Oscillator using ad829 works - trying to make it work using 741

  1. FOIWATER

    FOIWATER 391
    Gold Member

    Hello,

    I have a triangle wave generator working nicely with video op amps but im trying to substitute much slower op amps. I am required to get it to oscillate at 30 kHz. I performed calculations and got it to about 29.8kHz you can see it attached for the AD829's

    I tried substituting 741 for the AD829 and I am having issues getting it to work?

    I was wondering if someone could help me.

    Please see attached, thanks

    AD829 circuit.png

    AD829 waveform.png

    741 waveform.png
     
    Last edited by a moderator: Feb 6, 2014
  2. jcsd
  3. FOIWATER

    FOIWATER 391
    Gold Member

    PS: I am not sure how to calculate the slew rate limitation since the input is not sinusoidal.

    I am not sure how to overcome the issue.. any information appreciated
     
  4. SR_min = 10 * (+Vsat - -Vsat)/(1/2F) = 10 * (+12V - (-12V))/16.7μs = 24V/16.6μs = 10 * 1.44V/μs = 14V/μs
    So you need to change the op amp try TL071.
    Also I don't like your R2 and R3 resistor values and C3. Change R2 to 20K and correct C2 or R4 to get 30khz
     
    Last edited: Feb 6, 2014
  5. FOIWATER

    FOIWATER 391
    Gold Member

    Agreed Jony upon using cursors I see that it isn't at 30kHz although mathematically it should be I had to change some values.

    Thanks I will use another op amp.

    TL071 isn't something available immediately, what are your impressions of the LF356 JEFT opamp for this?

    Seems to simulate well, but I've heard it can be touchy in practice.
     
  6. LM356 has almost the same slew rate as TL071.
     
    1 person likes this.
  7. FOIWATER

    FOIWATER 391
    Gold Member

    Cool, Thanks once again Jony
     
  8. Try to change R2 to 20K or large and this will reduce the amplitude of a waveform and reduce SR stress.
     
  9. FOIWATER

    FOIWATER 391
    Gold Member

    yes R3Vcc/R2 gives the trigger points of the trigger but I need the amplitude large I am driving a mosfet and I need the 12 volts.. maybe Ill post the rest of the circuit tonight I will try to assemble it
     
  10. FOIWATER

    FOIWATER 391
    Gold Member

    Jony - I don't have slew rate issues anymore (even at 12 volts) but what I do have is an annoying cut off at the lower end. I was warned the LF356 is touchy. I have been backed into a corner and am resulting to overkill using a video op amp. for the integrator, and a LF356 for the shmitt trigger. In your opinion, would you think two LF356 would definitely be able to perform this function or is it possible it may be outside of its capabilities?

    I attached a picture of the annoying cutoff using the LM356 to integrate.. Any feedback appreciated as always
     

    Attached Files:

  11. NascentOxygen

    Staff: Mentor

    Can you display the comparator's output on the other channel? Superimposed.
     
  12. FOIWATER

    FOIWATER 391
    Gold Member

    I will take it later if that's OK, (not because I am lazy) but because the scope is in my room mates room and he isn't home

    Thanks for the reply N.O.
     
  13. What is your supply voltage? What type of a op amp you use as a Schmitt trigger?
    Also show as Schmitt trigger output and try to change R2 to 20K.
     
  14. FOIWATER

    FOIWATER 391
    Gold Member

    Supply is +-12, I am using the LF356 as the Schmitt trigger, I will post output of schmitt trigger tonight. I will change R2 to 20K and show it. I did change it here at home on your advice it works fine perfect trianglular output with two LF356's but the frequency of 30khz was hard to obtain. But I need 12 volt output to drive a mosfet what do you suggest
    I'll try to post more specific results so we can be more specific, my bad.
    I will do as you suggest, thanks
     
  15. NascentOxygen

    Staff: Mentor

    Are you needing the triangular output to drive the FET, or the squarewave? Are you wanting that driving waveform to be 12v pk-pk or 24v pk-pk?
     
  16. FOIWATER

    FOIWATER 391
    Gold Member

    Sorry,
    Using the triangular output as an input to a comparator, along with the signal from a PI controller (which takes the output from a buck converter) So the output of the comparator is actually driving the mosfet. As you said, square wave. Oh, I see. I can just have a 12 volt supplying my comparator....

    I want the driving signal to be 12 v peak, 24 peak to peak

    OK I will try to reduce the rail voltage in my oscillator and use the right components based on Jony's suggestion to get it working nicely. I will post back with more waveforms when I get a chance.

    Thanks
     
  17. FOIWATER

    FOIWATER 391
    Gold Member

    Do you know whether or not there is any disadvantage to going possibly even lower in amplitude than 6 volts? (ie, using a resistor even larger than 20K)

    I will let you know what I am trying to do here, I am trying to achieve this circuit I have attached.

    I assume the oscillator amplitude could be very low in fact?

    The idea is that the input voltage can change (lower than the 28 volt supply) and the output is to be a regulated 20v, 4A. A classic buck converter. Right now i have the oscillator working but has to be with AD829. But as you said, I wasn't realizing that its the 311 output that ultimately determines the amplitude of the gating signal. I guess I wasn't thinking clearly.
     

    Attached Files:

    • ckt.png
      ckt.png
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  18. NascentOxygen

    Staff: Mentor

    Changing the Schmitt's trigger levels by changing R2 has the effect of changing the amplitude of the triangular wave. It's the triangular wave that is used in your controller, so you may not be free to meddle with its amplitude without understanding the effect this will have on the operation of the 311 and how it controls the rest of the circuit.

    Does the published schematic really show R2 and R3 of equal value?
     
  19. FOIWATER

    FOIWATER 391
    Gold Member

    I apologize, this is my schematic
     
  20. FOIWATER

    FOIWATER 391
    Gold Member

    The resistor sizes are left out because I do not know how to set them lol

    I understand only how things should be laid out approximately.
     
  21. NascentOxygen

    Staff: Mentor

    Do you mean you are working off a published schematic but where none of the component values is specified? Or is the whole 10 IC circuit your own 'invention'?
     
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