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You are here: Home / Topics / Reverse bias and forward bias junctions of common collector configuration

Reverse bias and forward bias junctions of common collector configuration

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Electronics › Electronics › Reverse bias and forward bias junctions of common collector configuration

  • This topic has 12 replies, 3 voices, and was last updated 10 years, 9 months ago by Rafeek Ravneet.
Viewing 13 posts - 1 through 13 (of 13 total)
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    Posts
  • May 15, 2013 at 11:17 pm #2407
    Gaurav satish shah
    Participant

     

       I am talking about   NPN  transistor  here

        

        in common collector config,  collector is common to base and emitter

        there are two loops:

        1.   collector emitter loop 

        2.    base  collector loop

     

                   For transistor to be operational in active region , emitter-base junction should be 

                   forward bias and  base-collector junction to be reverse biased

     

                    But in base collector loop , base(p-region) is connected to positive terminal

                    of battery and  collector( n-region) to -ve terminal of battery

                   

                    dosent this mean that base-collector is forward biased

     

                    This is my query

                   

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

    May 16, 2013 at 4:28 am #9762
    AJISH ALFRED
    Participant

    Hi Gaurav,

    The common-collector configuration of an NPN transistor is given below;

     

    wysiwyg_imageupload:8516:

    You can see that the collector is connected to the positive itself and not to the negative as you have mentioned in your query.

    May 16, 2013 at 9:23 pm #9769
    Gaurav satish shah
    Participant

     

     

       

        Thanks a lot Ajish Alfred

     

        Do you have Boylestad  ?

     

        In this famous book,  page 144, common collector configuration is given

     

        same as i mentioned, ie. collector to -ve and base to +ve

     

       where  as in your case  ,  collector is  +ve  and base  -ve  which  should actually be the way  if transistor is to operate  in active region 

               

               Alfred, which book have you taken the diagram from

               and how are you able to paste it here, i want to know

        

          Thanks a lot, please refer boylestad also and tell me how can get diagrams in the box

    May 17, 2013 at 4:43 am #9771
    AJISH ALFRED
    Participant

    Boylestad can’t be wrong. I don’t have the book with me right now, but defenitely I’ll manage to get it from somewhere and try to answer what could be the reason they give the circuit in that way.

    Regarding the image, I copied it from a site and I used the image icon to paste which you can see in the tool bar over the text editing window.

    May 18, 2013 at 3:24 am #9779
    AJISH ALFRED
    Participant

    Hi Gaurav,

    I think this is the circuit which you are talking about,

     

    wysiwyg_imageupload:8591:

     

    The one which I’ve posted earlier was actually a ‘theoritical circuit’, but the Boylestad’s circuit is a practical one. In theoritical circuit the positive of two cells are connected together which should not do in practical cases, since it can cause lot of problems.

    In the above circuit the cells are connected in a normal way in which it should be and hence it is practical.

    Now in the above circuit we have to assume that Vee is greater than Vbb (Vee > Vbb). Consider the point at which the negative of the Vbb and the positive of Vee are connected together, the collector is also connected to this point. Since Vee > Vbb the potential there will be not zero or negative, but positive when compared with the positive of Vbb. In this case the base will be less positive than the collector and the collector is more positive than the base. Hence the collectro base junction is reverse biased.

    That was a very genuine and nice question from you and we expect more of this kind of queries.

    May 26, 2013 at 4:32 pm #9831
    Gaurav satish shah
    Participant

    Hi Ajish, 

             

           I read your comments on Boylestad’s  common collector configuration

     

           By applying same logic to common base configuration,

       

           

           

    wysiwyg_imageupload:8656:  At base , -ve of VCC and +ve of VEE are connected together.

     Now , VCC much greater than VEE. The approximate 

     values are VCC= 10V, VEE= 2V.

    As such , base will be -ve and at -8V.

     

    So base will be negative compared to emitter.

     

    Then , emitter-base junction will be reverse biased

    and transistor will not operate in active region.

     

    I am confused..

