Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Opamp multivibrator not oscillating

  1. Mar 19, 2017 #1
  2. jcsd
  3. Mar 19, 2017 #2


    User Avatar

    Staff: Mentor

    That's a very low feedback resistor, might not function very well in a real circuit.

    But I think your immediate problem is that the oscillator is not reliably self-starting. In a real construction you might just have to switch it on and off a few times, but in a simulation this may still not work. Can you put an initial voltage on C1? Say, 3.5V.
  4. Mar 20, 2017 #3

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    my GUESS is the oscillations are tiny and centered around 1 volt.
    Observations leading me to that suggestion:
    1. 100 ms/div timebase is WAY too slow to see 200 khz. Might as well use a calendar.
    2. At 200 khz , 100 nf is 8 ohms?(check my arithmetic)... So you're asking that poor little opamp to drive a load that's less than 30 ohms?

    I think you're trying to drive a railroad spike with a tack hammer . That's why it doesn't move enough to see with your 'scope settings where they are.

    to what sweep speed , vertical sensitivity and coupling mode should you set your 'scope to see if that's indeed what is going on ?

    Look at the datasheet for that opamp. At what load do they specify its drive capability ? How does yours compare?

    old jim
    Last edited: Mar 20, 2017
  5. Mar 23, 2017 #4
    I tried that but it still wont oscillate. I also tried a 125 ohm feedback resistor. Would the feedback resistor need to be higher ?, if it doesnt work then I'll just try building it on a breadboard

    Yes correct, but since impedance is 1/wC then I would have to make C smaller to increase the impedance (I cant change the frequency, it has to be 200k).

    Also the voltage is fixed at that value seen in the image. I checked because I moved that little green bar

    Not too sure what those are


    It says "Large-Capacitance Drive Capability . . . 10,000 pF" which is 10nF. I'm using a 100nF capacitor, so should be able to drive it ? or maybe not must it be smaller than 10n ?
  6. Mar 23, 2017 #5


    User Avatar
    Science Advisor

    The circuit is a Schmitt trigger and an integrator. That type of circuit should start without a problem. The resistor and capacitor that make the integrator need to be changed from 25R with 100nF to higher impedance components with the same time constant. e.g. 2k5 with 1nF.

    There is one problem I see. The Schmitt trigger is referenced to the ground connection in a single supply application. It should be referenced to the mid-point of the supply. So add another resistor with the same value as R3 = 30k to the circuit. Connect it from the R2 - R3 junction, to the Vcc rail. Then it will run. You may need to adjust R2 to correct the amplitude and frequency.
  7. Mar 23, 2017 #6


    User Avatar

    Staff: Mentor

    Yes, but a better approach may be to first get it working properly at some low frequency, then scale it upwards in steps, so that you can monitor its deteriorating performance as you increase the demands on it.
  8. Mar 23, 2017 #7
    Set Vee at -5v and see what happens.
  9. Mar 23, 2017 #8

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    wow you really need to spend some time learning to use the oscilloscope.

    multivibratorNotOscillating thread.jpg

    so R+jwXc needs to be higher, as Baluncore pointed out.
    The capacitor should charge to about midpoint of whatever AC is at output.

    Green Bar? What is its function ?

    Looks to me like you have trigger set to "none" so scope may be just waiting for instructions what to do.
    Were it an old fashioned analog scope i'd suggest set trigger to AUTO which in absence of valid trigger makes one sweep every line cycle.
    I'd expect this computerized scope to be showing the very last trace it ever drew, perhaps even before you connected it to your circuit.
  10. Mar 26, 2017 #9


    User Avatar
    Gold Member

    I'll have to remember that one!

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted