ve chosen is a ubiquitous NPN signal transistor, the 2N3904. It’s been around for a very long time, having been introduced by Motorola in the 1960s, and has become the go-to device for a myriad circuits. You can buy 2N3904s made by a variety of manufacturers all of whom publish their own data sheets, but for the purposes of this article we’ll be using the PDF 2N3904 data sheet from ON Semiconductor, the spun-off former Motorola semiconductor division. You might find it worth your while opening this document in another window  or printing it out for reference alongside the rest of this article.

Let’s take a look at all the knowledge enshrined in this datasheet, and the engineering eye you sometimes need to assign meaning to those numbers, diagrams, and formulas.

The front page of the ON Semiconductor 2N3904 datasheet.
The front page of the ON Semiconductor 2N3904 datasheet.

Give it to Me Straight

The front page of a data sheet will have the information the manufacturer considers to be most important. This should include the basic electrical properties of a device, a succinct description of what it does, and assuming it is not a device with a myriad pins, information about its external connections. Unfortunately some manufacturers seem more driven by marketing considerations than technical ones, so from time to time you will find data sheets whose front pages feel more like sales brochures, leaving you to have to hunt through the pages for the most basic of information. Happily the folks at ON Semiconductor seem to have a good understanding of what an engineer really wants from the front page of a data sheet, so straight away you have a table of the 2N3904’s maximum electrical ratings and an identification of its external connections.

In the case of a transistor, that table of maximum ratings is probably the single most important set of information for the designer in the whole sheet. You may have special requirements for which you need to know more about the device, but these are the most fundamental parameters that tell you a lot about what the transistor is suitable for, and those that (should you ignore them) can result in it releasing its inner store of magic smoke and costing you the price of another transistor. These are the voltage, current, and power dissipation figures you will have in front of you when you calculate the DC bias circuit for your application, in order to ensure that you’re operating the device within its electrical capabilities.

You needs these when you are selecting a device, for example if you are building an audio amplifier you might be interested in the device power dissipation for an idea of how much power it might be capable of delivering to a loudspeaker. In the case of a 2N3904 you’ll see that after allowing for the heat dissipation inherent to a transistor operating in a linear mode it can only yield a few hundred milliwatts, so a more powerful transistor might be a better choice as an audio power amplifier.

I Want All the Data

datasheet-3904-characteristicsTurning the page, on page 2 of the datasheet we find a much more comprehensive table of parameters for the 2N3904. Off characteristics, on characteristics, small-signal characteristics, and switching characteristics.

The off characteristics relate to the device in the off state, which is to say when the voltage between base and emitter is below the roughly 0.7 volts required to start current flowing between collector and emitter. The breakdown voltages are the same ones that were in the table of maximum ratings on the previous page, they are the maximum voltages a 2N3904 can take before it is damaged. The cutoff currents though are different, they release no magic smoke, instead they are the tiny currents that still flow even when the transistor is turned off. You’ll notice they are measured in nA, nano-amperes, a very tiny figure indeed.

What is an ‘h’ Parameter?

The h parameter model. User Rohitbd [GFDL or CC-BY-SA-3.0 ], via Wikimedia Commons
The h parameter model. User Rohitbd [GFDL or CC-BY-SA-3.0 ], via Wikimedia Commons
Moving to the on characteristic table, we encounter our first h parameter, the current gain hFE. The h parameter model is a mathematical model for describing the operation of a transistor. It’s something first-year electronic engineering students agonize at length over, but fortunately to use the figures it generates you do not need to know it in detail. In the case of hFE, this figure is the current gain of a transistor, or the ratio between the base current and the collector current it generates for a constant collector-emitter voltage. You will often see the hFE figure simply referred to as the transistor’s gain. In the case of the 2N3904 this has a maximum value of 300, so in a transistor with that hFE value if you put 1mA into the base you will be able to measure 300mA flowing into the collector if the collector-emitter voltage is 1 volt. It’s a slightly artificial figure in the way mathematical models sometimes can be, but it gives a straightforward idea of how good an amplifier this transistor is likely to be.

The collector-emitter and base-emitter saturation voltages are the voltages at which those connections are at maximum forward bias and will go no further in terms of voltage. The base-emitter path, and the collector-emitter path when the transistor is in the on state can both be considered as though they were forward biased diodes. One of the properties of a forward biased diode is that the voltage across it remains nearly constant no matter the value of the current flowing through it, and it is that constant voltage which is being referred to for the two paths through the transistor. If you think a constant voltage might cause the transistor to cease amplifying though, think again. The bipolar transistor is a current amplifying device, so once the junctions are at their saturation voltages the current flowing in the collector will still be hFE times that flowing in the base and amplification will still occur.

