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@Laywah - aside the deep theory, I wanted to share this with you, because up until just stumbling across it, I had never had this explained so easily to me!! Our more advanced members will probably say, well of course, but for me NPN and PNP where really nothing more than labels!!
NPN (Negative-Positive-Negative): Switches negative voltages with positive voltage control. It has the effect of an inverter.
PNP (Positive-Negative-Positive): Switches positive voltages with negative voltage control. Also an inverter.
from: http://makezine.com/projects/fun-with-transistors/
NPN (Negative-Positive-Negative): Switches negative voltages with positive voltage control. It has the effect of an inverter.
PNP (Positive-Negative-Positive): Switches positive voltages with negative voltage control. Also an inverter.
from: http://makezine.com/projects/fun-with-transistors/
I rather would say that this applies only to the very specific (theoretical) example you have mentioned (current source for biasing).
I think, in most cases, linear BJT amplfier stages are biased with a VOLTAGE (via a low-resistive voltage divider).
At least, it is the goal to create a "stiff" voltage at the base - however, there are some other constraints setting certain limits.
Ian Getreu
- Jun 17, 2010
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This paragraph may be considered as self-serving - if so, I apologize in advance for that.
I have written a book many years ago that clearly explains the operation and theory behind the bipolar transistor. It covers most of the questions and topics in this discussion - as far as the BJT model is concerned. The book is called "Modeling the Bipolar Transistor" and is available at lulu.com/iangetreu. It is oriented towards the simulation programs like SPICE, so the first half covers all the models from the simple Ebers-Moll to the Gummel-Poon model. The second half covers how to measure the parameters that are needed for a model. For comments on the book, see the posting on June18, 2010 on this site , about 6 pages back from the last posting.
To answer some of the questions that have been rolling around, the BJT is best thought of as a voltage-driven device. In the common-emitter configuration for example, the Vbe causes the B-E junction to be forward biased. This generates a diode-type current of electrons into the base. The "efficiency" of the BJT is determined by how much of those electrons are swept across the base into the reverse-biased C-B junction and are collected in the collector region (hence the names "emitter" and "collector"). The electrons lost in the base come out the base terminal. The measure of the efficiency is the ratio of collector current to base current (ie the beta). This is why the base must be very thin in order to have a BJT - just putting 2 dioides back to back does not create a BJT.
The analogy for gain can be considered by thinking of a waterfall that is controlled by an open or closed valve. If the valve is open, all the water flows. If the valve is closed no water flows. So a small amount of energy (that needed to open and close the valve) controls a large amount of energy (the water flow). I assume the anlaogy is clear - the water flow is the collector/emitter current and the valve is the base-emitter voltage.
As to the arrow directions on the symbols, it refers to the flow of "positive" current - which is opposed to the flow of electrons. Back in the old days, before people realized that current consisted of the flow of negatively-charged electrons, they chose the flow directions assuming the flow consisted of positively-charged things. We still use that flow assumption for current. So, even though electrons flow from the emitter to the collector via the base, the current is assumed to flow in the other direction. Once that's understood, it's no big deal.
As for holes (the absence of electrons), it is a useful concept. If you picture a sea of electrons and one hole and by applying a voltage, the electrons move to fill that hole, the hole thereby moves from one end to the other. When you understand the original assumption of the positively-charged particles flowing, you can start to understand (not completely) the usefulness of the hole concept. It is just a conceptual way of looking at things - if you don't like it you can replace the word "hole" with "the electrons flow the other way, effectively moving the absence of the electron in the other direction". It is more complex than that, since it really refers to the outermost electrons on the silicon atom.
I hope I have helped.
I have written a book many years ago that clearly explains the operation and theory behind the bipolar transistor. It covers most of the questions and topics in this discussion - as far as the BJT model is concerned. The book is called "Modeling the Bipolar Transistor" and is available at lulu.com/iangetreu. It is oriented towards the simulation programs like SPICE, so the first half covers all the models from the simple Ebers-Moll to the Gummel-Poon model. The second half covers how to measure the parameters that are needed for a model. For comments on the book, see the posting on June18, 2010 on this site , about 6 pages back from the last posting.
