op amp driving oscillator help

Discussion in 'General Electronics Chat' started by russpatterson, Mar 23, 2010.

  1. russpatterson

    Thread Starter Member

    Feb 1, 2010
    351
    16
    Hi There,

    I've got a ~3hz waveform coming out of an op amp (0 - 1.8V). I need to turn on an oscillator (~440hz) when the output of the waveform goes high (say anything above 1.0V).

    The problem I'm having is that when I hook the output of the opamp to the VDD of my oscillator circuit things don't work. I tried driving the oscillator current via a BJT (running a 10K resistor between the opamp output and the base) but as soon as I connect the output of the opamp to the 10K/base of the transistor I get a square wave on the output of the opamp that's unrelated to my incoming signal (square wave is ~ 1hz or below).

    So any advice on how to drive my osciallator circuit? Should I setup an opamp as a comparator and drive the BJT via that comparator?

    Power is a concern since I have 400uA at 1.8V to work with.

    Here are the two schematics (sorry for any irregularities, this is a work in progress). I had trouble with the IMG tag so here are some url's.

    http://images5b.snapfish.com/232323232fp63284>nu=3549>897>257>WSNRCG=3279;;<5;;348nu0mrj

    http://images5b.snapfish.com/232323232fp63263>nu=3549>897>257>WSNRCG=3279;;5874348nu0mrj



    Thanks!
     
    Last edited: Mar 23, 2010
  2. Audioguru

    New Member

    Dec 20, 2007
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    896
    Since you didn't post your schematic then probably nobody will guess at your connections and part numbers.
     
  3. russpatterson

    Thread Starter Member

    Feb 1, 2010
    351
    16
    Sorry. I posted some schematics in the original post.
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Simply click on Go Advanced then upload the schematics here instead of "over there".

    The input of the opamp might be destroyed if its input goes negative.
    The 10k base resistor probably does not supply enough base current for the switching transistor to turn on.
     
  5. russpatterson

    Thread Starter Member

    Feb 1, 2010
    351
    16
    Thanks for the reply. It's okay in this case if I lose the negative data. The transistor is turning on okay. I'm able to use it to power a 555 (configured to be an astable multivibrator) which I'm now using to produce the square wave.

    What is going wrong is that I get a good signal out of my opamp, then when I connect that output (from the opamp) to the 10K->transistor-GND then my wave form turns into this ugly square wave that seems "somewhat" connected to the signal I'm trying to amplify/filter with the opamp.

    I need to be able to turn on the 555 when I get the nice looking pulses shown in the good_wave.jpg pic. Check out the attached pics. The blue signal on the square wave pic is the output from the 555 @ ~200hz.

    If I just connect the opamp output through a resistor to ground I get the same behavior (ugly, unrelated square wave). Any ideas?

    Thanks!
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    896
    You make no sense.
    Your first post had a 2-transistors Mickey Mouse multivibrator oscillator with a transistor turning it on and off.
    Now you talk about using a 555 oscillator instead and an opamp somewhere without a schematic.
    Guess what. I am not a mind reader.
     
  7. russpatterson

    Thread Starter Member

    Feb 1, 2010
    351
    16
    I swapped the 2-transistor multivibrator oscillator with a low power 555 setup as an astable oscillator. I'll attach the schematic. The opamp section remains the same.
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You don't have a low power Cmos LMC555 oscillator that works from a supply as low as 1.5V, instead you have a high power ordinary LM555 oscillator that has a minimum supply of 4.5V.
    Your low resistor values cause the oscillator to use a fairly high current.

    Why are you turning the 555 on and off with its supply voltage instead of turning on and off its reset pin 4?
     
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