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bias of bjt current ot voltage controlled

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mona123

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bias of bjt current or voltage controlled

BJT is said to be current controlled device. But we apply voltage divider as the base bias. Isn't it then voltage controlled? Can someone clarify? Thanks.
 

Re: bias of bjt current or voltage controlled

mona123 said:
BJT is said to be current controlled device. But we apply voltage divider as the base bias. Isn't it then voltage controlled? Can someone clarify? Thanks.
Click to expand...

I remember a rather long and very detailed discussion about this question in this forum.
My opinion: Voltage controlled according to Shockley´s genial formula: I=Io*exp(Vbe/Vt-1).
For practical reasons, very often the simple relation Ic=Ib*beta is used - however, without a real physical background (which means: Ib does not CONTROL Ic).
 

If you regard a BJT as voltage controlled you are left with the fact that the input voltage must be in the range +.6 -> +.8V, else the transistor is off or burnt out. If you regard it as a current controlled device you will find it can amplify +.1 micro amp -> + 10 mA (at least), which is a very useable range.
Frank :)
 

Hello here the Voltage at the base is to give us the sufficient BASE CURRENT for the transformer to operate properly at the fixed Q point
 

chuckey said:
If you regard a BJT as voltage controlled you are left with the fact that the input voltage must be in the range +.6 -> +.8V, else the transistor is off or burnt out.
Click to expand...

chuckey, didn`t you ever hear about Ic stabilization using dc feedback?
 

BJT is said to be current controlled device. But we apply voltage divider as the base bias. Isn't it then voltage controlled? Can someone clarify? Thanks.
Click to expand...
Hi Mona
In addition to the posts of the other experts , here , i think this one can help you for more clarification :
Take a look into the out put characteristics curve of a transistor ( BJT ) . when you put 0.4 volts across the BE it will be nearby conduction mode ( in fact it has been conducted but in a pretty logarithmic region )
after that ( up to 0.6 or 0.7 ) you can test an interesting issue ! try to limit the current of the voltage across the BE junction ! ( put 0.6 across the BE bur with a limited current of 100 uA ) .
and don't use any load in collector or emitter ! tie the collector to the +5 volts DC supply and also connect the emitter to the ground directly ( you can put an ammeter in collector ) . then measure the collector current ! what you can see ? VBE is 0.6 volts ( approx ) but IB is low ( it has been limited via a resistor or something else ) . then try to increase the base current and see the effect !
What you'll se is something like this :
VBE is something like 0.6 ( for silicon transistors ) . but ib is limited ! it means the short circuit current of collector is dependent to the base current ! ( you can find a current gain for it that called Beta )
Best Wishes
Goldsmith
 


jeffrey samuel,

That is true a Common Base Configuration acts as a perfect current source in short time burst
Click to expand...

Time is irrelevant as to whether a BJT is a VCCC. The topology of the circuit is irrelevant as to whether a BJT is a "perfect" current source. A BJT is never a perfect current source. It always has some finite impedance.

The more stable your Configuration biasing is there is a higher efficiency in it
Click to expand...

That is irrelevant as to what I said. Bias stability has nothing to do with a BJT being a VCCC.

Ratch

P.S. In English, sentences are ended with a period "." . It makes them easier to read.
 

Time is irrelevant as to whether a BJT is a VCCC. The topology of the circuit is irrelevant as to whether a BJT is a "perfect" current source. A BJT is never a perfect current source. It always has some finite impedance.
Click to expand...
Hi Ratch
May i ask you a question please ?
When i'm seeing your posts , most of the times you've a common behavior in most of them . for instance , you don't apply some usual approximations to simplify your process . most of the times , if we don't apply some approximations in our circuits or our methods of design , we'll have unnecessary complexity .
I think we can simply , consider , when we want change a current we have to change a voltage . isn't it ? let me give an example :
In a simple CE amplifier , when we're applying input signal across the BE , what is happening there ? nothing special ! Just BE voltage has low variations ! thus base current will have low variations too . thus collector current will have high value of variations too ( for increasing or decreasing base current we have to increase or decrease BE voltage but variations of VBE can't be very high . )
ok ? hence some of the text books told that a BJT is a current controlled element , but the fact is another thing ! when we want control base current , we have to change BE voltage ! isn't it ?
What i've tried to say , is most of the times , we can easily design a circuit without involving ourselves with complexity and definitions and concepts !
It is very good to know fact of each thing but not involving ourselves with the facts !
Another example :
I've a design team . some days a go i tried to accept a new member in the team . i wanted to test him . i asked him for designing a circuit ! a simple RF amplifier with gain of 150 and FH of 3MHZ . after three hour he couldn't design it ! i saw his literatures . i saw something strange ! many many funny considerations ! he didn't want to use design rules and rules of thumb ! he didn't use any approximation ! hence at last he couldn't design what i wanted ! he told me may i see how you design this circuit ? i told him yes ! after just 5 minute i've designed the required circuit ! without any problem .

