Can someone explain me how a BJT works?

[duke37]

You brought in the term power, presumably this was power delivered to the output or in other words the load. This may be a resistor to generate a voltage or a motor or a speaker.

A heat sink is a component to take away heat from a hot component and spread it around. The most common is a block of aluminium which is finned. It may be assisted with a fan or water cooling.

 

[irq20xdfr]

Great! duke37... thank you for the clarification

 

[LvW]

Ok that explanation makes thing more clear
One thing
I am currently reading the book tech yourself electronics at the same time as I read the art of electronics. I know that one is not comparable to the other. Obviously I found more easy readable the first one.

Do you know if my approach is right ?
I use the first one to understand the concepts and the last one to get more exactly details and when I don't understand well I research the terms on Google View videos in YouTube and ask in forums. Is it ok ?

I`ve got the impression that you are trying to use several different knowledge sources to reveal the "secrets" of the transistors (BJT and FET).
In general, this is a good idea. However, I like to remind you that you have to be very careful because not everything which is printed or published in the internet is correct. Unfortunately, this is even true for textbooks. In particular, for the working principle of the bipolar transistor you will be confronted with two different explanations. Sometimes, the bipolar transistor is even described as a "controllable resistor" (which, undoubtly, is incorrect).
Very often, some simple explanations can conflict with scientific validity. Should you rely on wrong stuff just because it is simple? I don’t think so.
My recommendation: Don`t be confused; use reliable sources and - primarily - your own brain.
Good luck.

PS: For my opinion, "The Art of Electronics" (Horowitz/Hill) is a good and reliable source of information.

 

[Colin Mitchell]

The presenter appears to believe that Ib controls Ic, which it does not. Vbe controls Ic.

Ratch

Not true AT ALL. The voltage Vbe is a mere bi-product of the current entering the base and will vary considerably from one type of transistor to the other. You can NEVER measure Vbe and work out the value of collector current.
Ib controls Ic

 

[irq20xdfr]

Ok, i am first trying to understand how a diode and a PN juntion works completely, trying to figure out every term like bias, resonance, frecuency at a deep deep level and then continue with all the transistors stuff, specially i can't find a good site or resource where all equations are detailed explained, can you point me one? or is it really necessary for me to understand the whole equations thing? I am the kind of person that when don't understand something at a satisfying level keep looking for more info even when am not going to use it in the everyday work... should i keep myself with the concept explanation or should i continue to striving to understand the formulas using the Arts Of Electronics book?

 

[(*steve*)]

There are several mathematical models of the BJT (for example). Each differs in the degree to which it models the transistor. Do you want to dive straight in to a stack of formulae which model stuff you don't know about yet or start with simpler approximations (which may be fine for many purposes) and refine your knowledge as you learn more.

Let me offer some insight. You will likely never have an equation which describes in perfect detail what a particular transistor will do. In many cases, in the region you're operating the device, the manufacturing tolerances which cause devices to differ from each other may have an effect stronger than the difference between a smoke and a more complex model.

Think of your understanding of electronics as some level of abstraction. You might understand transistors as simple switches controlled by the base terminal. Then you might as to this understanding that unlike real switches they are never completely turned off or (more importantly) turned on. Later you add to this an understanding of the state between on and off.

When you read the arguments of Colin and Ratch, you may perceive them as polar opposites. However, they simply describe the operation at different levels of abstraction.

Ratch's position is closer to the physics of the device and he would argue he is correct.

Colin is operating at a higher level of abstraction, and he would argue that it is practically more applicable.

Both of them are correct, and both of them are wrong in not accepting the others position as valid.

The Art of Electronics has very little mathematics in it (aside from the early stuff on KVL and KCL). It teaches initially from a higher level of abstraction.

Compare this with many semiconductor books which start out teaching you about silicon, the effects of shopping in the crystal lattice, and models to describe the effect this has on voltage fields and the flow of charge via minority and majority carriers. These start off at a very low level of abstraction and may eventually tell you something useful if you want to (say) use a transistor to switch a load.

So even beyond your stated purpose of gathering all the equations, you need to decide where to start; will you start at the practical and move to the theoretical or vice versa?

