Voltage Divider Bias: Removing Electrons from the Base

In summary, Voltage divider bias means that we are not drawing any electrons from the base. If we maintain the base at a positive potential, then the electrons from Vcc can flow through the base to the collector and reach the positive terminal.
  • #1
sudar_dhoni
90
0
In Voltage divider bias ,
we are not drawing any electrons from the base.
Then how can the electron current pass through the base and go to the collector and reach the positive terminal.
Only if You draw electrons from the base by maintaining the base at positive potential,
then only the electrons from Vcc can flow through the base and to the collector and reach the positive terminal of Vcc.

The need for removal of electrons from the base is that.
In the first circuit shown,
when the base emitter is forward biased, the holes in the P region diffuse into the N region and the holes in the base are now filled by electrons as a result the base becomes the negatively charged. Now the electrons from Vcc negative terminal cannot pass through the transistor as they have to cross the base which is now negatively charged . But since the base is made positive the electrons that filled up the holes are attracted as a result the base now consists of holes and the electrons from - Vcc can now flow through the transistor .
Since in Voltage divider bias this provision( Vbb) is not there , then how can electrons be withdrawn from the base and how can current flow through the transistor.
 

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  • #2
Why do you think an amplifier set up with voltage divider bias will have no base current?
 
  • #3
Averagesupernova said:
Why do you think an amplifier set up with voltage divider bias will have no base current?

I am speaking only in terms of electron movement and not conventional current.
The first thing should happen in any transistor is that , we have to draw electrons from the base . Then only collector electron current begins to flow from Vcc (-) to Vcc +.
This does not happen.
If you feel this happens , then please explain it.
 
  • #4
I still refer you to my original post. Electron movement and conventional current are the same thing ignoring direction. Analyzing the circuit you have posted is simple enough with either electron flow or conventional current flow. So again I ask you why you think there is no current (or no electrons moving) into/out of the base of the transistor?
 
  • #5
Averagesupernova said:
I still refer you to my original post. Electron movement and conventional current are the same thing ignoring direction. Analyzing the circuit you have posted is simple enough with either electron flow or conventional current flow. So again I ask you why you think there is no current (or no electrons moving) into/out of the base of the transistor?

Actually conventional flow is not reality,
only electron flow is reality.
I have clearly stated in the beginning the need for considering electrons.They are the one who sit in the holes of base and block the Vcc electrons from crossing the base and going into the collector.
If you consider conventional current (flow of positive charges)
you cannot give an explanation like the one i have given.
Anyways where is the answer to my question.
you are giving a one line answer,
show the proof behind your explanation.If possible mark it on the diagram.
 
  • #6
Ok fine. We'll go with electron flow. NOW WHY DO YOU THINK THERE IS NO CURRENT LEAVING THE BASE OF THE TRANSISTOR? I keep giving you a one liner because you refuse to answer the question that I have asked. Maybe I'm misunderstanding you. Is this question specific toward the way the transistor is configured in your first post or are you referring to transistors in general?
 
  • #7
See attached image of NPN transistor. Electrons flow into emitter, and out through the base (into voltage divider) and out through the collector (through collector resistor) to Vcc.
Bob S
 

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  • #8
Bob S said:
See attached image of NPN transistor. Electrons flow into emitter, and out through the base (into voltage divider) and out through the collector (through collector resistor) to Vcc.
Bob S

BOB S How are you? Long time no see.

I'll start from transistor basics,
Firstly there are holes in base and electrons in emitter.When the base emitter junction is forward biased, the holes from base diffuse into the emitter (i.e electrons in the valence band of the emitter fill the holes in the base.)
As a result the base which had earlier holes and was electrically neutral now due the entry of new electrons has now become negatively charged.
The electrons from the -terminal of Vcc can't pass the transistor as the negatively charged base is blocking it.
Since the base is connected to positive terminal.It will attract the electrons from the base and as a result holes are once again generated in base.
Now the electrons form -terminal of Vcc can pass through the transistor and go back to the +positive terminal of Vcc. Thus only if you draw the electrons from base it allows the collector electron current to flow. i.e the base current controls collector current .This is the principle of amplification.

But you have said that the Electrons flow into emitter, and out through the base (into voltage divider) and out through the collector (through collector resistor) to Vcc.
But what i am asking is that the base electron current controls the other current.
If that is the case then electrons should start to flow only from base.
For that you require a positive voltage applied to base so as to attract electrons from the base. But we don't have a positive voltage applied at the base.
 
