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Basic question about transistor operation

Discussion in 'Electronic Basics' started by Imagineer, Mar 18, 2013.

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  1. Imagineer

    Imagineer Guest

    Hi everybody,
    could someone explain to me something very basic:
    the simplest transistor has 3 legs: gate, drain and source. If I were to apply voltage to the gate (above threshold), the transistor "opens" and now current between the source and the drain can flow. I have simulated it in LTSpice and also tested the same in a lab - and the drain current always proportional to the gate voltage. Is it suppose to be so and why?
  2. Daniel Pitts

    Daniel Pitts Guest

    There are several types of transistors which have slightly different
    characteristics. It sounds like you're talking about a FET transistor.

    Eventually you'll hit the saturation point and no longer will a voltage
    increase on the gate cause an increase to the source-drain current.

    Also, if I'm remembering correctly (I've just learned this stuff a few
    weeks ago), the relationship isn't entirely "proportional". I think it
    is either a squared or a logarithmic relationship, but I could be wrong
    about that.

    Take my comments all with a grain of salt, I'm just learning this stuff
    myself. I'm sure a more veteran member of this group will correct me if
    I'm wrong, so you may wish to wait until they do ;-)

    Hope that helps,
  3. Imagineer

    Imagineer Guest

    Thank you all, now I understand it beter.
    It was indeed a MOSFET transistor.
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