inverting operational amplifier

Discussion in 'General Electronics Chat' started by sharanbr123, Oct 16, 2014.

  1. sharanbr123

    Thread Starter Member

    Sep 29, 2014
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    0
    Hello All,

    I would like to get some inputs on the inverting operational amplifier. I have attached picture (available on public domain) to put forth my questions.

    1) With respect to inverting operational amplifier, how should Rf path be treated. Is it feedforward path from input to output? or it is feedback path from output to input? I am a little confused here ...

    2) It is given that Ib current into the inverting pin of op-amp is very small (almost 0). Is this because the input resistance is very high?

    3) If the real voltage gain is primarily due to resistances Rf and Rg then why the triangle symbol is called amplifier when the trick is due to these resistances?

    4) By the term amplifier, does it mean that input shape will be maintained at output (with invertion) and higher amplitude?

    Thanks a lot in advance ...
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,449
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    An ideal operational amplifier would have the following properties:

    1) Infinite input impedance
    2) Zero output impedance
    3) Infinite voltage gain
    4) Infinite bandwidth

    An ideal opamp by itself with these properties would not be a useful circuit except as a voltage comparator.

    In order to tame the beast, one applies negative feedback to bring the gain down to a usable value. That is the purpose of the feedback resistors.

    So...

    1) Feedback is from output to input.

    2) Input current is zero because input resistance is very high.

    3) The opamp is an excessive amplifier. The resistors are required to reduce the gain to something manageable.

    4) An ideal linear amplifier ought to reproduce the input signal faithfully with zero distortion. The output can be higher or even lower than the input voltage, i.e gain can be less than 1.
     
  3. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi 123,
    In Q2, which Ib are you referring too.?
    E
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,148
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    I cannot really answer except to point out that most op-amps cannot operate effectively at RF. They're too slow.
    Exactly right. 1MΩ is not unusual and there are o-amps with insanely high input impedance.
    The resistances manage the inputs seen by the amplifier, but the application of power supply energy in response to those inputs occurs inside the amplifier.
    That's often the goal but it doesn't have to be; an op-amp is often used to adjust other features of the waveform, including to differentiate or integrate it.
     
  5. sharanbr123

    Thread Starter Member

    Sep 29, 2014
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    Hello Eric, Ib is the current into the inverting input of the op-amp ... the figure also shows this ...
     
  6. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    Ib is the Input Bias current and is dependent upon the design of the input stage.

    I have found a tutorial that should help you.;)

    E
     
  7. sharanbr123

    Thread Starter Member

    Sep 29, 2014
    49
    0
    Thank you very much ...
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    IMHO, RF is the feedback resistor in this context, and does not stand for Radio Frequency.
     
    wayneh likes this.
  9. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    If you feel that triangle symbol is not necessary, then don't use it.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    Yup, I totally blew that one.
     
  11. Papabravo

    Expert

    Feb 24, 2006
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    'saright, I've had my share of missteps.
     
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