explain op amp in simplest term

Discussion in 'Homework Help' started by bug13, Mar 27, 2012.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi, op amp pops up from time to time in different circuit, and I really want to know what that is, and more importantly, what does it do and the real word application of it.

    However, when I search it on wikipedia, it gives me more questions than answers.

    So if someone can explain it to me as simple as possible, that will be great, thanks :)
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    An operational amplifier or op-amp is simply a black box that takes an input voltage and produces an amplified output voltage.

    Amplifiers were well understood in the days of vacuum tubes, long before transistors and integrated circuits were invented.

    Electronic engineers knew what they wanted in an ideal amplifier.
    Here is their wish list:

    1. Voltage gain = infinity
    2. Band width = infinity
    3. Slew rate = infinity
    4. Input impedance = infinity
    5. Output impedance = zero
    6. Input offset voltage = zero
    7. Input bias current = zero

    A real modern op-amp comes pretty darn close to meeting these ideal specifications.

    An op-amp comes with two inputs, a non-inverting input and an inverting input, which we can call Vin+ and Vin- for this explanation. This type of op-amp is called a differential amplifier or difference amplifier.

    The voltage output Vo = Av(Vin+ - Vin-), where Av is the voltage gain.

    If we apply a common mode voltage to both inputs, we may observe a non-zero output voltage Vcm

    The common-mode rejection ratio CMRR = 20 log (Vo/Vcm) and we want this to be infinite in an ideal op-amp.

    When we have an op-amp with close to ideal characteristics, we can ignore the limitations of the black box and the circuit behaviour will be totally dependent on external components and circuit configuration. That is, we can design a circuit as we please.

    Hope this is a useful introduction to op-amps. Sorry if it sounds too technical.

    Edit: oops, I just read Wikipedia's version of an op-amp and I basically repeated what is already there:

    http://en.wikipedia.org/wiki/Operational_amplifier
     
    Last edited: Mar 27, 2012
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  3. absf

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    So we can never achieve making a near perfect op-amp with today's technology or was is too expensive to make one?

    Allen
     
  4. MrChips

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    Did I say that? I said today's op-amps come very close to an ideal op-amp and for most circuit applications cost less that $1.
     
  5. bug13

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  6. jimkeith

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    Oct 26, 2011
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    The op amp does everything within its power to make both inputs equal in voltage.
     
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  7. crutschow

    Expert

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    That's only true when the op amp is connected in a circuit with proper negative feedback. ;)
     
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  8. #12

    Expert

    Nov 30, 2010
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    I'm with jimkeith as the simplest answer. An op-amp has, besides its power supply, 3 important pins. They are as said before and the action of the amplifier is to do whatever it takes to make both inputs be the same voltage.

    Bowing to crutschow, when you give the op-amp a proper negative feedback circuit, it can accomplish "nulling" its inputs, that is, making them equal voltage as each other. You can also give an op-amp positive feedback and take advanatge of its desire to null its inputs by putting it in a condition where it will seek the wrong polarity and its output will be "stuck" to one supply voltage or the other, depending on what it gets for an input.

    After you absorb that basic theorem, the rest of op-amp study is about dealing with their limitations. "Exactly which chip to choose for this job." There is no perfect op-amp. If there was, all other part numbers would stop being sold! What we're seeing lately are an incredible number of new op-amps available. Some must have their frequency range limited with a capacitor. Some have fairly miserable input offset voltages or bias currents. Only recently have "rail-to-rail" input and output op-amps become available, and I suspect there are limitations to that claim, too. All op-amps have a maximum current they can output. All are limited to outputing no more than their supply voltage.

    Designing with op-amps is about choosing the best you can find, within budget limits, and working within its limitations.
     
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  9. bug13

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    so what's the real world application of a op amp? some basic examples?
     
  10. MrChips

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    Real world? Op-amps are used everywhere, it is a basic building block like bricks and mortar, sugar and salt, flour and eggs.

    - radios, mp3 player
    - cell phones
    - power supplies
    - computers
    - communications
    - aerospace, healthcare, factories, business, stockmarket
    - cars, trains, boats, planes
    - any and every thing electronic
     
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  11. Jony130

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  12. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Thanks MrChips, and could you give some more basic example? like a example you will see in a op amp 101 book. thanks:)
     
  13. bug13

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  14. absf

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  15. bug13

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    what are TI and NS? I am new to electronics and don't really know what those are.
     
  16. absf

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    Dec 29, 2010
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    What would you recommend for a better substitute for LM358 that has low input offsets and output able to go as close to the rail as possible?

    Allen
     
  17. panic mode

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    Oct 10, 2011
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    TI - Texas Instruments
    NS - National Semiconductors
     
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  18. #12

    Expert

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    I wouldn't recommend an op-amp. There are too many parameters. What if I liked one that cost $20? Had the wrong voltage range? What I think is "close to the rail" isn't what you think it is?

    I would recommend that you go to www.mouser.com and learn how to use a parametric search engine. That way YOU can find out what's available and what they cost.
     
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  19. panic mode

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