  • #9
Averagesupernova said:
Ok fine. We'll go with electron flow. NOW WHY DO YOU THINK THERE IS NO CURRENT LEAVING THE BASE OF THE TRANSISTOR? I keep giving you a one liner because you refuse to answer the question that I have asked. Maybe I'm misunderstanding you. Is this question specific toward the way the transistor is configured in your first post or are you referring to transistors in general?

Ok according to you let electrons leave the base . I accept it .
Now show it how can it flow . By referring my doubt in the post no:8
 
  • #10
Sudar I don't understand anything you are saying at all. I get the sub-atomic structure of the transistor and the flow of electrons. That's nothing new to me.
If that is the case then electrons should start to flow only from base.
For that you require a positive voltage applied to base so as to attract electrons from the base. But we don't have a positive voltage applied at the base.

How can you say there is not a positive voltage applied at the base? Here is what happens. Vcc is +15 volts referenced to ground. Please don't say -Vcc. There is no negative Vcc. The voltage divider is set up to get a voltage on the base that is somewhere between Vcc and ground. Let's pick a value of 5 volts referenced to ground. The voltage on the emitter will be about 4.3 volts since the emitter is never more than about .7 volts lower than the base with an NPN transistor. (Forward biased P/N junction) So we now have an emitter current that is 4.3/Re. This same current minus the base current will flow in the collector also. I don't understand how you can say that there will be no current leaving the base. The voltage divider forms a voltage on the base that is MORE POSITIVE than the emitter and as long as the voltage across the base and emitter turns on the base/emitter junction, you will have current flowing in the base.
 
  • #11
Averagesupernova said:
Sudar I don't understand anything you are saying at all. I get the sub-atomic structure of the transistor and the flow of electrons. That's nothing new to me.


How can you say there is not a positive voltage applied at the base? Here is what happens. Vcc is +15 volts referenced to ground. Please don't say -Vcc. There is no negative Vcc. The voltage divider is set up to get a voltage on the base that is somewhere between Vcc and ground. Let's pick a value of 5 volts referenced to ground. The voltage on the emitter will be about 4.3 volts since the emitter is never more than about .7 volts lower than the base with an NPN transistor. (Forward biased P/N junction) So we now have an emitter current that is 4.3/Re. This same current minus the base current will flow in the collector also. I don't understand how you can say that there will be no current leaving the base. The voltage divider forms a voltage on the base that is MORE POSITIVE than the emitter and as long as the voltage across the base and emitter turns on the base/emitter junction, you will have current flowing in the base.

Please explain the concept of voltage divider .
I can't understand how a resistance can provide bias. i.e how can it supply a positive voltage.
 
  • #12
sudar_dhoni, based on your post history, I believe that many of the holes in your understanding of electrical concepts (and the reason many of us have trouble conveying things to you) stem from the fact that you seek in-depth understanding of advanced topics (e.g. transistors and amplifiers) without having learned or understood the fundamental concepts which underlie them.

What's worse (from my point of view, and presumably that of other posters) is that you use these complex topics (which, to me at least, presupposes a certain level of understanding / background) as a way to understand the simple topics.
 
  • #13
To the point however, a voltage divider can be used to supply a bias to a transistor because it is at higher potential than ground (assuming the transistor is not connected to anything lower than ground.) And the only way the passive resistors can do this is because they are connected to a power supply or battery which can pump (positive) charge back up to a higher potential (or equivalently, push electrons down to ground and allow them to float back up to the positive terminal)
 
  • #14
MATLABdude said:
To the point however, a voltage divider can be used to supply a bias to a transistor because it is at higher potential than ground (assuming the transistor is not connected to anything lower than ground.) And the only way the passive resistors can do this is because they are connected to a power supply or battery which can pump (positive) charge back up to a higher potential (or equivalently, push electrons down to ground and allow them to float back up to the positive terminal)

If you feel this difficult to explain,then atleast can anyone suggest a very good referrence book or webiste which explains my doubt very well.
 
  • #15
I'm still not 100% sure what your doubt is sudar. I thought I explained it reasonably well to someone who should have the voltage divider basics down pretty well. If you do not fully understand a voltage divider then I am wondering how you have ended up analyzing a transistor circuit that uses voltage divider bias (or any transistor circuit for that matter). Here's a trick you may want to try. Remove the transistor from the circuit leaving everything else as is. Then tell us what you think the voltages would be when measured between the nodes that connected to the three transistor leads. In other words, what would the voltage be between Vb and Ve, between Vc and Ve, and between Vc and Vb? For the sake of this excercise, let's assume that R1 is 2.2K, R2 is 1.1K, Re is 220 ohms and Rc is 470 ohms.
 