To answer some of the questions that have been rolling around, the BJT is best thought of as a voltage-driven device. In the common-emitter configuration for example, the Vbe causes the B-E junction to be forward biased. This generates a diode-type current of electrons into the base. The "efficiency" of the BJT is determined by how much of those electrons are swept across the base into the reverse-biased C-B junction and are collected in the collector region (hence the names "emitter" and "collector"). The electrons lost in the base come out the base terminal. The measure of the efficiency is the ratio of collector current to base current (ie the beta). This is why the base must be very thin in order to have a BJT - just putting 2 dioides back to back does not create a BJT.
The analogy for gain can be considered by thinking of a waterfall that is controlled by an open or closed valve. If the valve is open, all the water flows. If the valve is closed no water flows. So a small amount of energy (that needed to open and close the valve) controls a large amount of energy (the water flow). I assume the anlaogy is clear - the water flow is the collector/emitter current and the valve is the base-emitter voltage.
As to the arrow directions on the symbols, it refers to the flow of "positive" current - which is opposed to the flow of electrons. Back in the old days, before people realized that current consisted of the flow of negatively-charged electrons, they chose the flow directions assuming the flow consisted of positively-charged things. We still use that flow assumption for current. So, even though electrons flow from the emitter to the collector via the base, the current is assumed to flow in the other direction. Once that's understood, it's no big deal.
As for holes (the absence of electrons), it is a useful concept. If you picture a sea of electrons and one hole and by applying a voltage, the electrons move to fill that hole, the hole thereby moves from one end to the other. When you understand the original assumption of the positively-charged particles flowing, you can start to understand (not completely) the usefulness of the hole concept. It is just a conceptual way of looking at things - if you don't like it you can replace the word "hole" with "the electrons flow the other way, effectively moving the absence of the electron in the other direction". It is more complex than that, since it really refers to the outermost electrons on the silicon atom.
I hope I have helped.
Ian Getreu,
I agree with just about everything you said above. I can say more about charge flow (current), however. Current consists of charge carrier movement of any polarity. There are just as many positive charges in the universe as negative ones. Examples are semiconductor holes, electrochemical ions, air cleaners, smoke precipitators, etc. There is no reason to accept the movement of negative charges in metals as the definitive direction of current. The correct way to think about this is use the mathematical convention (some times called conventional current) and assume positive charges are moving in a conductor from a positive voltage to a negative voltage. Do the calculations based on that principle. Then, if the real charges are negative, simply reverse the calculated flow to find the real flow direction.
A lot of folks seem to think that a hole is simply a missing electron in the conduction band. The hole in the valance band is every bit as real as a electron. It has its own mobility, effective mass, drift velocity, diffusion coefficients, etc. In other words, a hole is just another subatomic particle. That would not be true if a hole were simply a missing electron. This conceptual dilemma is caused by the inadequacy of the models used to describe what is really happening at the quantum level. It is a marvel that the model works as well as it does.
Ratch
I agree with just about everything you said above. I can say more about charge flow (current), however. Current consists of charge carrier movement of any polarity. There are just as many positive charges in the universe as negative ones. Examples are semiconductor holes, electrochemical ions, air cleaners, smoke precipitators, etc. There is no reason to accept the movement of negative charges in metals as the definitive direction of current. The correct way to think about this is use the mathematical convention (some times called conventional current) and assume positive charges are moving in a conductor from a positive voltage to a negative voltage. Do the calculations based on that principle. Then, if the real charges are negative, simply reverse the calculated flow to find the real flow direction.
A lot of folks seem to think that a hole is simply a missing electron in the conduction band. The hole in the valance band is every bit as real as a electron. It has its own mobility, effective mass, drift velocity, diffusion coefficients, etc. In other words, a hole is just another subatomic particle. That would not be true if a hole were simply a missing electron. This conceptual dilemma is caused by the inadequacy of the models used to describe what is really happening at the quantum level. It is a marvel that the model works as well as it does.
Ratch
Ian Getreu
- Jun 17, 2010
- 6
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- 6
Ratch,
I totally agree with you - I was just keeping it simple.
Of course, if current flow is due to positively charged particles, the direction will agree with the conventional current direction. I was simply responding to the original post in which he asked why the arrow directions were chosen that way on the sybmols. And the answer is that conventional current was originally (aribtrarily) selected as if positive particles were flowing.
As for holes, I was responding to the question raised as to whether they were "real". As I said in the post, it is a lot more complicated than I described. For anyone interested in solid-state physics and the deeper understanding of how semiconductors work (not just BJTs) they are totally real.
Thanks for your comments in the thread.
Ian
I totally agree with you - I was just keeping it simple.
Of course, if current flow is due to positively charged particles, the direction will agree with the conventional current direction. I was simply responding to the original post in which he asked why the arrow directions were chosen that way on the sybmols. And the answer is that conventional current was originally (aribtrarily) selected as if positive particles were flowing.
As for holes, I was responding to the question raised as to whether they were "real". As I said in the post, it is a lot more complicated than I described. For anyone interested in solid-state physics and the deeper understanding of how semiconductors work (not just BJTs) they are totally real.
Thanks for your comments in the thread.
Ian
Perhaps not too important, however: In order to be one small step more exact: ....consists of charge movement of any polarity. (The carriers move very slowly).
More than that, I agree, of course, to Ian Getreu`s contribution.
Ian Getreu
- Jun 17, 2010
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As with most postings, the topic has veered away from the original question. But that's OK.
I would like to address the question of whether a BJT should be considered as current driven or voltage driven. To a certain extent, the answer is 'both" or "either".
This is almost like arguing Norton's/Thevenin's Theorems in which, if you have an ideal current source of 1A in parallel with a 1ohm resistor inside a box that has the 2 terminals coming out and then you have another identical box with an ideal 1V voltage source in series with a 1 ohm resistor and those terminals coming out, how do you tell the difference without opening the boxes? (Ignore the fact that you can't create ideal current and voltage sources). Mathematically, they are identical so, in theory, you can't tell the difference. [Aside: In practice, you can tell the difference. This is a well-known interview question. Answer at the bottom of this posting - to allow you time to try to figure it out yourself. It's a tough one.]
The problem with the current-driven and the voltage-driven advocates (including me) is that they have not stated their position accurately. In many circumstances, a simple current model in which you have Ib going in to the base and beta*Ib in the collector is more than adequate. This is a simple first-order model and I'm a big advocate of always using the simplest model that will do the job.
The real statement that the current-driven advocates should use is "For my applications (or for many applications) looking at the BJT as current-driven works and works well." Extrapolating from that to advocating that this is the best way to look at it for all applications is wrong.
And the same applies to the voltage-driven advocates. Stating that "since this works for me in my applications it is therefore the best way for everyone else to look at in their applications" is also wrong.
Here is my more well-thought out position: If a current-driven model works for you, use it. If a voltage-driven model works for you, use it. But don't insist that yours is the only right one.
The voltage-driven model (and by this I mean the voltage-driven way of looking at the BJT) is a more natural way of looking at the BJT from a solid-state physics and understanding of the basic operation perspective. For example, the normal, active region of operation is defined as the BE junction is forward biased and the CB junction is reverse-biased. I can picture how that is done by providing the appropriate voltages across these junctions. I cannot picture how that is done by looking at the base current.
The genius of the analysis by Herman Gummel and Sam Poon in their description of how the BJT works is the realization that the transistor saturation current (Is) - which is not the saturation current of the BE or CB junctions- is a fundamental parameter of the BJT. It inherently contains within it the explanation for beta versus Ic, why the output resistance variation with Ic can be modeled by one parameter (the Early voltage - named after Jim Early) and all the other second-order effects. It also inhently models all other regions of operation (inverse, saturation and off).
It also explains the reciprocity property that people talk about when they derive the Ebers-Moll model. Everyone says "It can be shown that alphaE*IsBE = alphaC*IsBC". [These equal Is]. But no-one ever shows that. Primarily because no-one knows how to. But the Gummel-Poon model shows that reciprocity is just saying the integral of the doping level in the base from E to C is the the same as the integral of that doping level in the base from C to E - which is intuitively obvious. If you don't understand this paragraph, don't worry - it's not important.
