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    Common Base output characteristic

    Hello people!

    I am reading some transistor theory, and curently i am reading about the common base connection. I have seen that we need to calculate some metrics to get the Ic (which equals -approx- to Ie) and Vcb values of the polarization, so that we can draw the load line on the Ic -> Vbc characteristic. This characteristic is something like this:

    Bipolar Transistor Common-Base Output Characteristics: Vcb-Ic Curves for Different Ie Values

    Funny thing is that, no matter how many transistor datasheets i have open (and i have open A LOT), i could not find not a single one datasheets which has this characteristic. I mean, i have seen the output characteristic for Common Emitter connection, but there is nothing for the Common Base....

    What is wrong here?????? Where can i find these characteristics for a transistor?

    •   Alt29th September 2011, 20:12

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    Re: Common Base output characteristic

    Dear friend!
    Hi
    It is not important. it is not difference between curves of Common base and common emitter(approx) . with the curves of CE amplifier , you can design a common collector or common base amplifier . it is simple . for example the out put curve is general.
    I hope that this become helpful for you.
    Best wishes
    Goldsmith



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    Re: Common Base output characteristic

    Quote Originally Posted by goldsmith View Post
    Dear friend!
    Hi
    It is not important. it is not difference between curves of Common base and common emitter(approx) . with the curves of CE amplifier , you can design a common collector or common base amplifier . it is simple . for example the out put curve is general.
    I hope that this become helpful for you.
    Best wishes
    Goldsmith
    thank you for your reply.

    Goldsmith, can you explain me how can i change the CE curves to gain the CB curves? I understand that Ic axis will remain as is, and the Ie characteristics are almost parallel to Vcb. But how does Vcb changes on the shaft? Does it follow the same as Vce?
    Thanks



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    FvM
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    Re: Common Base output characteristic

    I agree with goldsmith, that the common base output characteristic is similar to and can be derived from common emitter one. And as a matter of fact, you won't find a common base characteristic in a datasheet, at best some common base small signal parameters, where appropriate.

    But both characteristics aren't identical. They differ by Vbe for the horizontal axis and by factor B for the curve parameter. Because both aren't constant, the curve shape looks slightly different.

    I wonder however, if it's actually need exact output characteristic curves in common base circuit. I guess, it's more an exercise than an actual design problem. You can always derive individual points of the curve from the other datasheet specifications and are in so far able to define the load line. It's more important to understand how the transistor works and how data sheet specifications are related.



    •   Alt30th September 2011, 09:28

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    Re: Common Base output characteristic

    Quote Originally Posted by FvM View Post
    I agree with goldsmith, that the common base output characteristic is similar to and can be derived from common emitter one. And as a matter of fact, you won't find a common base characteristic in a datasheet, at best some common base small signal parameters, where appropriate.
    we both agree with goldsmith, but i was asking of a way to put the numbers on the horizontal axis. Do you know how can i put the numbers on this axis?



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    Re: Common Base output characteristic

    Common base configuration isn't used as much as the other two.

    Speaking for myself, I didn't understand how it could operate for the longest time. I took it to be 'just theory'. Not too practical but it was mentioned in the books just to be thorough.

    Most all transistor projects have the signal going into the base. It only seems natural. It matches the basic theory of transistor operation. Small current into base, high current through CE.

    I don't remember ever experimenting with a common base transistor circuit. They're used more in high frequency circuits. I have only built common E or common C.

    It took using an animated simulator for me to realize common base operation works as the signal rises and falls at the emitter leg. Thus effectively changing V at base. The range of operation is only a few tenths of a volt.

    Notice that reducing V at emitter causes more bias current, bringing higher current from the collector, having the effect of raising V at emitter. (The process began by reducing V at emitter.) Seems like a counter-productive way to go.

    As to the data graphs...

    They bring out the most basic traits about transistor operation. There are many more aspects of transistor operation that require a bit of experimentation to explore.

