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What does an simple OP AMP do, I do not understand.

Discussion in 'General Electronics Discussion' started by apples, Aug 18, 2012.

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  1. apples


    Jul 1, 2012
    What does the most basic OP AMP do. I think it is like a inverting op amp.

    Is it you feed it 12v in and it spits out say 24V???

    How can you make a higher voltage from a lower voltage?
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    If you input 12V and it outputs 24 then the power supply must be greater than 24V.

    How can you make a higher voltage from a lower one? The same way any ampifier makes a soft sound a loud sound -- by amplifying it.
  3. apples


    Jul 1, 2012

    Your comment makes no sense to me. First you say the voltage must be greater, then you say it doesn't have to , just amplify it???

    Confused here.
  4. davenn

    davenn Moderator

    Sep 5, 2009
    Hi Apples
    welcome to the forums :)

    no, steve didnt say it doesnt, he gave an example of voltage amplification
    you need to understand how a transistor operates ... basically, you use a small input voltage to control a larger output voltage.

    see this tutorial ....

    here is a tutorial on Op-Amp basics, have a good read and come back with any questions :)

    Last edited: Aug 18, 2012
  5. john monks

    john monks

    Mar 9, 2012
    An op amp or operational amplifier is a form of differential amplifier.
    It has two inputs and one output. The gain result from the differential voltage that occurs between the two inputs. One input is called the positive input because when this input voltage is made to go positive relative the the other input the output pin goes positive. The other input is called negative.

    An ideal op amp has:
    Infinite gain. That is for any change of differential input voltage the output voltage goes to an extreme, one of the power supply voltages.
    Infinite input resistance. That is no current is present at either input.
    Infinite bandwidth. That is it has no limit of how fast it can operate or no limited frequency of operation.
    Zero output impedance. Another words it can supply any current. There is no limit of load.

    An ideal op amp does not exist and compromises have to be made.
    You cannot get any voltage greater outside the supply voltages or "rails"

    The gain of on op amp circuit is set up by external components, usually resistors, but sometimes capacitors and inductors.

    The reason op amps are popular is because of the ease at which a high quality amplifier can be made. They have a very good frequency response, have low internal noise, have great immunity to temperature changes, and are very easy to manipulate for making band pass or notch filters.
    It is easy to make an inverting amplifier and a non-inverting amplifier. That is you can make an amplifier that has a gain of 10 or a gain of -10.

    I suggest you get a uf741 and play around with it.
    Last edited: Aug 18, 2012
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Perhaps another simpler explanation.

    An amplifier can be thought of as having connections to three things:

    1) an input
    2) an output
    3) a power supply

    Ignoring some very special cases, the output cannot exceed the bounds of the power supply. For example if the power supply was a simple 30V, the output could not go below 0, and could not rise above 30.

    How the amplifier actually transforms a small voltage (say 1V) on the input to a higher voltage (say 10V) on the output is another story, but essentially it involves (as Davenn says) the output being a controlled version of the power supply that is controlled by the input signal.

    Davenn also points out that this is voltage amplification. It's worth pointing out (although you possibly don't need to understand it right now) that the output voltage does not have to increase for the device to amplify.

    None of these things are particular to an op-amp. John Monks describes some things that are particular to op-amps, but understanding a much simpler amplifier first is important.

    What is also important is the theoretical model of the op-amp. The simplest theoretical model allows for the output to exceed the power supply (they do this by basically ignoring the power supply entirely). If you are using a simulation where the power supply to the op-amp is not shown (especially where it is not allowed for) you may be getting results that cannot happen in practice.
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