Again, if your application is not affected by these second-order effects or they can be adequately covered by simple modifications then the current-driven model is perfectly fine.
The bottom line (and I'm sorry it took so long to get there) is the current-driven model is just as good as the voltage-driven model for applications where the first-order effects are the only important ones (and that includes many, many applications). However, if second-order effects (beta v Ic, ro v Ic etc) are important in your application (usually when you are stretching the limits of the device or trying to stretch certain specifications), then the voltage-driven model is a more natural one to use.
Answer to the boxes question: One is warm.
I would like to address the question of whether a BJT should be considered as current driven or voltage driven. To a certain extent, the answer is 'both" or "either".
This is almost like arguing Norton's/Thevenin's Theorems in which, if you have an ideal current source of 1A in parallel with a 1ohm resistor inside a box that has the 2 terminals coming out and then you have another identical box with an ideal 1V voltage source in series with a 1 ohm resistor and those terminals coming out, how do you tell the difference without opening the boxes? (Ignore the fact that you can't create ideal current and voltage sources). Mathematically, they are identical so, in theory, you can't tell the difference. [Aside: In practice, you can tell the difference. This is a well-known interview question. Answer at the bottom of this posting - to allow you time to try to figure it out yourself. It's a tough one.]
The problem with the current-driven and the voltage-driven advocates (including me) is that they have not stated their position accurately. In many circumstances, a simple current model in which you have Ib going in to the base and beta*Ib in the collector is more than adequate. This is a simple first-order model and I'm a big advocate of always using the simplest model that will do the job.
The real statement that the current-driven advocates should use is "For my applications (or for many applications) looking at the BJT as current-driven works and works well." Extrapolating from that to advocating that this is the best way to look at it for all applications is wrong.
And the same applies to the voltage-driven advocates. Stating that "since this works for me in my applications it is therefore the best way for everyone else to look at in their applications" is also wrong.
Here is my more well-thought out position: If a current-driven model works for you, use it. If a voltage-driven model works for you, use it. But don't insist that yours is the only right one.
The voltage-driven model (and by this I mean the voltage-driven way of looking at the BJT) is a more natural way of looking at the BJT from a solid-state physics and understanding of the basic operation perspective. For example, the normal, active region of operation is defined as the BE junction is forward biased and the CB junction is reverse-biased. I can picture how that is done by providing the appropriate voltages across these junctions. I cannot picture how that is done by looking at the base current.
The genius of the analysis by Herman Gummel and Sam Poon in their description of how the BJT works is the realization that the transistor saturation current (Is) - which is not the saturation current of the BE or CB junctions- is a fundamental parameter of the BJT. It inherently contains within it the explanation for beta versus Ic, why the output resistance variation with Ic can be modeled by one parameter (the Early voltage - named after Jim Early) and all the other second-order effects. It also inhently models all other regions of operation (inverse, saturation and off).
It also explains the reciprocity property that people talk about when they derive the Ebers-Moll model. Everyone says "It can be shown that alphaE*IsBE = alphaC*IsBC". [These equal Is]. But no-one ever shows that. Primarily because no-one knows how to. But the Gummel-Poon model shows that reciprocity is just saying the integral of the doping level in the base from E to C is the the same as the integral of that doping level in the base from C to E - which is intuitively obvious. If you don't understand this paragraph, don't worry - it's not important.
Again, if your application is not affected by these second-order effects or they can be adequately covered by simple modifications then the current-driven model is perfectly fine.
The bottom line (and I'm sorry it took so long to get there) is the current-driven model is just as good as the voltage-driven model for applications where the first-order effects are the only important ones (and that includes many, many applications). However, if second-order effects (beta v Ic, ro v Ic etc) are important in your application (usually when you are stretching the limits of the device or trying to stretch certain specifications), then the voltage-driven model is a more natural one to use.
Answer to the boxes question: One is warm.
Ian Getreu,
"I would like to address the question of whether a BJT should be considered as current driven or voltage driven. To a certain extent, the answer is 'both" or "either"."