3904-datasheet-small-signalThe small-signal characteristics relate to how the transistor performs as an AC amplifier. First up is the gain-bandwidth product, fT. It might be tempting to think that since the fT of a 2N3904 is 300MHz that the device might be usable up to that frequency, but this is a misleading figure. In fact it refers to the frequency at which the gain drops to 1, so the likely maximum frequency at which the device is useful will be considerably lower. In the case of a 2N3904 you would find it to be useful somewhere beyond 100MHz, for example.

Below the fT figure are the capacitances of the different parts of the transistor which will probably be of little importance in the majority of applications, followed by the rest of the h parameters. Again these are likely to be of little interest unless you are putting a 2N3904 into a modelling package. You will notice hfe, the small-signal AC counterpart of the DC hFE we mentioned earlier.

And finally in this section we have the noise figure. This is not a figure that will trouble you in the majority of applications but it is worth taking a moment to consider. If you are working in an environment in which noise considerations are important – perhaps a radio receiver or a demanding audio application – you will need to pay close attention to ensuring that this number is as low as you can make it in particular in the early stages of amplification. In this case the 2N3904 with a 5dB noise figure is not a particularly low-noise transistor, but then again it’s a general purpose workhorse rather than a high-performance thoroughbred.

How Well Does It Switch?

Below the small-signal characteristics is a table of the switching characteristics. If you imagine a perfect square wave, you might imagine it would appear on your oscilloscope screen as a sequence of sharp right angles. Every transition should be instantaneous from low to high voltage. In practice of course it doesn’t work that way. it takes a short time to traverse the gap. These are the parameters that give you those timings, and ultimately that tell you what the fastest logic signals a 2N3904 can handle are. You’d play close attention to these if you were designing fast logic circuitry, but for simple DC or analogue use they would not be something you’d need to know.

Rise and fall time test circuits for the 2N3904
Rise and fall time test circuits for the 2N3904

On page 3 of the 2N3904 datasheet you’re into the really irrelevant stuff for most Hackaday readers. Surprisingly, in many sheets this page would be further towards the back of the bundle. Ordering information, something that will interest you if you are buying ten million 2N3904s from ON Semiconductor, but since you are likely to get your transistors from a stockholder like RS, Farnell, Mouser, or DigiKey this section has little relevance. Below that are the circuits used to measure the switching characteristics, yet again something of great interest to a designer using the device in fast logic circuitry but not so gripping for others.

The next three pages have all the transistor’s parameters expressed as graphs. Now you might think that this would be the main event of a datasheet, and in some sheets you’d be right, but in the case of a transistor sheet it’s all very interesting in an Art of Electronics kind of way but you don’t need to bury yourself in them. You already know the pertinent information surrounding a 2N3904 from the first couple of pages, these graphs fill in the edge cases and tell you in more detail about how the device behaves. Fascinating if you are learning about how transistors work, but in most cases of straightforward transistor design you will not gain much from studying them.

And finally at the back of the datasheet, the package information. You’d expect the ordering information to be here too, but for some reason ON Semiconductor put that on page 3. Package information is something you might not consider important, however if you are creating a PCB you may find yourself spending a lot of time in this part of a datasheet. If your PCB CAD package doesn’t already have the device in its library you may have to create it. Even if there is a CAD footprint you had better ensure the dimensions match the part you are sourcing. It is the dimensions on this page that will ensure you get it right. If your device is surface-mount you will usually find a recommended area for its PCB lands with full dimensions, something that can save you a lot of trouble with wrongly-dimensioned boards.

Beyond this Datasheet

We hope this piece has helped demystify the manufacturer datasheet for you if you were intimidated by them. It’s a shame we only have a Hackaday article in which to cover this topic, for if we had looked at all the graphs in detail this would have made a decent sized book chapter. The 2N3904 is hardly an accomplished device, but with luck you’ll now know a little bit more about this most basic electronic building block.

Since we feel that the information contained in electronic component data sheets is often buried and not always fully understood, we’d like to feature more articles like this one. The example here is a transistor, but there is no reason why any of the other devices we use every day could not also be explored in depth, analog ICs, digital ICs, even passive components. Which devices would you like to see given this treatment? What are some of your favorite quirks and tidbits from other datasheets? Let us know in the comments below, and send in a tip for future articles.

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