Hence i think it is ok to know the truth of each thing but applying the time consuming and without benefit actualities , is not ok !
Best Regards
Goldsmith
 

Re: bias of bjt current or voltage controlled

mona123 said:
BJT is said to be current controlled device. But we apply voltage divider as the base bias. Isn't it then voltage controlled? Can someone clarify? Thanks.
Click to expand...
The divider voltage is applied to the base, but that is with an emitter resistor. Normally the emitter resistor voltage in a normal bias circuit swamps the base-emitter voltage of the transistor (the purpose of this is to make the bias point relatively stable with respect to any transistor base-emitter voltage change).

As Ratch is very fond of arguing, the physics of a BJT is that it is a voltage-controlled device (but with a very low input impedance [hie or rpi]). But for many bias calculations and switching applications, its more convenient to use a current-controlled model . Thus the base current required to bias the transistor can be easily determined from the current gain (Hfe or Beta) of the transistor and the base bias resistors can then be appropriately chosen to minimize the voltage error caused by this base current.
 
Last edited:

The prime reason for considering it as a current control device, is that there is a low input impedance here in BJT, than in FETs. Thus the input current has lots of significance in the results.

Whereas the FETs are void of this limitation. and are purely controlled by the Voltage applied at input.

PS: Periods are rightly placed I guess Ratch:)
 

Thanks. This is interesting.

goldsmith said:
Hi Mona
In addition to the posts of the other experts , here , i think this one can help you for more clarification :
Take a look into the out put characteristics curve of a transistor ( BJT ) . when you put 0.4 volts across the BE it will be nearby conduction mode ( in fact it has been conducted but in a pretty logarithmic region )
after that ( up to 0.6 or 0.7 ) you can test an interesting issue ! try to limit the current of the voltage across the BE junction ! ( put 0.6 across the BE bur with a limited current of 100 uA ) .
and don't use any load in collector or emitter ! tie the collector to the +5 volts DC supply and also connect the emitter to the ground directly ( you can put an ammeter in collector ) . then measure the collector current ! what you can see ? VBE is 0.6 volts ( approx ) but IB is low ( it has been limited via a resistor or something else ) . then try to increase the base current and see the effect !
What you'll se is something like this :
VBE is something like 0.6 ( for silicon transistors ) . but ib is limited ! it means the short circuit current of collector is dependent to the base current ! ( you can find a current gain for it that called Beta )
Best Wishes
Goldsmith
Click to expand...
 

goldsmith,

May i ask you a question please ?
Click to expand...

By all means, go right ahead.

When i'm seeing your posts , most of the times you've a common behavior in most of them . for instance , you don't apply some usual approximations to simplify your process . most of the times , if we don't apply some approximations in our circuits or our methods of design , we'll have unnecessary complexity .
Click to expand...

I am all for approximations, but I don't know to what you are referring. When was I involved with approximations, design, and excessive complexity?

In a simple CE amplifier , when we're applying input signal across the BE , what is happening there ? nothing special ! Just BE voltage has low variations ! thus base current will have low variations too . thus collector current will have high value of variations too ( for increasing or decreasing base current we have to increase or decrease BE voltage but variations of VBE can't be very high . )
ok ?
Click to expand...

Yes, most of us know that already. What is your point?

hence some of the text books told that a BJT is a current controlled element , but the fact is another thing ! when we want control base current , we have to change BE voltage ! isn't it ?
What i've tried to say , is most of the times , we can easily design a circuit without involving ourselves with complexity and definitions and concepts !
It is very good to know fact of each thing but not involving ourselves with the facts !
Click to expand...

If a textbook says a BJT is a current controlled element, then it is wrong. I don't know what you are trying to say about controlling base current or designing something without involving facts about the design.

I've a design team . some days a go i tried to accept a new member in the team . i wanted to test him . i asked him for designing a circuit ! a simple RF amplifier with gain of 150 and FH of 3MHZ . after three hour he couldn't design it ! i saw his literatures . i saw something strange ! many many funny considerations ! he didn't want to use design rules and rules of thumb ! he didn't use any approximation ! hence at last he couldn't design what i wanted ! he told me may i see how you design this circuit ? i told him yes ! after just 5 minute i've designed the required circuit ! without any problem .
Click to expand...