I believe you are answering the question, "What is the best design method?" If you are asking about whether the BJT is current or voltage controlled, I don't think there is any ambiguity. The BJT in the active region responds only to Vbe. If you drive it with a current, then you are in effect putting a large external resistance in series with the BJT, and are implementing a BJT plus base resistor circuit instead of the BJT device alone. I believe the OP asked how the device works, not how it could be applied. Although the models are interesting and useful, they only show what the device does, not prove how it works.
Ratch
"I would like to address the question of whether a BJT should be considered as current driven or voltage driven. To a certain extent, the answer is 'both" or "either"."
I believe you are answering the question, "What is the best design method?" If you are asking about whether the BJT is current or voltage controlled, I don't think there is any ambiguity. The BJT in the active region responds only to Vbe. If you drive it with a current, then you are in effect putting a large external resistance in series with the BJT, and are implementing a BJT plus base resistor circuit instead of the BJT device alone. I believe the OP asked how the device works, not how it could be applied. Although the models are interesting and useful, they only show what the device does, not prove how it works.
Ratch
But if you drive the base with a current source, the large resistance becomes a very, very large resistance approaching infinity. A review of Circuits-101 should convince anyone that an infinite resistance is the same as an open circuit. So when driving the BJT with a current source the current appears to the BJT to magically come from an open circuit. In that case there is no base resistor circuit and one can measure current drive as an inherent device characteristic.
An ideal current source has an infinite voltage to propel the charge through the infinite resistance. By the definition, the ideal current source dispenses a defined constant current, therefore the circuit cannot be considered open. The Vbe of the BJT will be a particular value with a constant current source, and the same Vbe applied with a voltage source will cause the same current.
Ratch
Ian Getreu
- Jun 17, 2010
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Ratch,
You are right. I was primarily considering design.
If you have a design where the only important characteristic of the BJT is beta, the current gain, then looking at the device as current-driven is OK - from a design perspective.
If you have a design where you need to understand and/or design with or around the transistor then you have to understand how the BJT works. And, since you define the operation via forward-and reverse-biased junctions, only a voltage-based approach makes sense.
Base current is a result of the operation - it is not a driver.
I do disagree with your comment about the models. The best models, such as the Gummel-Poon, are based on the physics of the device so they do explain how the BJT works.
Laplace,
There are several things wrong with your argument.
1. You cannot define both the base current and the Vbe - one or the other. If you use a current source in the base, the BJT will adjust the Vbe to accomodate it.
2. It is not legitimate to say a large resistance = infinity. You just defined an ideal current source and no-one knows how to make one of those. So the base current does not come magically from an open circuit - never has, never will. No matter what you do, there is a base circuit (even if there is only an ideal current source).
3. As I mentioned above, the base current is a result of the BJT operation, not the cause. So the BJT has to adjust the Vbe and Vbc to achieve the base current you are forcing upon it.
You are right. I was primarily considering design.
If you have a design where the only important characteristic of the BJT is beta, the current gain, then looking at the device as current-driven is OK - from a design perspective.
If you have a design where you need to understand and/or design with or around the transistor then you have to understand how the BJT works. And, since you define the operation via forward-and reverse-biased junctions, only a voltage-based approach makes sense.
Base current is a result of the operation - it is not a driver.
I do disagree with your comment about the models. The best models, such as the Gummel-Poon, are based on the physics of the device so they do explain how the BJT works.
Laplace,
There are several things wrong with your argument.
1. You cannot define both the base current and the Vbe - one or the other. If you use a current source in the base, the BJT will adjust the Vbe to accomodate it.
2. It is not legitimate to say a large resistance = infinity. You just defined an ideal current source and no-one knows how to make one of those. So the base current does not come magically from an open circuit - never has, never will. No matter what you do, there is a base circuit (even if there is only an ideal current source).
3. As I mentioned above, the base current is a result of the BJT operation, not the cause. So the BJT has to adjust the Vbe and Vbc to achieve the base current you are forcing upon it.
That is a rather slim ledge to be clinging to. With a good current source, the difference between infinity and not measurable.
Ian Getreu,
I think we both agree on how the BJT operates.
