1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Filter circuits

  1. Mar 18, 2008 #1
    I did a lab using an active low pass filter with an op amp and at the end we were instructed to remove the capacitor that was in parallel with the resistor. The result was an extremely high voltage compared to the voltage with the capacitor in the circuit. We are asked to explain what kind of circuit this is w/o the capacitor, and why it behaves this way. I have re-read the entire chapter and am at a loss.

    With the capacitor in the circuit, the capacitor acts as a short when the frequency is high--> so the voltage goes to zero. So i can understand why the voltage would not go to zero quickly with the capacitor removd, but I have no idea what you call thei circuit and why it goes so high.

    Can anyone better explain this or point me in the right direction?

  2. jcsd
  3. Mar 18, 2008 #2
    I realise that you may be talking about a 'bog standard' filter with an op amp but could we have a circuit diagram. I don't want to go off on one about a circuit different to yours and confuse matters.
  4. Mar 19, 2008 #3


    User Avatar
    Science Advisor
    Homework Helper

    You say the result was an extremely high voltage. At all frequencies? I'll take a guess, that your initial active low pass filter looked like figure 1 in http://engr.astate.edu/jdg/Electronics/Lab1/04LowPassFilter.html [Broken] reference. When you take the cap out of that type of circuit, write out the transfer function and see what you get. If you do that properly, I suspect you will recognize the type of circuit you have.

    Last edited by a moderator: May 3, 2017
  5. Mar 20, 2008 #4
    Yes, thats is. It started out at about 28 volts and began to decay at about 4000Hz reaching near zero at around 1MHz.
    We have not dealt with transfer functions, can you be more specific?

    Last edited by a moderator: May 3, 2017
  6. Mar 20, 2008 #5
    It is the circuit in fogure 1 that the other poster linked to.

  7. Mar 21, 2008 #6
    Does the removal of the capacitor make it an amplifier? This would explain the amplified voltage.
  8. Mar 22, 2008 #7
    Yes, using that circuit without the capacitor changed it into being an amplifier, with the resistor between inverting input and output as the feedback, as you should know that in theory a Opamp has infinite input impedance and infinite gain, so you need a feedback resistor to control the actual gain of the overall system.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook