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Oscillator doesn't oscillate

  1. Aug 30, 2013 #1
    Hi
    Does anyone have an idea of why my oscillator doesn't oscillate? It's supposed to generate sawtooth. But the scope shows constant -13V. Actually the output of the oscillator has a more stable voltage than the input voltage source!

    It works IRL with ua741 opamp (but the 741 doesn't provide enough current for my application)
    It works in simulation with lm1875t opamp.
    While I was trying to debug and placing probes around the circuit all of a sudden it started to work... and after 30seconds or so it stopped just as sudden as it had started.

    Then I took the components off the board and made a square wave generator circuit that worked. I checked both opamps and the capacitor with the square wave generator, so the components seems not to be defective.

    Then I put them all back to the sawtooth generator circuit, and again it's output is constant -13V.

    So I took a new breadboard, all new components, this time I added some potentiometers in order to tweak it a bit. Hopefully that would get it started, but it didn't. The output is barely changing when I turn the potentiometers.

    I've tried 3 different capacitors; 2nF, 20nF, 200nF. It doesn't affect the output at all (except for the startup time when voltage drops from 0 to -13).

    Something elementary must be wrong. Could it be that the lm1875t is to fast at voltage changes so it adjusts for feedback too fast and becomes stable? (I have never before used any opamp as powerfull and fast as the lm1875t.)
    Any ideas where I'm failing?

    Attached is a sketch of the latest test circuit with potentiometers.
     

    Attached Files:

  2. jcsd
  3. Aug 30, 2013 #2

    berkeman

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    Where did you get the circuit from? It would be good to see a source circuit -- I can kind of see how this one is supposed to work, but a couple things about setting the hysteresis are not clear to me.
     
  4. Aug 30, 2013 #3

    berkeman

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  5. Aug 30, 2013 #4

    berkeman

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    So the main difference I see right off the bat is that the 2nd opamp's + input should be grounded (at least in the simplest implementation). Try turning your 160k pot so that it grounds the + input of the 2nd opamp...
     
  6. Aug 30, 2013 #5
    There are loads of implementations of this circuit out there.
    The 160k pot is the one that (in simulation) makes the difference in rise and fall time. When turned to ground the rise and fall times will be equal. Turning it away from ground changes the output towards sawtooth - in simulation.
    IRL turning the pots doesn't make any difference at all. That includes turning the 160k all the way to ground - still got the ultimate noiseless DC output.
     
  7. Aug 30, 2013 #6

    berkeman

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    Can you label all the nodes in the schematic with the DC voltages from your simulation?
     
  8. Aug 30, 2013 #7
    (Sketch in #1 was made in NI Multisim 30day demo, because it's the only simulator I found that has lm1875t in it's library.)

    I've now created a similar circuit in Lt Spice. Using LT1366 as opamp it worked. Using LT1210 the simulation behaves just like my RL lm1875t circuit. LT1210 is faster and more powerful than lm1875t. LT1366 is slower and less powerful. This strengthen my idea that the issue I'm facing IRL is that the opamp is to fast/powerful for the circuit. It sounds strange though.

    If that actually is the case, how can I modify the circuit to work with fast and powerful opamps?
     
  9. Aug 30, 2013 #8
    In simulation (in Ni Multisim) there is AC everywhere. Do you want me to lable simulation AC averange, or RL DC measurements?
     
  10. Aug 30, 2013 #9

    berkeman

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    Are you saying that the output is DC, but there are AC waveforms everywhere else? Maybe I'm misunderstanding the orignial problem...
     
  11. Aug 30, 2013 #10

    AlephZero

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    You could leave the oscillator working with your low power op amps, and use its output to drive your high power amp (e.g. as a unity gain amplifier).

    Trying to take a lot of power direct from an oscillator isn't a particularly good design idea, even if it does work. If your "large" load is not a pure resistance, you might be messing up the phase shifts in the oscillator circuit, which might stop it oscillating.

    From your descriptions it seems like your "non-oscillator" has got one of the op amp outputs locked up to a supply rail voltage, which will certainly stop it oscillating - but I've no idea why that is happening.
     
  12. Aug 30, 2013 #11
    Good point about lowpower oscillator + amplifier! That is probably the way to go.

    Currently it looks like I've fried 4pc lm1875t. Voltage from powersource drops to +/-1,5V once connected to any of the 4 op amps I've used for testing. No load is connected, both inputs are grounded. Just Vcc and Vee is connected.
     
  13. Aug 30, 2013 #12

    davenn

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    The LM1875 is specifically an audio power amplifier not an Op-Amp as such, its output is designed to drive a speaker, not the input to another LM1875
    why did you choose this device ?

    you should be using a more specific Op-Amp


    Dave
     
  14. Aug 31, 2013 #13
    I looked through the opamps listed on ebay and chose the first I came to that had enough output current and speed. I had no idea that some opamps can't drive another of the same kind.
     
  15. Aug 31, 2013 #14

    davenn

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    I dont think you understood what I said .... these are not really op-amps as such
    They are audio amplifier chips designed to drive speakers

    You should really be using something like TL072 or similar for the actual oscillator ... once you have an osc working ... then you could use the LM1875 or some other audio amplifier chip to boost the output

    cheers
    Dave
     
  16. Aug 31, 2013 #15
    I've now learned that oscillator based on lm1875 is not good.

    Is ua741 suitable for driving the oscillator before lm1875 amplifies it?
    (Those are the two kinds I have. If they're no good I'll need to order others, i.e TL072)
     
  17. Aug 31, 2013 #16

    jim hardy

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    741 should drive an audio amp quite nicely.
    I much prefer LM324. They're cheap and easy to use and okay for single or dual supply.
    And the most widely used opamp ever so they're widely available, even Radio Shack .
    Input and output specs are better than 741.
    http://www.ti.com/lit/ds/symlink/lm324-n.pdf

    ...........................................................................................

    I suspect that your trouble lies with LM1725's input.
    The datasheet is a bit sketchy as to input impedance
    http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=lm1875&fileType=pdf

    look at parameter "Input Bias Current"

    so long as the amplifier can balance its inputs it draws only a microamp or so of input current
    but the datasheet doesn't say what happens when input voltage differential becomes substantial as it does for U1. They don't expect us to use it that way. It's not a general purpose opamp.

    Observe that in order to oscillate , your circuit must push or pull enough current through R1 to change state of U1.
    and we don't know how much current that is.
    Furthermore your unequal supply rails(more + than -) are in the direction to make U1's output stay positive , and your symptom(-13V out) agrees with that. There's less negative to pull current through R1 than there is positive to push it through R2 .
    That's probably not an issue if R3 is set fairly high.

    Lastly C1 can pass a LOT of current into U2's inverting input.
    That can cause latchup in some amplifiers, see

    http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=slya014&fileType=pdf

    and that can be destructive.
    I'd stick a kohm or two between U2 pin2 and junction C1-R5

    It'd be very educational to get this working with 1725's.


    old jim
     
  18. Aug 31, 2013 #17

    AlephZero

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    Good point - the 1725 is meant to be a used as a linear amplifier not as a general purpose op amp.

    If you really want to use the 1725 for a "one chIp" circuit you might try a Wien bridge oscillator, which will use it as a linear amplifier (and it only needs one 1725, not two).

    One other comment from the data sheet - it says the 1725s must always be mounted on a heatsink, because the quiescent current can be high (100 ma). Even if they have thermal shutdown protection, they won't work till they cool down again!
     
  19. Aug 31, 2013 #18
    Thanks for the detailed description, Jim.

    I needed 6 lm1875's for the project (5 when using ua741 for the oscillator). I bought 10. Having killed 4 of them, I'm not into retrying what killed them. So I played around with LT1210 in LT Spice. The circuit with LT1210 gave a constant output of 4.8V (positive) in simulation. Adding a R7>300 on U2 pin2 and setting R6 (the 160k pot) at most 30% away from ground made the oscillation start. (The other pots were left on 50%.)
    I guess something similar would get the real lm1875 circuit going. But from what I've learned I'll let the ua741's do the oscillation and use lm1875 just for amplification.

    With this solution I'll run out of ua741's. Do you recommend that I order lm324 or TL074 or both?


    The powersupply I'm using is supposed to be 2x 15V, according to HP. The voltage in the sketch is what I actually measure from it. My original plan was to use a +/-12V from a pc powersupply, but it turns out that -12V drops to -7V already when 32mA is pulled from the terminal. I might need to make a supply circuit too.
     
  20. Aug 31, 2013 #19

    jim hardy

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    Yes PC's haven't much oomph on their -12 line.
    You can probably find a supply at thrift store. I just picked up a HP printer wall-wart transformer that has secondary 20VAC centertapped at almost an amp.

    I've never used TLO74 but its datasheet looks very good. Both opamps are specified with 2kohm load
    the 074's fet input stage should give you a better sawtooth at very low frequencies
    and I think they're pin compatible (at least in 14 pin dip package) so you could compare them easily. Its ~3 to 6 millivolts offset voltage won't matter much in your circuit, probably 741 is at least that bad.

    lm324's are so cheap i'd say get a handful for experimenting. Someplace I have a tube of fifty which should last me the rest of my life.
    lm324: http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_23683_-1
    22 cents apiece in quantity; but TL074 is only 39 cents

    I'd like to experiment with some 1725 power amps. I have been using LM12's (more current) for a three phase 60 hz power driver but they've got terribly expensive. It's worth making a new board.
    The LM12 is what you thought the 1725 to be - a general purpose opamp with 8 amp output capability, good for about 80 watts of audio. OPA541 is a possible replacement but even it's $25.

    keep us posted - this is a most interesting project.

    old jjim
     
    Last edited: Aug 31, 2013
  21. Aug 31, 2013 #20

    davenn

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    you really mean LM1875 ... not sure how the 1725 crept into this discussion
    I think Jim got sidetracked even tho he linked to the 1875 pdf :wink:

    As I have already covered the fact that the 1875 is not applicable to this process, lets get petterg
    going on the right path :smile:

    petterg ....

    The TL072 has 2 opamps in the same package ... now using the info from the datasheet, wire up the the 2 opamps in the package according to the circuit berkeman gave you right at the beginning of this thread .... NOTE the pin numbering will be different
    Page 4 of the datasheet has the pinouts for the TL072 ... top row , centre diagram

    Dave
     
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