     

     

     

     Boylestad common base configuration

     

     

     

     

     

     

     

     

     

     

     

    May 27, 2013 at 11:02 am #9833
    AJISH ALFRED
    Participant

    Hi Gaurav,

    I think you have’nt notice the ‘ground symbol’ in the circuit diagram.

    wysiwyg_imageupload:8704:

     

    Whenevr you find such a symbol it means that the potential at that particular point is zero. The potential at that point in the circuit becomes zero only when the Vee equals Vcc.

    There was no such ground symbol in the previous case and hence we have assumed that one of the voltage is much larger than the other, but here the case is different.

    Now the base is at zero potential and the emitter is at negative potential (lower potential compared to base), hence the emitter base junction is forward biased.

    May 30, 2013 at 9:00 pm #9866
    Gaurav satish shah
    Participant

     

     

     

     

         Hi Ajish , take a look at Boylestad,s common collector configuration

     

        Last time i had pasted photo of Boylestad’s common base configuration

     

     

     

    wysiwyg_imageupload:8811:

     

    So , here  collector at zero potential.

    base is positive  compared to collector

     

    collector base junction is forward biased

    transistor cannot operate in active region

     

     

    I am confused

    May 31, 2013 at 4:49 am #9867
    AJISH ALFRED
    Participant

    Hi,

    I found this circuit only from boylested for common collector

     

    wysiwyg_imageupload:8812:There is no ground in the above circuit. Tell me how can I find the circuit posted by you in the text book. Page no. or topic numnber, heading anything??

    Also tell me which edition of the book are you using. I think I’ve 7th edition with me.

    June 4, 2013 at 9:52 pm #9892
    Gaurav satish shah
    Participant

     

         

        Hi Ajish,

             The ckt I had pasted from Boylestad in last comments was in fact :

             

             I took its picture by camera and then transferred it to computer,

     

              so it is a true Boylestad circuit.

               well to get that circuit, I am using Ninth edition of Boylestad

     

               The copyright of this publishing is with  Pearson Education

         

               I am from India, and this book is an Indian subcontinent adaptation:

                                                                      copyright@2007 Dorling Kindersey(India) pvt. Ltd

     

                This edition is  manufactured only in India and authorized for sale in

                 India, Bangladesh,Bhutan, Pakistan,Nepal , Sri lanka, Maldives.

     

              As for page no,the diagram for common collector configuration occurs on

              

               page 144  after the second paragraph.

     

              Well, anyways, grounding the common terminal is must in transistor configurations

               and your case even grounding may be assumed.

    June 11, 2013 at 8:49 pm #9949
    Gaurav satish shah
    Participant

          

     

     

     

            Hi Ajish, is there a way out of this common collector configuration topic

            

            I am confused about it for a long time.

           

            The Boylestad  circuits I have inserted in last to last comments are genuine

     

            We need to find  how to go ahead about this. Do you have any  solution or ask 

     

             it to your professors.in fact there is dearth of  genuine professors in my area.

     

            I have many  other doubts too

     

    June 13, 2013 at 11:47 am #9960
    AJISH ALFRED
    Participant

    Hi Gaurav,

     

    Sorry for the late response.

     

    First of all you should know that the common-collector configuration of the transistor is hardly found in the electronic circuits, since it is not that practical to implement. Hence it is only a theoritical circuits and your observations are absolutely correct. 

    My answer is that the “circuit is not practical”. That dosen’t means the Boylestad is wrong, they have given you the simplest form of common collector circuit so that you can understand the working of different configuration of transistors.

    I’ve seen in lot of text books the last line of the circuit description as “this circuit is not practical to implement”. Is it mentioned same in the Boylestad also??

     

    Lot of adjustments need to be done with common-collector circuit to get it working.

    For the time being you can just learn the theory and the present circuit for the exam purpose.

    It would be great if you try out a common-collector circuit in your lab.

     

    May I know your other doubts.

    August 19, 2014 at 12:39 pm #12040
    Rafeek Ravneet
    Participant

    Hi Gaurav, have you find the solution yet? I’m also stuck on this exact problem .

     

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