Transistor Used as a Resistor

Discussion in 'General Electronics Chat' started by Faisal Ikhlaq, Sep 13, 2013.

  1. Faisal Ikhlaq

    Thread Starter New Member

    Sep 13, 2013
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    Dear All,

    I have studied in Circuit and Systems Design course that transistor can be used as a resistor in IC technology because transistor has high resistance and it takes less space on chip than resistor.

    Now i want to know how and why transistor has high resistance so that for same resistance value we prefer transistor in IC technology rather than normal resistor????
     
  2. edmundopt

    Member

    May 4, 2011
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    Can you be more specific ? Because if you check the datasheet of an IC, you still can see resistors in the internal schematic.
    Maybe you can post a IC schematic where that specific transistor is used as a resistor
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
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    The name transistor is a contraction of "transfer resistor".

    Of course the transistor can be used as a resistor that is exactly what it is - a controllable resistance.

    The resistance is between the collector and emitter - we control it by applying suitable signals to the base.

    However there is no requirement for that resistance to be high or low, it is up to us to set its value. And yess transistors can be cheaper to fabricate on chip than resistors.

    One question for you

    The transistor has two regions of operation the active region and the saturated region, which do you think applies in this case?
     
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  4. edmundopt

    Member

    May 4, 2011
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    Did not know that a fixed value resistor(2pins) is more expensive to produce than a transistor(3 pin) inside a IC, learning every day! I still see a lot of resistors inside IC.
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    You don't see resistors in an IC. What you are looking at is the circuit schematic.
    The physical resistor structure is fabricated using the similar structure that is used to fabricate transistors. The resistor is not constructed from a 3-pin transistor. It is made from the conduction channel similar to that of a FET.
     
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  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Actually fabricating very high value resistors on chip is difficult. It is even difficult in discrete components.

    A transistor can be connected in a mode known as an active load. This appears to other parts of the circuitry as a very high impedance.

    Faisal, did you think aboput my question?

    Here is another.

    Why whould we want a very high impedance load?
     
  7. Faisal Ikhlaq

    Thread Starter New Member

    Sep 13, 2013
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    We want high load so that maximum voltage drop should be across the load.

    Actually question was related to Differential MOS pair where we prefer to use current source for biasing of the amplifier other than resistor on the Emitter.

    So there was written that Current Source has high resistance as compared to resistor. So that's why i asked about transistor because current source is made up of transistor.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    Hah! Now we are in a completely different universe.

    You are not talking about IC transistor fabrication.

    You are talking about replacing a resistor current source with an active current source.
    Why did you not say that in the first place?

    In a differential long tail amplifier, there is a current source on the tails of the amplifier. A single resistor is a cheap way of creating a current source.

    You can create an active current source using a transistor. An ideal constant current source has infinite impedance.

    This has nothing to do with replacing a resistor with a transistor.
     
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  9. Faisal Ikhlaq

    Thread Starter New Member

    Sep 13, 2013
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    Thanks for explaining.

    Actually this is my actual question that why an ideal constant current source has infinite impedance.

    What is the basic concept and reason behind that.
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    Firstly, think about a non-ideal voltage source connected to an unknown load of resistance R.
    A non-ideal voltage source has internal resistance Rs.

    What is the voltage across the load R if the voltage source is supplying a voltage of Vs?

    What do you need Rs to be for an ideal voltage source?

    Now do the same analysis for a non-ideal current source.
     
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  11. Faisal Ikhlaq

    Thread Starter New Member

    Sep 13, 2013
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    Thanks alot.... :)
     
  12. Jibby

    New Member

    Sep 12, 2013
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    Transistor can be operated like a dynamic resistor
     
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