Looking for a good model of transistors - Page 1
Forum section: electronics / beginners

author: Legion
author_rank: Frequent Contributor
timestamp: April 25, 2014, 01:43:55 pm
content: I'm trying to get a better understanding of transistors. My model up to now has been very simple - all transistors are voltage controlled (even BJTs) and are essentially no different than a rotary pot where the base of the transistor represents the dial. For me to understand something I need to be able to visualize it working in my head.While this model has worked for me so far, it's oversimplified. When trying to learn more the problem I keep running into is either the info is too shallow (ie. all the samey YouTube videos that talk about P and N type doping); or if it has more detail it doesn't present a proper model of what's going on, just a collection of facts. Worse, often times those facts are contradictory.So, for example, if it tells me that Vc has to be at least 0.2V above Ve, I want to know why, not just remember that as a matter of fact.Any recommendations for videos, books or articles that you've found to provide a particularly useful model for working with transistors?

author: IonizedGears
author_rank: Regular Contributor
timestamp: April 25, 2014, 06:32:06 pm
content: Well, BJTs are not voltage controlled devices, they are current devices. FETs are voltage controlled.Also, "The Art of Electronics" is a great book for everything electronics and will serve as a reference for many things you may need some info on.~Ionized Gears

author: w2aew
author_rank: Super Contributor
timestamp: April 25, 2014, 06:49:41 pm
content: Quote from: IonizedGears on April 25, 2014, 06:32:06 pmBasically true, but the transconductance of a BJT (vt/Ic) is much more predictable and consistent compared to the current gain Hfe (Beta) of the device. Most good designs strive to be largely independent of Beta via use of feedback, etc.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: April 25, 2014, 06:54:27 pm
content: Quote from: IonizedGears on April 25, 2014, 06:32:06 pmAcording to a website mentioned in this thread:https://www.eevblog.com/forum/chat/how-transistors-really-work/(Linked directly)http://amasci.com/amateur/transis.htmlQuoteSo, which is correct?I guess I need to get my "Ben Streetman" book out and re-read.

author: c4757p
author_rank: Super Contributor
timestamp: April 25, 2014, 07:00:55 pm
content: Quote from: IonizedGears on April 25, 2014, 06:32:06 pmBJTs can be modeled as either. They're more accuratelycharge-controlled devices, though, as the charge carriers which exist in the base (caused by either base voltage or current, depending on how you look at it) are what allow emitter current to flow.It's much more accurate IMO to consider the base current a result of the same effect that causes the collector current, rather than the thing thatcontrolsthe collector current.But none of this is really relevant to understanding basic circuits.*** You should be able to use a voltage or current model depending on the situation - whichever gets you results.***This sentence is not directed specifically at the OP, I see you're looking to go past these simplifications. There's not much I can add beyond what free_electron did in the thread Excavatoree suggested - go read that

author: LvW
author_rank: Frequent Contributor
timestamp: April 25, 2014, 07:35:44 pm
content: It is correct that BJT`s can be modeled either - current or voltage controlled.However, we shouldn´t mix MODELS with PHYSICAL reality. At least, one should know if there is a fundamental difference.Hence, as far as the physical phenomenon is concerned the bipolar transistor is a VOLTAGE-controlled device.This can be shown very easily - without too much theory (positive/negative charges, carriers, diffusion effects, recombination,...).Remember the working principle of a classical pn diode, where the thickness of the depletion zone is undoubtly controlled by the applied voltage.The same applies to the depletion layer of the BJT which determines the current Ic. The thickness of this zone is, of course, also controlled by the voltage Vbe.Because the BJT is a non-ideal voltage-controlled current source, there will be a current Ib which cannot be avoided (and which is roughly a fixed percentage of the collector current).But such a small current never can control another current which is much larger.However, this does not mean that during design of a BJT stage we would not be allowed to TREAT the BJT as current-controlled. Of course, in practice we use the simple relationship Ic=beta*Ib. But this does not imply any control mechanism (in the sense: cause and effect).In a previous reply somebody did mention "The art of Electronics". Also in this book the voltage-control property of the BJT is mentioned.

