2 stage op amp

Discussion in 'The Projects Forum' started by Nano001, Oct 14, 2010.

  1. Nano001

    Thread Starter Active Member

    Jan 12, 2010
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    Hi. I have a 2 stage op amp design that I have some questions on. The 1st stage output is the output from a photodiode signal. This signal is then fed into a level shifted DC blocking capacitor to block out the background white light from the photodetector. This is is then fed into a second stage op-amp. The output from the 2 opamps are shown in blue (first) and green (second). I was wondering if it is possible, with the configuration I have, to pull the output to ground (it currently goes low to .7V) and tune the duty cycle to 50%. There are 2 more op-amps on this chip (LM324) that I can use. I would rather not add extra components such as flip flops or schmitt triggers for pulse shaping. Is there anyway I can acheive this? Thanks.
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. wayneh

    Expert

    Sep 9, 2010
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    Could you just put your output thru a diode, and then pull it down with a, eg. 10K resistor? Dropping across the diode would lose ~0.7v. Of course, it will be lopped off the high signal also.
     
  4. Nano001

    Thread Starter Active Member

    Jan 12, 2010
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    Bertus-I put my ouput through a LM311 comparator. The pulse gets cleaned up quite a bit, however the duty cycle is still not 50%. But you are right I can use the on chip opamp as a schmitt trigger to clean up teh pulses also. Will this give 50% duty cycle, or close to it? Since the comparator doesn't I feel the schmitt trigger will not either. I will try though.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    The input to the second opamp should be through a resistor because the (-) input is a dead short in the inverting opamp circuit.

    The (-) input of the second inverting opamp does not need the biasing parts because it is biased by the 15k feedback resistor.
     
  6. DickCappels

    Moderator

    Aug 21, 2008
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    You can get the output of the LM324 to swing to ground by adding a pull-down resistor from the output to ground. If the output of the circuit does not have to source significant current, such as if you are only driving a scope probe, the resistor can be many thousands of ohms.
     
  7. Nano001

    Thread Starter Active Member

    Jan 12, 2010
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    Thank you everyone for your input. I will implement and test.
     
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