How area of transistor increases load current in current mirror circuits?

Discussion in 'General Electronics Chat' started by antonyfrancisp, Mar 14, 2016.

  1. antonyfrancisp

    Thread Starter New Member

    Mar 14, 2016
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    In the current mirror chapter of All About Circuits, in the last part it is said that to get twice load current increase area of the transistor. How is this possible. How does emitter current in a current mirror depend upon area of the transistor?
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    This discussion involves creating the circuit, i.e. building the transistors, in the silicon itself. This is the task of the integrated circuit (IC) designer. Most of us will never see transistors or circuits at that level, we buy them only after they are packaged. Therefore, we do not have the ability to change the physical size of the individual transistor, but can choose a different transistor to match the specification of our individual circuit.

    The paragraph above uses examples of ways to get more current than what is generated by the current mirror voltage reference. First, add a second transistor to help drive the load. In the IC designer world, this is the same as changing the physical size of a single transistor in silicon to get the same effect.
     
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  3. StayatHomeElectronics

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    Sep 25, 2008
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    Think of it as: if you change the transistor area you are also changing the transistor gain.
     
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  4. antonyfrancisp

    Thread Starter New Member

    Mar 14, 2016
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    Thank you... now i am kind of getting it :)
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    Transistor beta is primarily a function of the doping concentrations in the base and emitter regions.
     
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  6. StayatHomeElectronics

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    Sep 25, 2008
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    Thank you, I misstated what I was trying to say since I was trying to keep it simple. Changing the transistor area changes its current carrying capability.
     
  7. antonyfrancisp

    Thread Starter New Member

    Mar 14, 2016
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    And gain also??
     
  8. StayatHomeElectronics

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    Sep 25, 2008
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    The larger current going through the transistor is due to a multiple loads (larger current draw) now being attached to one place instead of two as with two transistors. Changing the area allows the transistor to handle the larger load.
     
  9. dl324

    Distinguished Member

    Mar 30, 2015
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    Yes, but in the current mirrors you were referring to, larger area gives more current at the same junction voltage.
     
  10. antonyfrancisp

    Thread Starter New Member

    Mar 14, 2016
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    Actually aren't current mirrors acting like constant current sources(for a constant junction voltage). As per I understood without changing the junction voltage, current won't increase.
    Is there a condition that Rbias and Rload should be equal in order to attain this constant current?
    Is that why when load increases current increasing?
     
  11. antonyfrancisp

    Thread Starter New Member

    Mar 14, 2016
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    Suppose if we attach 2 more parallel loads to the transistor which demands more current.
    The current we will be getting in the loop will be (Vsupply-Vbe)/Rbias. As long as Vbe is constant, this will be constant.
    So what I am getting is this current gets divided and goes into different loads as per corresponding loads. Is that right? current mirror doubt.PNG
     
  12. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    Yesterday is one of those days that I should have taken my own advise and stayed at home.

    For a given Vbe, the collector current of a bjt is fundamentally tied to the area of the transistor. It has a proportional relationship.
     
  13. Bordodynov

    Active Member

    May 20, 2015
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    See
    Mirror.png
     
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