  • #16
I think you are trying to run before you can walk. If you can't apply simple electronic theory, relating to resistor networks then how can you expect to have a clue about more advanced electronics?
You ask for a source - try Wikkers or your local bookstore or library. The shelves will have loads of college books which will take you through the subject in a conventional way - which has been shown to work pretty well for millions of students. Or are you 'above' such things?
 
  • #17
sudar_dhoni said:
If you feel this difficult to explain,then atleast can anyone suggest a very good referrence book or webiste which explains my doubt very well.

http://en.wikipedia.org/wiki/Ohm's_law
 
  • #18
Averagesupernova said:
I'm still not 100% sure what your doubt is sudar. I thought I explained it reasonably well to someone who should have the voltage divider basics down pretty well. If you do not fully understand a voltage divider then I am wondering how you have ended up analyzing a transistor circuit that uses voltage divider bias (or any transistor circuit for that matter). Here's a trick you may want to try. Remove the transistor from the circuit leaving everything else as is. Then tell us what you think the voltages would be when measured between the nodes that connected to the three transistor leads. In other words, what would the voltage be between Vb and Ve, between Vc and Ve, and between Vc and Vb? For the sake of this excercise, let's assume that R1 is 2.2K, R2 is 1.1K, Re is 220 ohms and Rc is 470 ohms.

I know how a voltage divider works without a transistor.
But with a transistor i can't understand it.
First of all you all can't understand my doubts.Since you can't understand it you tell that i don't know the basics.
I know all the basics. Without knowing the basics i can't come all this way upto transistors.
If you can't understand my doubt, please tell it. Dont tell that i don't know the basics and act smart. I know how a voltage divider , rheostat, potentiometer works in ordinary D.C circuits. But when we introduce transistor its different.Please read my first post carefully. I have clearly mentioned the working of transitor i.e how base current controls collector current.
I myslef have found the answer which i will represent diagramatically

In diagram E represents the electric field of the battery.
Please confirm whether this is right or wrong.
The eletric field from the positive terminal of the battery Vcc attracts the electrons from the base. As a result electrons leave the base and the rest of the process takes place.
 

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  • #19
"If you feel this difficult to explain"!
I think it is you who find it hard to accept things.
A number of us have tried to give an answer. Clearly we are totally incapable so I suggest you just read a book or look at Wikkers or just Google for a tutorial on simple electrical circuit theory. Anyone who reckons they understand solid state Physics to a level where they can question conventional theory must be more than capable of doing that.
 
  • #20
sophiecentaur said:
"If you feel this difficult to explain"!
I think it is you who find it hard to accept things.
A number of us have tried to give an answer. Clearly we are totally incapable so I suggest you just read a book or look at Wikkers or just Google for a tutorial on simple electrical circuit theory. Anyone who reckons they understand solid state Physics to a level where they can question conventional theory must be more than capable of doing that.

Ok i will follow conventional theory itself with water analogies.
Could you please give me a water analogy for minority carriers, leakage current .
If you give me this answer i'll surely follow conventional theory.
I am not against conventional theory.In fact if i can't find a proper answer in terms of electron movement , i think the transistor as a valve as mentioned in this site
http://www.satcure-focus.com/tutor/page4.htm
Its a beautiful way of thinking a transistor.But it can't explain about the reverse leakage current, minority carriers ,etc.
IF you give me a solution to that all my problems are over and i will start to follow the conventional way.
 
  • #21
All I see in your last attachment (post #18) sudar is a box with a bunch of arrows that mean nothing. You have not given me an answer to the question I asked in post #15. Until you do, it is my opinion that you know nothing about voltage dividers at all. If I were an instructor and you were one of my students you would be on a sure way to a failing grade. The reason why? Because you ask a question, and no matter what kind of reply comes from me (or anyone else for that matter) it isn't good enough. If the answer doesn't come in the form you prefer it you claim that no one understands you. When you can give me answers to the questions I asked in post #15 (actual numbers in volts, not some electric field mumbo-jumbo) I will turn my abrasive attitude towards you off.
 
  • #22
There just has to be something wrong when someone asks for a "water analogy". Does flowing water have any effect analogous to an Electromagnetic Field?
 