Remember that in theory the is no difference been theory and practice, but as Colin and Ratch's differing views attest, in practice there is.

Oh, I should also point out that LvW is an educator with significant experience and he probably understands this stuff better than most of us here.

 

[Ratch]

Not true AT ALL. The voltage Vbe is a mere bi-product of the current entering the base and will vary considerably from one type of transistor to the other. You can NEVER measure Vbe and work out the value of collector current.
Ib controls Ic

You picked the wrong subject to aver is true, because it can be proven definitively that a single BJT is a transconductace amplifier. A BJT is not a current amplifier which you and the video presenter seem to think is the case. This means that the Vbe voltage controls the collector current, not the Ib. The proof of that fact is the physics of the transistor operation, which I explained in post #13 of this thread. In that explanation, I showed why Vbe controls the collector current, and Ib is a waste current by-product that is an indicator of the collector current, but does not control it. Didn't you read that post? Although it can be done, it is not practical to define the collector current in terms of Vbe. Likewise, it is not practical to define the collector current in terms of Ib and the highly variable parameter beta, either. Nevertheless, Vbe controls Ic.

So, if you want to effectively defend your position, you must either point out the mistakes of my post #13, or present your particular assertion using semiconductor physics.

Ratch

 

[Ratch]

Ok, i am first trying to understand how a diode and a PN juntion works completely, trying to figure out every term like bias, resonance, frecuency at a deep deep level and then continue with all the transistors stuff, specially i can't find a good site or resource where all equations are detailed explained, can you point me one? or is it really necessary for me to understand the whole equations thing? I am the kind of person that when don't understand something at a satisfying level keep looking for more info even when am not going to use it in the everyday work... should i keep myself with the concept explanation or should i continue to striving to understand the formulas using the Arts Of Electronics book?

There are plenty of books and literature on how diodes and transistors work. Go as deep as you want. I also gave you a thumbnail explanation in post #13 of this thread. Did you read it? Any questions? No one can tell you how to study this subject. No one source has all knowledge on this subject. Be careful of sources like the internet, where anyone can publish anything without fact checking. As I said previously, study methods are not what I have an expert knowledge of.

Ratch

 

[hevans1944]

is it really necessary for me to understand the whole equations thing?

That depends on whether you care if your "understanding" is pretty much correct or just a poor approximation to reality. It could be argued that any understanding is just an approximation to reality, but some approximations are better than others. For example, in post #24, Colin disputes the claim that Vbe controls Ic, claiming instead that Ib controls Ic. Ratch stands by the Ebers-Moll explanation where Vbe controls Ic.

Both viewpoints are models of transistor behavior, and neither one is an exact representation of reality. If we assume that Vbe is constant (as Colin does) and apply a variable voltage source to the base-emitter junction through a current-limiting resistor, we will find that the the collector current, Ic is approximately equal to Ib multiplied by a constant, hfe or beta. If you then measure the base-emitter voltage, Vbe, you will find that for various collector currents the measured value varies with collector current. It will, in fact, closely follow the Ebers-Moll model. In either case, the voltage applied to the base-emitter junction is controlling the collector current.

If you want to follow further down the rabbit hole, Alice, you need to Read This Wikipedia introduction to BJTs. Be sure to check out the references at the end of the article and brush up on your math skills. Math is the language of science. Without it, almost any explanation of reality is just pretty pictures and hand waving, good enough perhaps to satisfy the masses, but useless in predicting what will happen. Hypothesize. Experiment. Measure. Explain. Predict. Wash, rinse, and repeat. That's the "Scientific Method" in a nutshell.

There is absolutely no way to "prove" a theory (an hypothesis). And it takes just one counter-example to dis-prove any theory. Applying the scientific method over long periods of time, using independent investigators testing a particular hypothesis or theory, will lend credence to that theory, but it never proves it. Age simply makes theories that have not yet been dis-proven palatable and acceptable. Who woulda thunk some young upstart German would offer up the General Theory of Relativity as a counter-example to Newtons Three Laws of Motion? And get a Nobel Prize, not for that particular theory, but for a theory of how photoelectrons are produced in matter. Sure, photo-electricity is useful, but without Einstein's General Theory of Relativity there would be no way to pin-point with millimeter accuracy any location on Earth using a simple radio receiver and a constellation of man-made, orbiting, Global Positioning System (GPS) satellites. You want some "interesting" math? Go read up on how that works.

