Simple Function Generator

Discussion in 'Homework Help' started by Mr_Edward, Jul 18, 2014.

  1. Mr_Edward

    Thread Starter New Member

    Jul 17, 2014
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    hello everyone

    so we have this laboratory project where we must build a function generator without the use of 555 timers..

    This is the circuit that I got.


    i've been looking everywhere for an easy to understand circuit for wave generation and this is the best one i could find.

    so is there any way for me to vary the amplitude of the resultant wave of the circuit?

    i got this circuit here:

    "Circuit tutorial - sawtooth generator w/ current sources, diode switches and hysteresis comparator"
    https://www.youtube.com/watch?v=ibnz5UjQ4u0

    Best Regards,
    Edward
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    function generator

    Sine? Square? Triangular?

    Fixed frequency? Adjustable frequency?
     
  3. Mr_Edward

    Thread Starter New Member

    Jul 17, 2014
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    we've been asked to do Sine, Square and Triangular with variable frequency, duty cycle, and amplitude. i've got the duty cycle adjustment from the pots on the current source and drain but i can't seem to find a place to vary the amplitude.

    any tips on how?
     
  4. Alec_t

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    Sep 17, 2013
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    Yes. Hints: (a) Consider the purpose of R7. (b) Are you familiar with the use of a potentiometer?
     
  5. Mr_Edward

    Thread Starter New Member

    Jul 17, 2014
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    i've tried using it on R7 and it just enlarges and shrinks the wave screwing up the frequency..

    i shorted the middle terminal of the pot to one of its other legs or am i missing something?
     
  6. Alec_t

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    Sep 17, 2013
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    Think about where you could add another pot.
     
  7. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    Just a quick note...

    Is that circuit schematic complete? It looks like the second 9v battery has no connection to ground. Is it missing a ground?
     
  8. Brownout

    Well-Known Member

    Jan 10, 2012
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    You'll need to add a variable gain stage.
     
  9. shteii01

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    Feb 19, 2010
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    Pot for R7?
     
  10. Brownout

    Well-Known Member

    Jan 10, 2012
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    Unfortunately, that changes the frequency and amplitude. I think the OP wants independent control over the amplitude.
     
  11. shteii01

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    Feb 19, 2010
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    Yes. I saw Alec_t suggest it. I was repeating what they said.

    About that R7. It is connected to op amp. Is that the amplifier op amp or is it something else? It sounded like one of the functions of that op amp was to amplify the signal, but like OP said, it was changing the signal. Would that mean that OP needs a buffer between amplifier and the rest of the circuit?
     
  12. Brownout

    Well-Known Member

    Jan 10, 2012
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    Its wired as a comparator. It just goes from the + limit to the - limit. Changing R7 changes the hysteresis, the switching point, but that won't change the amplitude because its already at the limits.
     
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  13. Mr_Edward

    Thread Starter New Member

    Jul 17, 2014
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    the circuit works... and the 2nd 9v is connected to a 2nd ground to the right.. I still haven't cleaned this circuit yet so it looks like a jumble of wires..
     
  14. Mr_Edward

    Thread Starter New Member

    Jul 17, 2014
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    i'm thinking of changing the comparator with a RS latch then connecting the Q' terminal to the diode switches.. a friend of mine suggested it but then again the problem lies on changing the amplitude..

    can a non-inverting amplifier be connected after the unity gain op amp?
     
  15. ericgibbs

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    Jan 29, 2010
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    Mr Ed,
    I would suggest a non inverting OPA, with a gain control, be used to vary the amplitude of the circuits output.

    E
     
  16. Alec_t

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    Sep 17, 2013
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    Another hint: U2 doesn't have to be a unity-gain amp.
     
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  17. ericgibbs

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    hi,
    This is one option, the output OPA could also be used to give some Offset.

    E
     
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  18. Mr_Edward

    Thread Starter New Member

    Jul 17, 2014
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    i haven't tried that configuration yet..thank you very much.

    can an integrator be connected to produce a sine wave? or do i have to build another circuit like colpitt's or the like?


    what did you use for your simulation btw?
     
  19. Alec_t

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    An integrator will give only an approximation of a sine wave.
     
  20. ericgibbs

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