understanding operational amplifier

Discussion in 'General Electronics Chat' started by PG1995, Aug 24, 2011.

  1. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Hi

    Please note that I'm a beginner so please keep your replies simple and straightforward so that you could be understood and your effort fully appreciated. Thank you.

    1: Why is operational amplifier or op-amp so called? Is there also something like non-operational amplifier?

    2: As I see it operational amplifier could be modified to do different things with the input signal. For instance, it could be designed into an inverting amplifier, non-inverting amplifier, etc.

    3: Is a simple oscillator also made from an op-amp?

    4: Can you please provide me some link which shows me in images that how different type of amplifier, such as inverting amplifier or differential amplifier, affects the input signal? The Wikipedia page only gives math formulas.

    Thank you very much for your help and time.

    Best wishes
    PG
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You will need to delve into the history of operational amplifiers and how they came to be. Here is a good reference:
    http://www.analog.com/static/import... Amp Applications Book (PDF)/P2 ChH_final.pdf
    The name "Operational Amplifier" was assigned to that general class of amplifiers in 1947.

    Yes, they are very versatile (flexible) which is why they are so commonly used.

    They certainly can be. Texas Instruments has at least one application note for various sine-wave oscillators using operational amplifiers.
    Here is the relevant chapter from the application note:
    http://www.ti.com/lit/ml/sloa087/sloa087.pdf

    Try this Java-based circuit simulator:
    http://www.falstad.com/circuit/
    You must have Java installed on your computer for it to work.
    The simulator will pop out of the page. If you have a popup blocker turned on, you won't be able to see the simulation.

    Then on the menu, click on Circuts, then Op-Amps, and select circuits under that menu. There are a number of simulations that show the results in an animated manner. You can speed them up, slow them down, stop them, and re-start them. You can also change component values to see what happens.
     
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  3. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    Thank you very much, Sgt. Your reply was very helpful. That applet simulation site is good one. Thanks for it.

    Best wishes
    PG
     
  4. PG1995

    Thread Starter Active Member

    Apr 15, 2011
    753
    5
    1: A op-amp can be modified to do different things. For instance, it could be modified into a summing amplifier who does addition. But, is there anything in itself such as op-amp? What does it do? Or, is it simply a catch all term for all related amplifiers such as differential amplifier, summing amplifier, etc.?

    2: Are there other kinds of amplifiers besides op-amp?

    3: I have always been under the impression that an amplifier is used to amplify voltage. Can an amplifier also be used to amplify current?

    4: The word 'feedback' simply suggests something that is returned to a machine, system, or process to improve output. I have seen in some circuits that a wire connects the output of the amplifier with one of its input and I presume it's called feedback. How would you explain this feedback and its use? Please keep it simple.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    The opamp itself is not modified; external components are added to the basic "building block" that is an opamp to cause the circuit as a whole to behave in a desired manner. One needs to be able to understand, or least apply, the mathematics necessary to go from the basic building block to accomplishing the desired function.
    An opamp is a basic building block. There are various sub-classes of opamps, but I'm not going to get into that; I don't have the time to explain them all.

    Yes, there are. One very common type is an audio amplifier, such as an LM386. On the surface, they seem identical to an opamp; even using the same symbol. However, they are frequently wired internally for a default amount of gain, and other parameters.

    Yes. Many opamps are rather limited as far as current output; somewhere between 10mA and 50mA would be a typical limit - but your mileage will vary, depending on the exact model. There are "power" opamps that can output quite a bit more current. An L272 is a very common power opamp, capable of 700mA source or sink in the output. However, it is rather slow.

    One has to consider a great many parameters to select the best opamp for a particular application.

    Most opamps have extremely high gain; several hundred thousand to one or more. Without feedback, even a very minute change in the input signal could cause the output to go from one power rail to the other.

    This is not how operational amplifiers are best used; if you need that kind of response, you need a different class of device, namely a "comparator", which uses the exact same triangular symbol as an opamp - just to confuse you. ;) Opamps are used when you wish to have a continuously variable a.k.a. analog output. Comparators are used when you want a Yes/No True/False (digital logic) output.

    It's a matter of using the correct class of component for the task at hand. You could install a wood screw by using a hammer, but the results would be so much better if you had used a drill and screwdriver instead.

    If the output of an opamp is connected to its' inverting input (-) with no resistive divider to ground, power, or other input, then the opamp is being used as a buffer aka "voltage follower"; whatever signal is applied to the non-inverting input (+) will be reproduced at the output of the opamp.

    The buffer aka voltage follower is perhaps the most basic function of an operational amplifier. It also illustrates how an opamp functions, which is very important: the inverting input attempts to adjust the opamp output so that the feedback to the inverting input matches the signal seen at the non-inverting input. Once you fully grasp that basic concept, opamp circuits start making much more sense.

    I previously linked to chapter 15 of Texas Instruments' "Op Amps for Everyone" application guide.
    A link to the entire guide is here:
    http://www.ti.com.cn/cn/lit/an/slod006b/slod006b.pdf
    You should save it to your computer, and read through it in detail several times. Try the circuits on a breadboard to see how they work. You will need to work through the math presented.
     
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