Early Effect, Ebers-Moll, Common Emitter Resistance, reverse alpha/beta and saturation current

Question

Ive been reading Sedra/Smith's Micro Electronic Circuits book, and it has a lot of info which I like, however I dont know how much of it is actually practial and useful, for instance it talks about Ebers-Moll equations and models, which I know are useful to understand how transistors work and how the practical equations are derived, but then it talks about stuff like Early voltage and dynamic resistance using Early equations, reverse alpha and beta coefficients, saturation current, etc... and while im solving the drill excercises I wonder if im not just wasting my time on this.

I cant seem to find anything on a transistor datasheet that remotely points to stuff like an Early Voltage parameter, or saturation current, or reverse alpha and similar parameters.

Ive also been reading Boylestad/Nashelsky book which is also great and more practical per se, but it doesnt even mention half of the things present in Sedra/Smith's book, which is why I wanted to go deeper with hopes that it would give me better electronics foundations, which is why im reading Sedra/Smith's book, but even if I understand most of the content of the latter, I fail to see the practical application of most of it...

So, my question is, how useful is to know and learn all these complex stuff? will it improve my design knowledge knowing things like how the doping on the semiconductor, combined with the electron and hole diffusion coefficient and the width of the base interact with the saturation current in the exponential model?

Do people actually use the long Ebers-Moll equations or Early Voltage in real circuit design?

Or do most people just use the simplified and practical equations and leave the other stuff to the phsycisists ?

Answer 1

IMHO it's very useful to know all this stuff, even if you don't use it from day to day, as it deals with the fundamentals of what makes everything else you use IC-wise work.
Whether it will be of "practical use" depends on your field, nowadays especially there is much less discrete design, but if you are designing e.g. a high quality audio amp (I personally work on a lot of all discrete analogue audio gear as well as uCs FPGAs, etc, but I think this probably not so common) or designing analogue ICs, precision test gear, etc then knowing how to work with the above is essential. You generally wouldn't sit down and use all the equations you mention for a design, more recognise the effects of them and know how to compensate (or indeed make use of them)

Knowing the fundamentals gives you grounding you need to make confident design decisions, track down problems and understand why things in practice often look nothing like they do in SPICE.

Answer 2

All The Time -> you need to take these factors into account to ensure low distortion, gain drop off, operational mode etc. The difference that you are noticing is that simply, most advanced designs cannot be done using discreets, and discreet transistors have enough mismatch issues as it is that you don't need to worry about Early voltage etc. You're not going to be designing an amplifier now a days with 20 transistors using discreets, it's so much easier , faster and better to just grab someone else's design in the form of an op-amp. And 20 would be a small transistor count amplifier.

The transistor datasheets and the spice models for IC processes are mostly complete. And include a whole shopping list of parameters well beyond what you mention including temperature coefficients, process variation etc. etc.

Sedra and Smith is notable as being a good book for starting IC design. If this is an area you want to get into you should concentrate on MOS systems. My favorite book for that is "Operation and modelling of the The MOS transistor" by Yannis Tsividis.

If you are using transistors as discreets then you may not need Sedra and Smith but you may and it can't hurt.

Studying BJT's and the topologies for certain operations does map across to MOS systems. So it isn't a waste of time as the concepts map across. i.e a differential pair is a differential pair regardless of the base process. Of course there will be different issues (relating to early voltage, matching etc.) but the starting point can be the same even if you end up with different designs in different processes at the end.

Answer 3

This answer is NOT intended to contradict the two good answers so far (which you definitely should take note of), but to add perspective to them. This answer relates to the use of discretes in analog design in "everyday" applications - if you are doing ASIC design or Oli's amplifiers for utter audio aficionados you may want to ignore what I say.

You can do very many useful things with analog discretes with a far less complete working model than you are looking at. Very simple "rules" will allow you to build real world circuits in say 95% of cases you are liable to meet with no explicit reference to probably any of the 'big names' ideas that you have mentioned. And, that's before you add a good SPICE model that deals with much of the detail unseen.

If you use ICs for the parts that justify their use and discretes "around the edges" or for small designs then you will almost never (if ever) have to enter the arcane darkness. What does happen is that things may not work as well as desired or quite as desired in limiting cases - you can learn to deal with such through experience, or by consulting texts or long ago learned and since archived knowledge as and if required.

I am happily at home in the world of analog design. The underlying fundamental concepts of semiconductor operation were part of my undergraduate training (which was a very long time ago :-) ). . And I can delve into such arcanery as is called for in more extreme cases, or when things don't behave as I believe they ought. But exceedingly close to 100% of the time I am able to happily work on the level that common sense and long experience has imparted. As it's vanishingly hard to start off with "long experience" you'll need to keep an eye on where the edge of the world is until you gain some. But with time the "edge" vanishes away into darker corners and you can do very well with more simplistic models.

Answer 4

If you haven't seen Horowitz&Hill "The Art of Electronics" take a look at it. It has as wealth of practical detail, but does describe (at a fairly basic level) things like the Early effect, and how it affects practical circuits. It may be just the thing to make the connection between theory and practice.