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Current - vs - Voltage

Discussion in 'Electronic Basics' started by Scott Brehler, Jun 2, 2007.

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  1. Hi all,

    This has been burning in the back of my mind for quite some time, and I've
    not been able to come up with an obvious/simple answer.

    What is the difference between something being described as voltage
    controlled as opposed to current controlled? I.E. FET's(voltage
    controlled) - vs - BJT's(current controlled). If current and voltage are
    directly proportional to each other (given a constant resistance)why is one
    device described one way and the other another way? It seems that you could
    always use voltage (or current) to describe either device and come up with
    the same calculations.
    Is it because the resistance of the device is variable that one is used
    instead of the other? I refer to the transistors, but I've heard of other
    devices described as current controlled or voltage controlled too. What
    makes this distinction?

    Thanks in advance for any input.

  2. Phil Allison

    Phil Allison Guest

  3. Bob

    Bob Guest

    In general terms, something that is a voltage controlled device usually (but
    not always) has a high input impedance and the "controlled" quantity is
    most-easily described by the input voltage. A current controlled device has
    the opposite characteristics -- that is (usually) a low input impedance and
    the controlled thing is most-easily described by its input current. That's
    not to say that you couldn't describe a voltage controlled device by its
    input current, but it usually doesn't make sense to do this (see below).

    When describing a FET with a simple model, its output current (drain-source)
    is typically related to the input voltage (gate-source). The gate to source
    impedance is extremely high so it's not appropriate to talk about its input
    current. The output vs input quantities that are the most constant (from
    unit to unit) are its output current vs its input voltage. This concept is
    key in the characterization of a FET's transfer function. The ratio between
    output current and input voltage is called the transconductance of the FET.

    When describing a bipolar junction transistor (again a simplified model),
    its output current (collector-emitter) is characterized as a function of the
    input current (base-emitter). The base-emitter impedance is fairly low. The
    ouptut vs input quantities that are most constant and linear are its output
    current vs. its input current. The ratio between the output current and the
    input current is called the Hfe and/or beta of the BJT.

    Hope this helps.

  4. Any real device requires both voltage and current to control
    it. But usually, one of those input variables generally has
    a more linear relationship to the controlled output than the
    other, so it is easier to think about that being the
    controlling variable. This has little to do with the
    physics of the situation, and more to do with simplified
    mental models of what is going on.

    For example, BJTs require a voltage base to emitter to
    enable collector current, but there is an exponential
    relationship between base to emitter voltage and collector
    current, so it takes a higher level of mathematics to deal
    with that relationship than it takes to deal with the more
    linear base current to collector current relationship, even
    though that relationship is less precise than the voltage to
    current relationship is. So the simplest description of a
    BJT is that it is a current controlled device. and you have
    to add that the base to emitter voltage varies much less
    than the current and has a nonlinear relationship to the
    current (sort of as an after thought).
  5. Circa Sat, 02 Jun 2007 03:09:05 GMT recorded as
    <lb58i.17192$> looks like "Scott
    Aye, there's the rub. You are used to thinking in a linear manner, and
    semiconductors are non-linear devices. John's response about users
    preferring to examine the most linear relationship between control and
    output is quite excellent. We all like to think in linear terms, because
    the math - and the imagining of the operation - is simpler that way. Fewer
    curvy lines means easier algebra.
  6. A way of thinking about this is that some things respond relatively
    linearly with voltage, some with current. Some respond linearly to
    either. By 'respond,' I mean that the desired outcome behaves
    linearly to changes in the input. This doesn't mean, in any way, that
    the internals in the black box being controlled must all operate that
    way. Just the observable, desired behavior.

    It is one thing to use 'x' and control the value of 'y' with y=k*x and
    quite another to try and control the value of 'y' with y=A*e^(kx). If
    you have a choice, you'll go with the first equation. It's just

  7. It's also just plain easier to control precisely, in practice, too.
    Not only in one's imagination.

  8. Thank you for the GREAT replys. I had sneaking suspicions, but wasn't able
    to put it quite so elegantly as those of you. I am greatly enlightened now!

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