  • #23
Averagesupernova said:
All I see in your last attachment (post #18) sudar is a box with a bunch of arrows that mean nothing. You have not given me an answer to the question I asked in post #15. Until you do, it is my opinion that you know nothing about voltage dividers at all. If I were an instructor and you were one of my students you would be on a sure way to a failing grade. The reason why? Because you ask a question, and no matter what kind of reply comes from me (or anyone else for that matter) it isn't good enough. If the answer doesn't come in the form you prefer it you claim that no one understands you. When you can give me answers to the questions I asked in post #15 (actual numbers in volts, not some electric field mumbo-jumbo) I will turn my abrasive attitude towards you off.

Ok let it be that i am a dumb.
But i am sure that you don't know anything about electronics . You have till now blabbered some rubbish not answering to the point and simply finding fault in me. Actually the truth is you don't know the answer and you cleverly managed to escape by critisizing me . Thats all you did throughout this thread. You yourself see what you have contributed in this thread. SImply one line answers and that too you were asking me something else without answering my question. If you don't know anything please don't answer. You are an empty vessel that's why you make noise.
 
  • #24
That implies that we are ALL wrong and you are right, young man. You might just examine your own understanding before you start getting rude and saying that people are ignorant. That is not the best way to earn respect or to get knowledge.

If you want to invent your own Science then go ahead (and make sure that it is at least as consistent and as full as the legit form) but you cannot pick and choose between bits of established Science then say it is wrong. You are demanding an answer in a form that doesn't exist - or else (to be charitable) it could be that you have not actually made yourself clear. If you want to resolve this then it is only you who can do anything about it. Start at the beginning and work through the Science in a thorough manner. I advise against 'fun' models like water flow.
P.S. Politeness costs nothing.
 
  • #25
sophiecentaur said:
That implies that we are ALL wrong and you are right, young man. You might just examine your own understanding before you start getting rude and saying that people are ignorant. That is not the best way to earn respect or to get knowledge.

If you want to invent your own Science then go ahead (and make sure that it is at least as consistent and as full as the legit form) but you cannot pick and choose between bits of established Science then say it is wrong. You are demanding an answer in a form that doesn't exist - or else (to be charitable) it could be that you have not actually made yourself clear. If you want to resolve this then it is only you who can do anything about it. Start at the beginning and work through the Science in a thorough manner. I advise against 'fun' models like water flow.
P.S. Politeness costs nothing.

I never wanted to be rude sir .
That guy spoke unnecessary words stating that i would get fail grades and all.
What bothers him whether i would fail or pass.
I did not get angry because he cursed me that i would get fail grades . I got angry because he still did not tell the answer and he and yourself are always finding fault on me.
You both don't know the answer. Then please write that i don't know the answer politely . You and that guy were the one who went arrogant and starting mocking at me that i did not know the basics.
If you don't know anything please don't reply.
p.s Accept things
 
  • #26
"If you don't know anything please don't reply."
I know that I have a lot to learn.
I know that answers don't always come in the form I should like.
I have learned to figure things out for myself when the answers aren't given to me on a plate.
PS I don't have to accept impoliteness. Your responses have been impolite and over demanding.
 
  • #27
sudar_dhoni said:
You and that guy were the one who went arrogant and starting mocking at me that i did not know the basics.
If you don't know anything please don't reply.
p.s Accept things
Sudar-
I am not trying to be arrogant. You need to understand basic dc electrical circuits; voltage sources (power supplies), volts, amps, and resistors. Then you need to understand basic solid state device concepts. In my case, I was educated when vacuum tubes were popular, and my understanding of cathodes, grids, plates, and space charge helped me understand npn transistors. Please review the voltage divider in my post #7. See
https://www.physicsforums.com/attachment.php?attachmentid=23038&d=1263409258
Electrons are flowing out of the base into a voltage divider. The positive base-emitter bias is causing electrons to flow from the emitter to the collector. This is very similar to electron currents flowing out of a hot vacuum tube cathode to the plate, and being controlled by the grid..

