Reverse bias and forward bias junctions of common collector configuration

[Gaurav satish shah]

 

   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

               

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[AJISH ALFRED]

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.

[Gaurav satish shah]

 

 

   

    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

[AJISH ALFRED]

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.

[AJISH ALFRED]

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.

[Gaurav satish shah]

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

 

 

 

 

 

 

 

 

 

 

 

[AJISH ALFRED]

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.

[Gaurav satish shah]

 

 

 

 

     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

[AJISH ALFRED]

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.

[Gaurav satish shah]

 

     

    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.

[Gaurav satish shah]

      

 

 

 

        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

 

[AJISH ALFRED]

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.

[Rafeek Ravneet]

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

 

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