What does the fact that your new team member was incompetent have to do with the fact that a BJT is a voltage controlled current source?

Hence i think it is ok to know the truth of each thing but applying the time consuming and without benefit actualities , is not ok !
Click to expand...

Again I ask, what does all this have to do the a BJT being a VCCS?

Ratch

- - - Updated - - -

jeffrey samuel,

The prime reason for considering it as a current control device, is that there is a low input impedance here in BJT, than in FETs. Thus the input current has lots of significance in the results.
Click to expand...

Not true. The impedance has nothing to do with a BJT being a current or voltage control device. Both the collector current and the base current are dependent on Vbe, so the collector and base current are proportional to each other. Both are tied together through Vbe. Therefore the base current can be useful as an indicator or measure of what the collector collector is. But the base current does not control the collector current. Vbe controls the collector current.

Whereas the FETs are void of this limitation. and are purely controlled by the Voltage applied at input.
Click to expand...

Well, both Ib and Ic are purely controlled by Vbe, so why does impedance matter?

Ratch
 
Last edited:

Hi Ratch , again

If a textbook says a BJT is a current controlled element, then it is wrong. I don't know what you are trying to say about controlling base current or designing something without involving facts about the design.
Click to expand...
No that's not incorrect ! and a voltage controlled term isn't incorrect too !!!! so what is incorrect ?
Recall : base current depends on BE voltage . and collector current depends on base current and as base current depends on BE voltage then collector current is also controlled with BE voltage .
(IC= gm VBE can demonstrate it too ! )
Hence i think we can easily neglect the word voltage dependent to be more free in design process . and of course teaching process .
What does the fact that your new team member was incompetent have to do with the fact that a BJT is a voltage controlled current source?
Click to expand...
No the fact is , he didn't try to be a designer ! he just have tried to involve himself with equations and definitions instead of learning right processes ! that was an example of this issue ! ( but his problem was not this issue ! his problem was with how to earn that gain in that BW !

An interesting issue is , i don't consume my time with definitions ! i just have learned to do something like this :
We need a circuit with these specs ! here are specs ! ....... . design such a circuit for handling this aim ! it is what i've learned in my life !
I'm trying to don't confuse anybody and of course don't confuse myself .


When was I involved with approximations, design, and excessive complexity?
Click to expand...
Well let me back to this issue ! time to time i saw something like this !
For example as i can remember in a concept in negative impedance in transformer ! in a thread ( i think you can remember that . ) and in an input impedance calculation in a CE amplifier ( can you remember those two or three pages ( your literatures ? )
My friend , i'm just trying to show the path .

Best Regards
Goldsmith
 

goldsmith,

No that's not incorrect ! and a voltage controlled term isn't incorrect too !!!! so what is incorrect ?
Click to expand...

It is hard to understand what you are trying to say when you use double negatives. Therefore I will ignore the above sentences.

Recall : base current depends on BE voltage .
Click to expand...

Yes, that is true.

and collector current depends on base current and as base current depends on BE voltage then collector current is also controlled with BE voltage .
Click to expand...

No, that is plain wrong. Collector current does not depend on base current. I carefully explained that in my last message to Jeffrey Samuel.

Hence i think we can easily neglect the word voltage dependent to be more free in design process . and of course teaching process .
Click to expand...

Certainly not in the teaching process. Then knowledge would be incomplete. I believe it was pointed out in another thread some time ago that it was essential in designing differential amplifiers to know that base voltage affected collector current. And of course everyone knows that temperature change affects Vbe which in turn affects collector current. Therefore voltage dependency cannot be discarded.

No the fact is , he didn't try to be a designer ! he just have tried to involve himself with equations and definitions instead of learning right processes ! that was an example of this issue ! ( but his problem was not this issue ! his problem was with how to earn that gain in that BW !
Click to expand...

So what did that have to do with a BJT being a VCCC?

An interesting issue is , i don't consume my time with definitions ! i just have learned to do something like this :
We need a circuit with these specs ! here are specs ! ....... . design such a circuit for handling this aim ! it is what i've learned in my life !
I'm trying to don't confuse anybody and of course don't confuse myself .
Click to expand...

Of course you have to know and learn definitions. Otherwise you would not be able to communicate with anyone else. They would not know what you are talking about. I think you are confused about what a definition is. The question is not about what a VCCS is. It is about whether a BJT can be described by that definition. That is not the same as wondering about what a definition is.