Bob S
 
  • #28
Wow! This is amazing. Sudar, so far all you have told us is that you don't think a voltage divider biased transistor can have base current. After all, that IS the whole premise of this thread. I started out by asking you why you thought that it didn't have base current and I was accused of giving a one-liner. I tried to explain that I would like you to answer my question in order to suitably explain to you why the base DOES have base current. The only thing I got was from post #8
But we don't have a positive voltage applied at the base.
.
-
I then tried to explain in post #10 how there can be base current. In post #11 you said
Please explain the concept of voltage divider .
. But in post #18 you said
I know how a voltage divider works without a transistor.
But with a transistor i can't understand it.
-
At this point I assumed that you didn't truly understand voltage dividers since you had several oppurtunities to give me an answer to what the voltages would be in the circuit without the transistor inserted and you have yet to do that. I know what they will be, there is a good reason I picked the values that I did.
-
In post #21 I got a bit testy with you again. I was trying (and still am) to get you to understand why you are not getting the help you seek.
-
In post #23 you said this
But i am sure that you don't know anything about electronics . You have till now blabbered some rubbish not answering to the point and simply finding fault in me. Actually the truth is you don't know the answer and you cleverly managed to escape by critisizing me . Thats all you did throughout this thread. You yourself see what you have contributed in this thread. SImply one line answers and that too you were asking me something else without answering my question. If you don't know anything please don't answer. You are an empty vessel that's why you make noise.
-
Now this is downright funny. In post #10 I explained to you how there can be base current. But apparently you haven't accepted it. Not sure why. In post #15 I didn't really explain anything but told you that you may be able to understand it easier if you remove the transistor from the schematic and calculate the voltages then. Not sure if you have or not, but I haven't seen any values come from you concerning this so I am still assuming you don't know.
-
Yours truly, The Empty Vessel.
 
  • #29
Sudar dhoni,

Enormous numbers of circuits like the one which is under discussion here have been made. They have been analysed, either the hard way with pencil-and-paper, or more lately using tools like PSpice. They have been built, tested, used, and sometimes repaired.

In all of this, the engineers and technicians involved have been able to use the accepted circuit models to obtain workable solutions. There is nothing speculative about this, and really I find it hard to see how you can simply deny that a current flows.

Perhaps I should stop following this before my brain goes into saturation!
 
  • #30
I have my own mental image ('profile') of this guy. There are lots like him and they all want it 'their way'. They all display scientific naivite, then arrogance followed by petulance and then rudeness. If they drive in the same way - they will likely get punched on the nose by some neanderthal who they will manage to upset.
On the other hand, WE manage to be SOOO civilised about it!
My only consolation is that they mostly grow up into reasonable adults in the end.
 
  • #31
sophiecentaur said:
I have my own mental image ('profile') of this guy. There are lots like him and they all want it 'their way'. They all display scientific naivite, then arrogance followed by petulance and then rudeness. If they drive in the same way - they will likely get punched on the nose by some neanderthal who they will manage to upset.
On the other hand, WE manage to be SOOO civilised about it!
My only consolation is that they mostly grow up into reasonable adults in the end.

LOL. Yeah, we can't educate EVERYONE, we need the neanderthals to remind some folks they are not 'all that'. Seriously though, if sudar comes back with something besides 'you haven't told me the way I want to hear it', I'll have more input if it's accepted.
 
  • #32
But of course.
I never hold a grudge.
 

1. What is voltage divider bias?

Voltage divider bias is a method used to remove electrons from the base of a transistor in order to control its amplifying properties. It involves using a resistor network to divide the input voltage and create a stable bias voltage at the base.

2. Why is it necessary to remove electrons from the base?

Removing electrons from the base is necessary in order to control the flow of current through the transistor. By removing electrons, the base is kept at a lower potential than the emitter, allowing for proper amplification of the input signal.

3. How does voltage divider bias work?

Voltage divider bias works by using two resistors in series to divide the input voltage. The voltage at the junction between the two resistors becomes the bias voltage for the base of the transistor. This bias voltage is lower than the input voltage, creating a potential difference that allows for the removal of electrons from the base.

4. What are the advantages of using voltage divider bias?

One advantage of using voltage divider bias is that it provides a stable and predictable bias voltage for the base of the transistor. This allows for better control of the transistor's amplifying properties and helps to prevent fluctuations in the output signal. Additionally, voltage divider bias is a simple and cost-effective method compared to other biasing techniques.

5. Are there any disadvantages of using voltage divider bias?

One potential disadvantage of voltage divider bias is that it can be affected by changes in temperature or variations in the power supply. This can lead to changes in the bias voltage, which can affect the performance of the transistor. Additionally, voltage divider bias may not be suitable for high-power applications as it can result in a significant power loss in the resistors.

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