"I do disagree with your comment about the models. The best models, such as the Gummel-Poon, are based on the physics of the device so they do explain how the BJT works."
I am not convinced. Looking over the Gummel-Poon (G-P) model, I see it considers the current generators, resistance, capacitance, and temperature. In other words, it considers and combines the effects of those parameters into a model. All models do that. G-P is quite sophisticated in that respect. By the physics of the device, I mean discerning that a BJT is bipolar, operates by diffusion which in turn is controlled by a voltage, has different concentration dopings and sizes for each of its three parts. These are not revealed by a model, and you cannot explain how a BJT does what it does no matter how familiar your are with G-P. You can only tell me what the device does.
Ratch
I think we both agree on how the BJT operates.
"I do disagree with your comment about the models. The best models, such as the Gummel-Poon, are based on the physics of the device so they do explain how the BJT works."
I am not convinced. Looking over the Gummel-Poon (G-P) model, I see it considers the current generators, resistance, capacitance, and temperature. In other words, it considers and combines the effects of those parameters into a model. All models do that. G-P is quite sophisticated in that respect. By the physics of the device, I mean discerning that a BJT is bipolar, operates by diffusion which in turn is controlled by a voltage, has different concentration dopings and sizes for each of its three parts. These are not revealed by a model, and you cannot explain how a BJT does what it does no matter how familiar your are with G-P. You can only tell me what the device does.
Ratch
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It doesn't matter. A good current source contains a resistor external to the BJT, and the current source is not open. Great applications can be made with current sources and a BJT, but the BJT is still controlled by Vbe.
Ratch
I think, a good visualization is the following:
Using the classical base biasing scheme, we have a voltage divider consisting of a resistor (R1) and the parallel combination of R2 and the B-E path.
If we drop R2 we still have a voltage division between R1 (of course, with another value) and the B-E path.
The problem seems to be narrowing and clarifying itself. With that in mind, a warning should be prominently posted for the unsuspecting.
Electrically they are equivalent. But at open circuit the Norton version is dissipating at least (Imax*Imax*Req) and the Thevenin is dissipating nothing (somewhat ideal sources). So you can tell which is which by which one is dissipating the most/least heat at open circuit.
Merlin3189
- Aug 4, 2011
- 250
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At the risk of further red herrings, can I ask about something which has long puzzled me about these holes.
Ratch said,
"A lot of folks seem to think that a hole is simply a missing electron in the conduction band. The hole in the valance band is every bit as real as a electron. It has its own mobility, effective mass, drift velocity, diffusion coefficients, etc."
(Put aside his distinction between conduction band and valence band holes.)
If this were true, how would you explain the behaviour of a hole?
When a hole moves from A to B,
B looses the charge of one electron and looses the mass of one electron.
So it would seem that a hole has a charge of +1.602176565(35)×10−19 C and a mass of -9.10938291(40)×10−31 kg
When in an electric field, a hole moves - but which way?
Due to its positive charge there is a force away from positive and towards negative.
As a massive particle I guess it obeys Newtons laws, so a = F / m
Since m is negative, then a has the opposite sign to F and so the acceleration is in the opposite sense to the force and holes should move towards the positive!
If I understand the hole merchants, this is the opposite of what they claim.
Ratch said,
"A lot of folks seem to think that a hole is simply a missing electron in the conduction band. The hole in the valance band is every bit as real as a electron. It has its own mobility, effective mass, drift velocity, diffusion coefficients, etc."
(Put aside his distinction between conduction band and valence band holes.)
If this were true, how would you explain the behaviour of a hole?
When a hole moves from A to B,
B looses the charge of one electron and looses the mass of one electron.
So it would seem that a hole has a charge of +1.602176565(35)×10−19 C and a mass of -9.10938291(40)×10−31 kg
When in an electric field, a hole moves - but which way?
Due to its positive charge there is a force away from positive and towards negative.
As a massive particle I guess it obeys Newtons laws, so a = F / m
Since m is negative, then a has the opposite sign to F and so the acceleration is in the opposite sense to the force and holes should move towards the positive!
If I understand the hole merchants, this is the opposite of what they claim.
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