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Op Amp circuit

  1. Mar 11, 2009 #1
    When I connect 3V negative and 3V postive power supply to an OP07 op amp and I Connect a voltmeter to OUT of op amp and GROUND the voltmeter shows -1.8 volts
    why? I have connected anything to input I even tried shoring the input and this voltage is too high to be offset voltage. I Tried two OP07s, a tl071cp and a 741 they all show voltage between 1.5 and 1.8, why is it showing this high voltage?
     
  2. jcsd
  3. Mar 11, 2009 #2
    Connect the op amp output to the neg input, and the pos input to ground. This is a voltage follower circuit, and the output ideally should be zero volts (same as ground). because of the input offset voltage, You should read about 100 microvolts on the output to ground.
     
    Last edited: Mar 11, 2009
  4. Mar 11, 2009 #3
    Ok I will do that but then where do I connect the input signal it will be an antenna or dynamic mic.
     
  5. Mar 11, 2009 #4

    berkeman

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    Staff: Mentor

    Bob was answering your question about how to measure the input offset voltage. To connect a microphone or other sensor, un-ground the + input, and connect one lead of the mic to it, and the other end of the mic to ground. The opamp will serve as a follower for the mic voltage (a current and power amplifier, but not a voltage amplifier).

    Quiz Question -- how do you turn the opamp stage into a voltage amplifier, either inverting or non-inverting in configuration?
     
  6. Mar 11, 2009 #5
    ok thank you, I don't know how to make it a voltage amp. Using a resistor somewhere but I don't know where
     
  7. Mar 11, 2009 #6

    berkeman

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    Staff: Mentor

    Then it appears that you have some fun learning about how to use opamps ahead of you. What learning resources do you have available? What course are you working on, or is this self-study?
     
  8. Mar 12, 2009 #7
    I'm just reading online about op amps.
    How do I make it a voltage amplifier??
     
  9. Mar 12, 2009 #8
    Now that you have connected the mic signal to the positive input, disconnect the wire from the output to the negative input and in its place put a 100k resistor. Connect a 1k resistor from the negative input to the common (power supply) ground. Now connect a 1k resistor from the positive input (where the mic signal is already connected) to the same common ground. Now connect the mic ground to the common ground.

    You need to write out and solve the circuit equations

    Use V(out) = output
    V(in+) = positive input
    V(in-) = negative input
    V(sig) = signal

    Use opamp equation V(out) = G[(Vin+)-V(in-)] where G is 400,000 (for OP07)

    So V(in-) = V(out)*1k/(100k + 1k) = V(out)/101

    V(in+) = V(sig)

    So V(out) = G*V(sig) - G*V(out)/101

    re-arranging;

    V(out) = 101*V(sig) / (1 + 101/G) =approx 101*V(sig) for G>>101 voila!

    Note that the circuit gain depends on the two feedback resistors and not on G.

    Now if the opamp has a unity gain-bandwidth of 600 kHz, then the circuit gain should be close to 101 for frequencies below 6 kHz.
     
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