Well let me back to this issue ! time to time i saw something like this !
For example as i can remember in a concept in negative impedance in transformer ! in a thread ( i think you can remember that . ) and in an input impedance calculation in a CE amplifier ( can you remember those two or three pages ( your literatures ? )
My friend , i'm just trying to show the path .
Click to expand...

Show me the path, then. Provide a link to that thread, and point out where you think I was wrong. In the future, call me out on it like you did here.

Ratch
 
Emphasizing the nature of BJT as voltage controlled device is matter of transistor theory. It's also simply the way how the commonly used models (Shockley, Ebers-Moll, Gummel-Poon) are describing it's operation. The description involves a certain level of abstraction and shouldn't be held against those cases, where transistor operation appears to be controlled primarly by a base current. In return, the latter examples don't invalidate transistor theory.

Also the transistor model doesn't tell me, if base-emitter voltage or base current should be used as independent quantity in a particular circuit design problem. This has to be decided according to circuit properties.

In other words, don't mix up basic models of transistor operation and calculation methods used in practical design.
 
Re: bias of bjt current or voltage controlled

crutschow said:
....................
As Ratch is very fond of arguing, the physics of a BJT is that it is a voltage-controlled device (but with a very low input impedance ..............
Click to expand...

...very low...? Some years ago I have learned that there are no "low" and no "high" values - unless I make a comparison between two figures.
Example: The input resistance measured into the base (common emitter) is very high if compared with the input resistance into the emitter (common base), for example 3 kohms vs. 25 ohms.
 
No, that is plain wrong. Collector current does not depend on base current. I carefully explained that in my last message to Jeffrey Samuel.
Click to expand...
Hi again Ratch
Recall :
Think about it , carefully , please :
Why collector current has change when we are increasing VBE ? if we look into the semiconductor crystals of an NPN transistor , we'll see the BE voltage will has effect on conduction of a transistor . but why collector current has changes ? because base current has changes ! why base current has changes ? because BE voltage has changes ! i hope you get what i mean and what i'm trying to refer .


So what did that have to do with a BJT being a VCCC?
Click to expand...
My mean is , why we should involve ourselves with some definitions ?
For example , when we have two voltage divider resistors as base bias resistors . be can easily consider , VB is given by a simple divide between two resistors ! but the fact is we can't do it because of base current ! but because base current is negligible , we can do it !!
What i mean is , up to know i didn't pay any attention to this issue , because it didn't have any benefit in design process . but i agree each peoples , should know the fact of this issue ! hence i'm not disgree with knowing this effect . but i'm completely disagree with thinking about this , in design process !
Show me the path, then. Provide a link to that thread, and point out where you think I was wrong. In the future, call me out on it like you did here.
Click to expand...
What thread ? your threads ? why you don't search into your created threads ? of course all of us that are here , should tell each other ( ourselves ) about our problems or wrong ways ! thus we can increase our abilities .
Hence an advise :
Just using a paper and a pen and just theorem can't help ! and it won't have any benefit for you . try to design something ! then you'll learn while you're designing something ! it is a big lesson that many years ago , i've learned it . ( i believe when a project is involved with some complexity ! a thing has problem ! it is why i design all of my projects as simple as possible ! and i'm sure that practical issues and experiments are big professors )
I hope you get what i mean .

Emphasizing the nature of BJT as voltage controlled device is matter of transistor theory. It's also simply the way how the commonly used models (Shockley, Ebers-Moll, Gummel-Poon) are describing it's operation. The description involves a certain level of abstraction and shouldn't be held against those cases, where transistor operation appears to be controlled primarly by a base current. In return, the latter examples don't invalidate transistor theory.

Also the transistor model doesn't tell me, if base-emitter voltage or base current should be used as independent quantity in a particular circuit design problem. This has to be decided according to circuit properties.

In other words, don't mix up basic models of transistor operation and calculation methods used in practical design.
Click to expand...
Hi dear FvM and dear LvW
Yes i completely agree with you . statements of you and LvW , all of the times , make everything reliable . thanks you gentlemen and experts for that .

Best Regards
Goldsmith
 
Last edited:

Re: bias of bjt current or voltage controlled

LvW said:
...very low...? Some years ago I have learned that there are no "low" and no "high" values - unless I make a comparison between two figures.
Example: The input resistance measured into the base (common emitter) is very high if compared with the input resistance into the emitter (common base), for example 3 kohms vs. 25 ohms.
Click to expand...
True enough. ;-) I meant as compared to a high impedance, voltage operated transistor such as a FET which has orders of magnitude higher input resistance than either of those BJT configurations.
 

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