Einstein's work that led to his Nobel Prize ushered in quantum mechanics, probably much to his later dismay because God does play with dice... and loads them, too... which Albert could never accept. Maybe someone will discover how a Universe seemingly built on chaos and the Second Law of Thermodynamics can evolve a species like us... or maybe the human race is just an anomaly, background noise, of no significance at all.

Good luck with your studies. Electronics is a fascinating adventure.

 

[Ratch]

There are several mathematical models of the BJT (for example). Each differs in the degree to which it models the transistor. Do you want to dive straight in to a stack of formulae which model stuff you don't know about yet or start with simpler approximations (which may be fine for many purposes) and refine your knowledge as you learn more.

Let me offer some insight. You will likely never have an equation which describes in perfect detail what a particular transistor will do. In many cases, in the region you're operating the device, the manufacturing tolerances which cause devices to differ from each other may have an effect stronger than the difference between a smoke and a more complex model.

Think of your understanding of electronics as some level of abstraction. You might understand transistors as simple switches controlled by the base terminal. Then you might as to this understanding that unlike real switches they are never completely turned off or (more importantly) turned on. Later you add to this an understanding of the state between on and off.

When you read the arguments of Colin and Ratch, you may perceive them as polar opposites. However, they simply describe the operation at different levels of abstraction.

Ratch's position is closer to the physics of the device and he would argue he is correct.

Colin is operating at a higher level of abstraction, and he would argue that it is practically more applicable.

Both of them are correct, and both of them are wrong in not accepting the others position as valid.

The Art of Electronics has very little mathematics in it (aside from the early stuff on KVL and KCL). It teaches initially from a higher level of abstraction.

Compare this with many semiconductor books which start out teaching you about silicon, the effects of shopping in the crystal lattice, and models to describe the effect this has on voltage fields and the flow of charge via minority and majority carriers. These start off at a very low level of abstraction and may eventually tell you something useful if you want to (say) use a transistor to switch a load.

So even beyond your stated purpose of gathering all the equations, you need to decide where to start; will you start at the practical and move to the theoretical or vice versa?

Remember that in theory the is no difference been theory and practice, but as Colin and Ratch's differing views attest, in practice there is.

Oh, I should also point out that LvW is an educator with significant experience and he probably understands this stuff better than most of us here.

Abstractions and models are fine for design work, but the subject of this thread is "How a BJT works". Only one of us can be correct about this subject. Both of us are correct in not accepting each other's position, if we believe our own position is correct. I hope to prove my position is correct when Colin presents his arguments. Speaking of models, they only tell you what the device does. Models do not tell you how a device works.

Ratch

 

[Ratch]

That depends on whether you care if your "understanding" is pretty much correct or just a poor approximation to reality. It could be argued that any understanding is just an approximation to reality, but some approximations are better than others. For example, in post #24, Colin disputes the claim that Vbe controls Ic, claiming instead that Ib controls Ic. Ratch stands by the Ebers-Moll explanation where Vbe controls Ic.

Both viewpoints are models of transistor behavior, and neither one is an exact representation of reality. If we assume that Vbe is constant (as Colin does) and apply a variable voltage source to the base-emitter junction through a current-limiting resistor, we will find that the the collector current, Ic is approximately equal to Ib multiplied by a constant, hfe or beta. If you then measure the base-emitter voltage, Vbe, you will find that for various collector currents the measured value varies with collector current. It will, in fact, closely follow the Ebers-Moll model. In either case, the voltage applied to the base-emitter junction is controlling the collector current.

If you want to follow further down the rabbit hole, Alice, you need to Read This Wikipedia introduction to BJTs. Be sure to check out the references at the end of the article and brush up on your math skills. Math is the language of science. Without it, almost any explanation of reality is just pretty pictures and hand waving, good enough perhaps to satisfy the masses, but useless in predicting what will happen. Hypothesize. Experiment. Measure. Explain. Predict. Wash, rinse, and repeat. That's the "Scientific Method" in a nutshell.

