help help help

Discussion in 'General Electronics Chat' started by avionicsgirl, Oct 31, 2004.

  1. avionicsgirl

    Thread Starter New Member

    Oct 30, 2004
    7
    0
    if you are in here just post a message i need to ask a question.
     
  2. avionicsgirl

    Thread Starter New Member

    Oct 30, 2004
    7
    0
    ok the question i need answered.



    the negative feedback loop has a resistor in parallel with a capacitor. what is the resistor there for and how do you figure the value?
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Assuming you mean an op amp's negative feedback line, the combination of the resistor and capacitor act together to integrate the op amp's response to a step input. Let's say the input sees an abrupt 1 volt change. The output, which would normally swing at the maximum rate, will instead respond more gradually. The slewing rate will be close to the RC time.

    The classic use of the integrator is to change a square wave input into a triangle. The secret is to make sure that the RC time exceeds the period of the square wave so the output voltage never gets to a stopping point. With a two op amp rectifier, an integration cap changes the output from absolute value to RMS.
     
  4. Maxx

    Active Member

    Oct 30, 2004
    43
    0
    Usually a negative feedback resistor would in simple terms set the gain of a circuit and the capacitor is often used to aid stability and help with upper frequency rolloff.

    This is just a very quick generalised answer as i am only trying to guess at a circuit you might have in mind.

    Plus i'm half asleep and should have been in bed hours ago, so my appologies if i'm away with the fairys at the moment :p
     
  5. Steel

    Member

    Oct 27, 2004
    25
    0
    Putting a cap in the circuit will also build a filter (depending on the placement) and make it an active filter.
     
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