OpAmp Triangular wave oscillator

Discussion in 'Analog & Mixed-Signal Design' started by Thanos_husk, Nov 25, 2018.

  1. Thanos_husk

    Thread Starter New Member

    Nov 6, 2018
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    Hi!
    I need to create a triangular wave oscillator for a project. I was wandering if i can get such a waveform using only one OpAmp. I found this schematic witch takes the waveform from the red dot , but i can't figure out how should i choose the resistor and capacitor values. I need it to have a frequency of 38kHz and input voltage of 20V dc (as shown in the schematic).
    Any help would be awesome.
    Thanks a lot.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    That simple circuit would need to have an opamp supply voltage greater than 20V to get a 20V 'triangle' wave. The wave is actually an exponentially rising and falling wave rather than a true triangle, and would become distorted as soon as any significant load is applied to the red dot point.
    What will you drive with the 38kHz?
     
  3. Thanos_husk

    Thread Starter New Member

    Nov 6, 2018
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    The wave's Vp-p should be around 0.5 to 1V. Not even close to 20.
     
  4. danadak

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    Mar 10, 2018
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  5. Thanos_husk

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    Nov 6, 2018
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  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    So why the 20V rail? It would certainly help for getting a nearly linear chunk of the exponential waveform, but otherwise seems excessive.
    Methinks you are going to need another opamp to buffer the triangle wave in order to drive anything with it.
     
  7. danadak

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  8. Thanos_husk

    Thread Starter New Member

    Nov 6, 2018
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    First of all. Sorry but i can't understand the attachment that you sent. Second , i've already found the link that you sent. But i need to use the one i attached at my question. Not this one.
    Thanks a lot
     
  9. Thanos_husk

    Thread Starter New Member

    Nov 6, 2018
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    I just need to take a triangular wave form , filter it with a first order low pass filter and then turn it into a sin wave with a second order low pass filter.
     
  10. ebp

    Well-Known Member

    Feb 8, 2018
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    The short answer is that you cannot produce a triangle wave with linear rise and fall with a single op amp.
     
  11. Thanos_husk

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    Nov 6, 2018
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    I don't want it to be fully linear.
     
  12. Ylli

    Member

    Nov 13, 2015
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    Look again at danadak's link, https://pcbisolation.com/blog/triangle-wave-generator/ . Remove C2, R5 and R6 from that and it is the same as your original posted circuit. Although I didn't read it, the analysis is probably correct and will apply to your original circuit.
     
  13. danadak

    Well-Known Member

    Mar 10, 2018
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    The attachment is the development tool for PSOC UP. Ignore it if
    you have no experience with processors. Note it can generate
    two waveforms at same time, tri and sine if you want.

    If the goal is to make sine then many oscillators can be done with one
    OpAmp all generating a sine, phase shift osc, colpitts, wein bridge....

    Regards, Dana.
     
  14. grahamed

    Active Member

    Jul 23, 2012
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    As has been said what you ask for is not possible.

    For a small voltage range, and given that the output voltage swing of the OPA is much higher (and that is why you do need 20V supply despite what may have been said) the unloaded waveform will be tolerably triangular.

    However any loading of the point, especially by something like a low-pass filer, will destroy the linearity. You must buffer the signal (generally using another OPA). If this is some kind of homework or test exercise then I would complain about its validity, otherwise I'd use an additional OPA.
     
  15. Veracohr

    Distinguished Member

    Jan 3, 2011
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    If the ultimate goal is a sine wave, why not make a sine wave oscillator? That's perfectly feasible with one opamp.
     
  16. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    Sure it is.

    As pointed out in posts #9 and #11, the triangle waveform does not have to be linear. In fact, the more of an exponential curve there is to it, the less filtering it will take to turn it into a sine wave.

    ak
     
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