There is absolutely no way to "prove" a theory (an hypothesis). And it takes just one counter-example to dis-prove any theory. Applying the scientific method over long periods of time, using independent investigators testing a particular hypothesis or theory, will lend credence to that theory, but it never proves it. Age simply makes theories that have not yet been dis-proven palatable and acceptable. Who woulda thunk some young upstart German would offer up the General Theory of Relativity as a counter-example to Newtons Three Laws of Motion? And get a Nobel Prize, not for that particular theory, but for a theory of how photoelectrons are produced in matter. Sure, photo-electricity is useful, but without Einstein's General Theory of Relativity there would be no way to pin-point with millimeter accuracy any location on Earth using a simple radio receiver and a constellation of man-made, orbiting, Global Positioning System (GPS) satellites. You want some "interesting" math? Go read up on how that works.

Einstein's work that led to his Nobel Prize ushered in quantum mechanics, probably much to his later dismay because God does play with dice... and loads them, too... which Albert could never accept. Maybe someone will discover how a Universe seemingly built on chaos and the Second Law of Thermodynamics can evolve a species like us... or maybe the human race is just an anomaly, background noise, of no significance at all.

Good luck with your studies. Electronics is a fascinating adventure.

I would like to correct some of the statements above. I base my understanding of how a BJT works on the physics of the semiconductor, not on models. Models tell you what a device does, but not how it works. I consider evolution a pseudoscience. The idea that high complexity can come from simplicity is pure bunk. No one has ever found an example of that happening in the physical universe. If you see a complex entity like a life form existing, consider an alternative explanation.

Ratch

 

[LvW]

Not true AT ALL. The voltage Vbe is a mere bi-product of the current entering the base and will vary considerably from one type of transistor to the other. You can NEVER measure Vbe and work out the value of collector current.
Ib controls Ic

Colin - some month ago (in April 2016) we had a similar discussion in this forum
https://www.electronicspoint.com/posts/1690795/

In the course of this discussion I have asked you several technical questions (in response to your claim of BJT current control). Unfortunately, you did not give any answer. More than that, it would be really interesting and helpful if you would support your claim with some explanations/justifications.

 

[irq20xdfr]

Ok, thanks everybody...
I tried to understand the Vbe and V-etc things...
I suppouse thats why they teach you in a complete course of electronics, i just try to understand that field as a second field. I am currently reading the Teach Yourself Electronics book(very simple but i think it´s ok, id like to know what model uses Vbe or current) and im planning to read the chapters about that in the Arts Of Electronics book, also im planning to take a look to the course about MOSFET's: https://www.coursera.org/learn/mosfet/

Thank you so much for your contributions. I am going to continue in my research to understand the math in a simple but fully way, you know, like in understand the basic formulas behind but not to dive too much on it, i am not the math kind guy, i plan to, but currently am not. I just want to know how a transistor works in order to understand how a floating gate works so i can understand more about memories and EEPROMS and accomplish my final project that is read a memory in a modem to reverse engine the software, that's what i do and what im good at, that's why i don't want to get into the math too much and i need some complete but reasonable simple resource or resources to understand how a BJT and FET works.

Could you please point me to some extra resources besides those i already have, that
1. Explain how a BJT works in a conceptual way (not math)
2. Explain the math behind in a simple way
3. Explain the difference between control through Vbe or Ic (at least that's more or less what i understood)

PS. The only aproximation that i had on the vbe or current thing was in here, i dont know if it´s complete...
http://amasci.com/amateur/transis2.html that guy took a physics type point of view...

And sorry for my english, i see too much people from many countries here and i completely understand what they write... between work and other things i try to do my best in reading all that you post and spell right..

Thank you!!

 

[Ratch]

Ok, thanks everybody...
I tried to understand the Vbe and V-etc things...
I suppouse thats why they teach you in a complete course of electronics, i just try to understand that field as a second field. I am currently reading the Teach Yourself Electronics book(very simple but i think it´s ok, id like to know what model uses Vbe or current) and im planning to read the chapters about that in the Arts Of Electronics book, also im planning to take a look to the course about MOSFET's: https://www.coursera.org/learn/mosfet/

Thank you so much for your contributions. I am going to continue in my research to understand the math in a simple but fully way, you know, like in understand the basic formulas behind but not to dive too much on it, i am not the math kind guy, i plan to, but currently am not. I just want to know how a transistor works in order to understand how a floating gate works so i can understand more about memories and EEPROMS and accomplish my final project that is read a memory in a modem to reverse engine the software, that's what i do and what im good at, that's why i don't want to get into the math too much and i need some complete but reasonable simple resource or resources to understand how a BJT and FET works.

Could you please point me to some extra resources besides those i already have, that
1. Explain how a BJT works in a conceptual way (not math)
2. Explain the math behind in a simple way
3. Explain the difference between control through Vbe or Ic (at least that's more or less what i understood)

PS. The only aproximation that i had on the vbe or current thing was in here, i dont know if it´s complete...
http://amasci.com/amateur/transis2.html that guy took a physics type point of view...

And sorry for my english, i see too much people from many countries here and i completely understand what they write... between work and other things i try to do my best in reading all that you post and spell right..

Thank you!!

Bill Beaty's site called Amazing Science, that you reference above is basically correct. One of his virtues is that he does not use the scientific slang lingo of "current flow". Instead, he uses the correct description of "charge flow". One thing does bother me about that site, however, is that he never mentions diffusion. Diffusion is an important aspect of semiconductors that determines BJT operation, and is one of the major differences between BJTs and FETs. The base voltage (Vbe) controls the diffusion rate in a BJT.

Good luck studying just about any science without the math. One of the uses of mathematics is a bookkeeping method for science. The more you use math in science, the more in depth you can study science. Otherwise, scientific knowledge becomes too verbose and clumsy to apply. "Simplified" study texts don't go much into the innermost details, do they?

Ratch

 

[hevans1944]

And sorry for my english,

Your English is okay.

Although this forum is based on English, it attracts worldwide participants. Some of us sometimes struggle to understand when people write, and English is not their mother-tongue, but everyone is welcome here. My southern neighbors speak Spanish fluently, but unfortunately I do not. When I have visited Mexico, the people I met always politely tried to accommodate the gringo. I hope that most of us here will try to do the same thing: accommodate, understand, and help.

 

[irq20xdfr]

Bill Beaty's site called Amazing Science, that you reference above is basically correct. One of his virtues is that he does not use the scientific slang lingo of "current flow". Instead, he uses the correct description of "charge flow". One thing does bother me about that site, however, is that he never mentions diffusion. Diffusion is an important aspect of semiconductors that determines BJT operation, and is one of the major differences between BJTs and FETs. The base voltage (Vbe) controls the diffusion rate in a BJT.

Good luck studying just about any science without the math. One of the uses of mathematics is a bookkeeping method for science. The more you use math in science, the more in depth you can study science. Otherwise, scientific knowledge becomes too verbose and clumsy to apply. "Simplified" study texts don't go much into the innermost details, do they?

Ratch

Thank you Ratch, as you said, i understood clearer the concept of current thanks to that site. Also i got a glimpse about the voltage and current relationship in transistors.

I will be waiting for more articles from all of you to get a more deeper concept about the things a posted in previous message.

Thank you again!

 

[irq20xdfr]

Your English is okay.

Although this forum is based on English, it attracts worldwide participants. Some of us sometimes struggle to understand when people write, and English is not their mother-tongue, but everyone is welcome here. My southern neighbors speak Spanish fluently, but unfortunately I do not. When I have visited Mexico, the people I met always politely tried to accommodate the gringo. I hope that most of us here will try to do the same thing: accommodate, understand, and help.

Hye hevans1944, thank you for your words, make me feel more confortable...

I noticed that most of the quality information is in english forums and articles.
I will continue reading my books an articles as well as your posts and resources to get a full understanding about that and finally understand how a DRAM, EEPROM and FLASH memories work.

Thanks!