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.
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.
Hi MonaBJT 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.
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.
mona123,
A BJT is a voltage controlled current source (VCCC). Give me a reason why you think otherwise, and I will show that you are wrong.
Ratch
Read: Post #13 of this thread. https://www.edaboard.com/threads/77209/
Post #12 of this thread. https://www.edaboard.com/threads/245930/
That is true a Common Base Configuration acts as a perfect current source in short time burst
The more stable your Configuration biasing is there is a higher efficiency in it
Hi RatchTime 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 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).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.
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
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 .
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 !
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 !
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.
No that's not incorrect ! and a voltage controlled term isn't incorrect too !!!! so what is incorrect ?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.
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 !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?
Well let me back to this issue ! time to time i saw something like this !When was I involved with approximations, design, and excessive complexity?
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 .
Hence i think we can easily neglect the word voltage dependent to be more free in design process . and of course teaching process .
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 .
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 .
....................
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 ..............
Hi again RatchNo, that is plain wrong. Collector current does not depend on base current. I carefully explained that in my last message to Jeffrey Samuel.
My mean is , why we should involve ourselves with some definitions ?So what did that have to do with a BJT being a VCCC?
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 .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.
Hi dear FvM and dear LvWEmphasizing 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.
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....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.
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