1. iPromise

    Thread Starter New Member

    Aug 11, 2013
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    I read that Transistors amplify the voltage from input.

    But how does amplifying voltage help create logic gates preforming operations (AND gate, OR gate, XOR gate, NAND, etc.)?
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Transistors also function as switches, and it is in that mode that they may be arranged as logic gates. See two examples attached.
     
  3. iPromise

    Thread Starter New Member

    Aug 11, 2013
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    Thanks for the quick and clear answer!

    I'm beginning to love this forum.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    2,373
    A bi-polar transistor is considered a current amplifier, a FET could be considered a voltage amplifier.
    Max.
     
  5. onlyvinod56

    Active Member

    Oct 14, 2008
    362
    1
    consider an npn CE mode. if a relay coil is connected between collector and Vcc.

    the collector current Ic = (Vcc - Vce)/R.relay.
    How can the BJT amplifies the current as already the current is decided by the relay coil resistance??
     
  6. ScottWang

    Moderator

    Aug 23, 2012
    4,855
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    The normally BJT amplifies the current is working in the linear area, the Ic is depends on the Ib and count on hfe=Ic/Ib, and as you said to used the relay that is working in the logical area, hfe=Ic/Ib=10 for Vce saturation.
     
  7. jegues

    Well-Known Member

    Sep 13, 2010
    735
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    BJTs are often described as both a current controlled and voltage controlled current sources, but under varying circumstances.

    The current control model, I_{C} = h_{FE}I_{B} is often used when we are in the linear region of operation, as this is "good enough."

    However, other models like the Eber-Molls model, I_{C} = I_{S} \left(e^{\frac{V_{BE}}{V_{T}}}-1 \right) are used to address the fact that transistors are indeed transconductance devices, meaning that the collector current is determined by its base-to-emitter voltage.
     
    Last edited: Nov 14, 2013
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