author: wbeaty
author_rank: Contributor
timestamp: April 25, 2014, 07:47:07 pm
content: Quote from: IonizedGears on April 25, 2014, 06:32:06 pmEngineering texts disagree with you. See especially Horowitz and Hill's Art of Electronics.Quote from: IonizedGears on April 25, 2014, 06:32:06 pmUm... Art of Electronics specifically teaches that BJTs are voltage controlled. It's accompanying lab-manual goes into great deatil about this. And just so there's no question:When AOE author Win Hill was questioned about current-controlled BJTs, he had this to say:http://cr4.globalspec.com/comment/720033/Re-Voltage-vs-Currenthttp://cr4.globalspec.com/comment/720374/Re-Voltage-vs-CurrentAlso, when there was an attempt on Wikipedia to use Art of Electronics to "prove" that BJTs are current-controlled, Win Hill came in on the BJT talk-page and put a stop to it. As he says, AOE does start out with Ic=hfe*Ib, then goes on to teach the more general voltage-control theory.Dr. Hill points out that many common BJT circuits are based upon on the volt-control model, and Ic=Ib*hfe doesn't explain these. The current-gain model is a simplification. It's also a loosely-controlled variable in manufacturing, so circuits based on current-gain will fail when different transistors having identical part numbers are used. (Also fails if temperature varies by only a couple of degrees!) Current gain is obviously a very convenient rule of thumb, but brief investigation of BJT physics will show that Vbe directly determines Ic. Then of course Ib can affect Vbe. So Ib only controls Ic through the actions of Vbe changes.OP, see where the contradictions arise?Grade-school electronics texts tend to teach physics misconceptions, and Electrical Engineers fight an uphill battle to stomp them out in online forums. Unfortunately most EE texts are non-intuitive, math not verbal/visual, so the non-EE audience gets little benefit.Why does Vc need to be 0.2V above Ve?I picture it as follows. Ideally, the Vc-Ve difference should go all the way down to zero. It's because the ~0.7V diode drop across Vbe should be exactly equal to the ~0.7v diode drop across Vcb for the same current. Notice that the two 0.7V voltages are connected in series opposite. So they should subtract to zero: Vce is zero even while Vbe is 0.7v. However, BJTs are designed to give very low base currents. To do this we highly dope the emitter diode junction, and lightly dope the collector junction. The two diodes are not the same, so the junction voltages don't perfectly subtract to Vce=0. If you wanted BJT operation below Vce=0.2, you'd want a very symmetrical transistor with two identical junctions. But this gives you fairly high base currents in normal operation.Great books on such topics are:D. Ashby, "Electrical Engineering 101: Everything You Should Have Learned in School...but Probably Didn't"Horowitz/Hill Art of Electronics, read an online copy viagoogle books

author: Legion
author_rank: Frequent Contributor
timestamp: April 25, 2014, 08:36:29 pm
content: Quote from: IonizedGears on April 25, 2014, 06:32:06 pmI have AOE and I find it's description of transistors confusing and contradictory. They specifically point out that hFE is not a reliable statistic to use, but then proceed to setup circuits based on that parameter. I find a lot of things are stated as a matter of fact instead of explained.

author: LvW
author_rank: Frequent Contributor
timestamp: April 26, 2014, 10:30:22 am
content: Quote from: Legion on April 25, 2014, 08:36:29 pmOne really should know that not everything that is written down (in textbooks) is true.As another example of contradictory explanations, here are two quotations from "R.C. Jaeger: Microelectronic Circuit design":1.) "The three terminal currents are all exponentially dependent on the base-emitter voltage of the transistor",2.) "....injection of a small current into the base of the transistor produces a much larger current in both the collector and emitter terminals."