oscillator

Discussion in 'General Electronics Chat' started by TAKYMOUNIR, Jul 25, 2013.

  1. TAKYMOUNIR

    Thread Starter Active Member

    Jun 23, 2008
    351
    1
    Hi i try to simulate this circuit by using pspice but it did not work so can some one help me what is the problem
    thanks
     
    • osc.PNG
      osc.PNG
      File size:
      17 KB
      Views:
      64
  2. LvW

    Active Member

    Jun 13, 2013
    674
    100
    What means "did not work"?
    As a most import aspect in simulating a circuit - one should approximately know in advance what the result should be.
    In case you know that - it would be wise to inform us about your expectations.

    What do you want? An oscillator (with sinusoidal output) or a multivibtator (relaxation oscillator)? What frequency? And what do you get from the simulator? Nothing? You see - a substantial answer can be expected only in case of a good and detailed question with all necessary information.
     
    TAKYMOUNIR likes this.
  3. TAKYMOUNIR

    Thread Starter Active Member

    Jun 23, 2008
    351
    1
    the out put should be square wave and frequency about 30khz
     
  4. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    I believe that r6 might be meant to feed hysteresis back positive amp input and not connect to the positive supply.

    I have managed to play with the values and get the amplifier to start to work in LTSpice by making that change.



    It oscillates but the waveform is very triangular. That could be because I wasn't using an LM139 as the Op Amp.
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    @ Patato Pudding,
    The resistor at the output to the power supply is needed for the LM139, as the comparator has an open collector output.

    @TAKYMOUNIR,
    Patato Pudding is right about the positive feedback resistor.

    Take a look at this picture, taken from the National datasheet:

    [​IMG]

    Bertus
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    Apparently most oscillator types are kicked off by resistor noise or some imbalance due to tolerances on individual components.

    Usually - in a simulation; you have to give it a prod to get it going. Probably a voltage source programmed for a single narrow pulse.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    This is typically not necessary in a hysteretic oscillator, which this.
    It oscillates in simulation for me in LTspice. The frequency is about 185kHz.
     
  8. LvW

    Active Member

    Jun 13, 2013
    674
    100
    Yes - that´s correct. R6 must be changed to provide positive feedback.
     
  9. Shagas

    Active Member

    May 13, 2013
    802
    74
    Hmm I sometimes see this kind of resistor in a wien bridge oscillator . (output to positive rail with a 10k)
     
  10. LvW

    Active Member

    Jun 13, 2013
    674
    100
    Yes? I never did!
    Such a resistor (if it is not a pull-up resistor) acts primarily as a load - that`s all.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    R4 provides the positive feedback. R6 is the pullup for the open collector output of the LM139.

    The circuit is a hysteretic oscillator, similar to the one posted by bertus in post #5. The only difference is the addition of R7 and D1, which change the duty cycle.
     
    Last edited: Jul 28, 2013
  12. LvW

    Active Member

    Jun 13, 2013
    674
    100
    Oh yes - sorry for my mistake. I overlooked the positive feedback path (perhaps because it is shown at the bottom of the drawing).
     
Loading...