Operational Amplifier

Discussion in 'Homework Help' started by thisonedude, Aug 13, 2014.

  1. thisonedude

    Thread Starter Member

    Apr 20, 2014
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    So i am just beginning to learn about op amps and i have a few questions that have been bothering my about this thing.

    How can the amplifier have no current in the inputs? If it has no current, then it has no voltage at the input, which mean it has no voltage at the outputs. So how does it work?! What is the purpose of an op amp?! How can it function if it has no current!?
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    It has "no current" going in because it has REAAAAAAAAAAAAALY high resistance.

    It has voltage at the input because voltage is a FIELD. It does not need a medium/channel.

    For the rest you can read wiki.
     
  3. thisonedude

    Thread Starter Member

    Apr 20, 2014
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    Wait but for the input voltagae i though V =IR, If the curren is 0, then shouldn't the voltage also be zero?
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    Who every told you that an op-amp has no input current is wrong.

    The input current could be very small, like micro-amps.

    But the op-amp still works because it is an amplifier.
     
  5. thisonedude

    Thread Starter Member

    Apr 20, 2014
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    In terms of an ideal op amp...every book and every youtube video i've seen or read say that no current flows in or out of the inputs.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    That is for an ideal op-amp. In the real world no op-amp is ideal.

    An ideal op-amp has the following characteristics:

    • Infinite input resistance
    • Zero output resistance
    • Infinite gain
    • Infinite bandwidth
    No such op-amp exists in the real world.
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Hey Dude,

    do this simple thought experiment:

    Imagine you have a 9V battery (fresh)

    Connect a voltmeter + to + and - to -
    It reads ~9V, right?

    Now connect a 100K resistor across the battery terminals: If you are real observant, you might notice that the voltmeter reads just a little less, but ignore that for now...,
    it still reads ~9V, right.

    If your view of an opamp input resistance held, then the battery should have read zero with no resistor connected, and ~9V with the 100K connected, but that didn't happen...
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    This is not a thought experiment but a real world op-amp example.

    The TL081 is a popular JFET op-amp.
    The input resistance is 10^12 ohms.

    10pA in the input is an incredibly small amount of current which for most purposes is zero current.

    Yet the input voltage is 10V.
     
  9. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    I think your error is lack of fundamental understanding of the devices you are talking about. To be fair, it is very often assumed that you already know these stuff and most people really don't know it.

    V=R*I applies to passive devices. The reason it applies to passive devices is that they are passive. The only way to actually see what the Hek they are doing is to provided some kind of input, and then observe the result.

    Operational Amplifier is NOT a passive device. It plays by a different set of rules.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    If R is infinite then current would be zero, even if there is a voltage. Voltage can exist by itself (see battery) and does not require current but current does require voltage (except in a superconductor where R is zero).
     
  11. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    You can postulate the existence of voltage without any current, and you can also postulate the existence of current without any voltage. Voltage and current are considered independent of each other in lumped circuit analysis. It's only when they are connected to something that they may both appear and then have a defined relationship.

    If voltage is 'connected' to an open circuit, then no current flows. If current is 'connected' to a short circuit, then no voltage appears.

    An ideal op amp input looks almost like an open circuit, and because it is 'almost' like an open circuit to simplify the analysis we often postulate that the input current is zero even though it is not. The real bottom line here is we have to ask the question: "Is the input current comparable to other currents in the circuit?". If the answer is no then we can ignore it and pretend it is zero, but if the answer is yes then we can not ignore it.

    This is why you will see much 'theory' that says the input does not draw any current, because that theory already assumed that the input current was much less than any other current in the circuit that could affect it such that this applied theory becomes no longer valid.
     
    Last edited: Aug 14, 2014
  12. to3metalcan

    Member

    Jul 20, 2014
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    It's also worth mentioning that just because "no" current is flowing INTO the op-amp, that doesn't mean no current is flowing anywhere...most op amp circuits tie the input to ground or virtual ground with a resistor...current is still flowing through the resistor or the previous leg of the circuit to create the voltage at the input. The op amp just doesn't "steal" any noticeable amount of that current.
     
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