inverting amp with negative feedback

Discussion in 'The Projects Forum' started by Joe24, Aug 26, 2008.

  1. Joe24

    Thread Starter Active Member

    May 18, 2007
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    Hello,

    consider an amplifier configured as an inverting amp with negative feedback. in the feedback path, there is of course a feedback resistor. now, if i place a capacitor in series with this feedback resistor, what happens? what is the purpose of this capacitor? i ask because i saw a circuit like this but not sure why the cap is placed there.
    thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Do you have a link to the schematic, or could you post it?
     
  3. Joe24

    Thread Starter Active Member

    May 18, 2007
    52
    0
    Ok I have attached the schematic.

    To give a bit of a background on the circuit:

    This is part of a bigger circuit, but for now I am only interested in this part. At the inverting input, the voltage source is the control voltage that ranges from 0 - 10 V. The voltage at the non-inverting input is the output voltage a some other circuit in the overall system. This AMP compares these two voltages and produces an output.

    What is the role of the capacitor??

    Thanks
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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  5. Audioguru

    New Member

    Dec 20, 2007
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    An opamp has a DC voltage gain of about 200,000.
    If there is a very small difference beteen the input voltages in your circuit then the opamp will have its output saturated near the positive supply voltage or near the negative supply voltage.
    You do not want a capacitor in series with Rf. without the capacitor then the opamp will have negative feedback at all frequencies including DC.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    To add to the above;

    A capacitor blocking the DC feedback path of an operational amplifier is generally a bad idea. The outputs of opamps are not designed to be operated in a continually saturated mode; they will overheat and have a very short life span. A capacitor can be used in parallel wth the feedback resistance to improve transient response, but should not be used in series with the feedback resistance.

    The circuit could be implemented using comparators, which are designed to be operated with their outputs in constant saturation. The reliability of the circuit would increase considerably.
     
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