Simple Op-Amp Circuit

Discussion in 'The Projects Forum' started by lfgrdwill, Jan 19, 2012.

  1. lfgrdwill

    Thread Starter New Member

    Dec 9, 2011
    27
    0
    Hello guys,

    I am trying to build a simple circuit using a stand alone educational op-amp (desktop type) with a DC power supply, couple resistors and a few lead wires. The op-amp is to maintain constant voltage across a potentiometer as the resistance of the this pot is varied using a T-type feedback network.

    The op-amp model is this one https://www.thesciencesource.com/store/index.php?product_id=596&type=&category=

    Our model lacks the inverting/non-inverting switch. If i make the proper connections to the positive and negative inputs of the op-amp, will it act in the inverting mode that i need it to?

    The max gain is 100 and the circuit parameters i measure with this thing hooked up and running are not even remotely matching up to the ideal op-amp analysis. All the ground connections are connected to the negative sign connection on the power supply, and my circuit begins with a source, corresponding to the positive sign on the power supply.

    Thank you for any suggestions. Is there a simple way i could verify that the op-amp is even working?
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    Make sure you aren't saturating the amplifier. For example, if you have +/-15V internal supplies, and the gain set to 100, then the biggest signal you can input will be +/-15/100 = +/-.15V or 150mV. any more than they will saturate the output.
     
  3. lfgrdwill

    Thread Starter New Member

    Dec 9, 2011
    27
    0
    Would the adjustment knob on this stand-alone op-amp be the open loop gain or closed loop gain?
     
  4. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    As far as I can tell, the amplifier should be closed-loop. The adjustment knob is one of the closed-loop gain determining resistors.
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    All external ground connections for external signals should go to the common point for the ±6V power supplies, not the negative connection. The op amp zero point is a 0V between the two supplies. If you reference your signals to the minus connection all the signals will be offset by -6V which likely saturate the op amp.
     
  6. lfgrdwill

    Thread Starter New Member

    Dec 9, 2011
    27
    0
    When i set the DC power supply to 10 Volts, i measure this set voltage across the - and + power supply connections. If i measure across the common ground and the + sign on the power supply connections, i get 0 ish volts.

    One 5000 ohm (R1) resistor is hooked up to the + connection on the power supply and a resistor. The other end of this resistor goes into the negative input of the op-amp. Out from the negative op-amp input is a 1000 ohm resistor (Rf), then a potentiometer (Rw, opposite end of the pot goes to negative power supply input), and out from the Rw input to a 200 ohm resistor (R2), then R2 is connected to the op-amp output.

    Still can't get this pot to maintain constant voltage and i am going in circles.

    I have included an image.
     
  7. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    I don't think you should be connecting external feedback resistors. Those should already be inside the box???
     
  8. lfgrdwill

    Thread Starter New Member

    Dec 9, 2011
    27
    0
    They are already inside the box, which i just realized. I thought this set-top op-amp box only represented the triangle in a simplified diagram.

    But i have to hook up some resistors (R2, Rf, Rw) if i want to watch the voltage stay constant across Rw.
     
Loading...