Op Amps

Discussion in 'Homework Help' started by abd2005, Nov 14, 2005.

  1. abd2005

    Thread Starter New Member

    Nov 14, 2005
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    On an inverting op amp, how does Rf and Ri form a voltage divider? Where Rf is a feedback resistor and Ri is an input resistor to the inverting input of an op amp. The non-inverting input is grounded.
     
  2. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    732
    In both configurations, inverting and non inverting, Rin and Rf always both are a voltage divider between output and output.

    If Rin is grounded you read that as 0V (I think your problem is THIS). The other end, obviously, is Vout.
     
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    In a basic inverting op-amp circuit as you describe, the 'voltage divider' is between the signal input at Ri & the op-amp output at Rf.

    The op-amp will act to change it's output voltage so the middle point of the divider (inverting input) is always at the same voltage as it's none-inverting input (i.e. 0V or Ground in your example).

    e.g if Ri is 1K and Rf is 2.5K, a -1V input would cause the opamp output to settle at 2.5V, a -1.5V input would give 3.75V out etc.

    The inverting input of the op-amp in this configuration is sometimes called a Virtual Earth. If you put a meter or scope on it, it's at 0V regardless of the signal through the circuit, a long as everything is in the working range of the op-amp.

    The op-amp circuit Gain is Rf / Ri
    If Ri is larger than Rf you will get a fractional gain.
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    atferrari and rjenkins have both provided very good information regarding the operation of an inverting opamp.

    The only correction I would make is in the inverting opamp circuit gain expression that rjenkins has indicated. I am pretty sure he meant to write "the opamp circuit gain is -Rf / Ri". It is the occurrence of the negative sign in the gain expression that accounts for the inversion that takes place in the inverting opamp.

    hgmjr
     
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