    I think I learned more by twisting a potentiometer and watching two meters (and later my oscilloscope) to see how a simple transistor circuit behaves. Great way to get a notion about the regions labelled 'shut-off' and 'saturate', etc.

    In other words, understanding the data graphs becomes secondary to practicalities such as finding a good operating range for the transistor circuit you're putting in a project.



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    FvM
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    Re: Common Base output characteristic

    but i was asking of a way to put the numbers on the horizontal axis
    I have been exactly answereing the question about correspondence between both characteristics.
    They differ by Vbe for the horizontal axis and by factor B for the curve parameter. Because both aren't constant, the curve shape looks slightly different.
    Strictly speaking, the non constant point involves, that you have to redraw the curves point-by-point. In a rough estimation, you can assume an arbitrary constant Vbe value of e.g. 0.7 V and a constant B. Then shift the X axis by Vbe and change the Ie notation to Ib.



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    LvW
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    Re: Common Base output characteristic

    Quote Originally Posted by BradtheRad View Post
    ...........
    Most all transistor projects have the signal going into the base. It only seems natural. It matches the basic theory of transistor operation. Small current into base, high current through CE.
    ...............
    It only seems "natural"? Are you really convinced and - can you justify this?
    The basic theory of transistors is founded on the BE pn-junction and the corresponding voltage VBE. And - in principle - it does not matter (as far as Ib or Ic are concerned) if you fix the base potential and vary the emitter potential or vice versa !. This again shows that the current Ic is not "caused" by IB but by VBE. Don`t mix mathematical "rule of thumbs" (IC=B*IB) with physical facts.



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    Re: Common Base output characteristic

    Quote Originally Posted by LvW View Post
    It only seems "natural"? Are you really convinced and - can you justify this?
    The basic theory of transistors is founded on the BE pn-junction and the corresponding voltage VBE. And - in principle - it does not matter (as far as Ib or Ic are concerned) if you fix the base potential and vary the emitter potential or vice versa !. This again shows that the current Ic is not "caused" by IB but by VBE. Don`t mix mathematical "rule of thumbs" (IC=B*IB) with physical facts.
    Thanks. Please forgive my mindset.

    By saying 'it seems natural' I display a habit of thought reinforced by tutorials telling me about cause and effect in transistors, namely that base current is the cause and CE current is the effect.

    This is a helpful website that starts by saying common base mode has strange operating characteristics. It goes into some depth (scroll about 40% of the way down).

    Lessons In Electric Circuits -- Volume III (Semiconductors) - Chapter 4
    Last edited by BradtheRad; 3rd October 2011 at 19:53.



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    Re: Common Base output characteristic

    Quote Bradtherad: Thanks. Please forgive my mindset.
    By saying 'it seems natural' I display a habit of thought reinforced by tutorials telling me about cause and effect in transistors, namely that base current is the cause and CE current is the effect.


    Yes, I know that several internet contributions as well as even some textbooks speak about "base current controlled devices". But I think a forum like EDABOARD is a good place to discuss different opinions and also to correct some unclear or even false and misleading explanations given elsewhere.

    Quote: This is a helpful website that starts by saying common base mode has strange operating characteristics. It goes into some depth (scroll about 40% of the way down). :Lessons In Electric Circuits -- Volume III (Semiconductors) - Chapter 4

    Strange operating conditions? What does this mean?
    This webside is a good example for my doubts mentioned above: It does not explain HOW the base current controls the collector current - instead, it simply claims that Ib controls Ic. I suppose the author has no explanation (because there is no explanation for this false claim).

    By the way, I have detected another wrong statement on the refrenced website:

    While positive feedback drives an amplifier circuit toward a point of instability (oscillations), negative feedback drives it the opposite direction: toward a point of stability.
    Of course, this is wrong. Negative feedback has a lot of advantages, but it does not make the amplifier more stable. Just the opposite is true. I suppose the author has mixed stability of the dc operating point and dynamic stability.

    LvW



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