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Inverting and Non-inverting amplifiers

Discussion in 'General Electronics Discussion' started by codeman_nz, Mar 26, 2014.

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  1. codeman_nz

    codeman_nz

    3
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    Mar 26, 2014
    Hi everyone,

    With the inverting amplifier the input voltage goes through the input resister into the inverting input and the non-inverting input is connected to ground. But with the non-inverting amplifier the input voltage goes straight to the non-inverting input and the inverting input is connected through the resister to ground.

    Why the difference?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,491
    2,833
    Jan 21, 2010
    It is because one is an inverting amplifier and the other is a non-inverting amplifier.

    In theory, both inputs should have a resistor (or the same impedance to the 0V rail) to help eliminate errors caused by input offset currents.

    Post an image if you think I've understood your question poorly.
     
  3. codeman_nz

    codeman_nz

    3
    0
    Mar 26, 2014
    Sorry I did not explain myself properly. Why does the feedback resister go to the inverting input in both? Why must it go to the inverting input?
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,491
    2,833
    Jan 21, 2010
    Ah!

    That's because it's negative feedback (to control gain) and that *always* goes to the inverting input.

    Negative feedback makes the output work against the input. The resistors are chosen so that it cancels out at exactly the gain you require.

    If you have positive feedback (to the non-inverting input) the output adds to the input which forces the output to swing completely to one rail or the other. This is not what you want in an amplifier.
     
  5. codeman_nz

    codeman_nz

    3
    0
    Mar 26, 2014
    Ah, that explains it. Thanks
     
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