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Why Opamp with potentiometer?

Discussion in 'General Electronics Chat' started by sagh, May 5, 2014.

  1. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Yes but one is specified as -15v rather than just plain old 15v. That also makes one of them negative.
     
  2. sagh

    sagh Member

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    Thank you so much for your help, I really appreciate it. I have no clue why the current mirror is used, either. As you said it doesn't play an important role on increasing the
    gain. besides I don't understand what Jony said!
     
  3. sagh

    sagh Member

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    When I try to regulate offset voltage of op amp, the output voltage starts oscillating.
    I want to see this kind of oscillation in pspice simulation but I have two problems,
    First, I don't know how can i find an op amp with offset pins because almost all of these
    sort of ports are unmodeled. Second, I don't know how to apply noise to the circuit in
    pspice to see the oscillation. I ran into a big problem with circuit. Please Help :-((
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    But the negative battery is shown as a positive battery.
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    If you built the circuit on a solderless breadboard then the capacitance between the many rows of contacts and the many long jumper wires all over the place might cause oscillation.

    The positive and negative supplies need a bypass capacitor very close to pin 4 and pin 11 of the opamp and close to ground to prevent oscillation.
    The unused opamps should be properly disabled to prevent them from oscillating.
     
  7. sagh

    sagh Member

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    You mean I should add capacitor to negative and positive supplies of both 2 op amps?
     

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  8. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    Yes but that should not matter for the simulation.

    Either of these...

    For the positive supply:
    1. Positive battery with positive terminal to positive supply pin of IC
    2. Negative battery with negative terminal to positive supply pin of IC

    and for the negative supply:
    3. Positive battery with negative terminal to supply pin of IC
    4. Negative battery with positive terminal to supply pin of IC

    He happens to be using choices #1 and #4 which should be ok.

    I've included a drawing with the related terminals circled in red for negative and green for positive.
     

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  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Sorry, I forgot that the power supply pins of the opamp have current mirrors that also might make them oscillate.
    Try adding a 0.1uF ceramic capacitor to ground at pin 4 and pin 11, but then the current mirrors might oscillate.

    Maybe you should get rid of the current mirrors and make a normal opamp circuit.
     
  10. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    What is the frequency of oscillation? That could tell us what is causing this.

    An interesting trial would be to connect the output of the current mirror to the inverting terminal rather than the output of the op amp.

    Also, how do you have your offset null pot connected? This op amp seems to require both 'ends' of the pot to go to the negative supply line.
     
  11. sagh

    sagh Member

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    what do you mean? in this circuit the output of current mirror connects to a resisitor,
    this resisitor goes to either output of op amp or the inverting input. I don't get what you
    meant :-((
     
  12. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    I'm sorry, i worded that wrong. I meant this:

    An interesting trial would be to connect the output of the current mirror to the inverting terminal of the first op amp, rather than having the output of the first op amp connect to the inverting terminal. The resistor 10k would still go on the output of the first op amp, but the feedback would now come from the output of the current mirror rather than the output of the op amp.

    Just in case that is still not clear, another way of saying this is to do this:
    Disconnect the output of the first op amp from the inverting terminal but not from the 10k resistor. That leaves then inverting terminal open.
    Next connect the output of the current mirror to the inverting terminal, then try running it up and see what happens. Leave the output of the current mirror connected to the 1k too and the second op amp can stay in place also. It's just that now the feedback for the first op amp comes from the output of the current mirror rather than the output of the first op amp. That's the only difference.

    This is just something to try to see how it works that way.

    To simulate this more exactly and maybe get more real world results with either circuit, build up the op amp circuit itself using the data sheet internal diagram for the op amp which uses transistors. You can use very low power signal type transistors for the parts they show in the diagram.
     
  13. sagh

    sagh Member

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    I would be grateful if you check the image I 'm not sure you meant this circuit or not. I ran this in pspice and got (-13v) as output voltage. In previous circuit I got 1.04 V
    I don't get why is there such a difference?
     

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    Last edited: May 12, 2014
  14. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    That looks right but it looks like it is causing a latch up so something else would have to be added to prevent that. The idea was to feed back the output of the current mirror in order to ensure it was corrected, if needed. We'd have to investigate this more to see why it latches up.

    In the mean time, are you saying that you get 1.04v out before this change? So it doesnt oscillate anymore then?
     
  15. sagh

    sagh Member

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    I simulated both of these circuits in Pspice and These results (1.04 and -13) have been got from simulation. In real world when I assemble on soldress breadboard and
    want to regulate the offset voltage after some time it starts oscillating! :-(
    I still don't know what's the reason of oscillating.
     
  16. sagh

    sagh Member

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    Frequency of oscillation is 2Mhz, frequency is high for this circuit I don't know what should I do to reduce it :-((
     
  17. sagh

    sagh Member

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    Would anyone please tell me why I get zero volt from this circuit in Pspice when one of input is (1v) and the other is (-1v)?!
     

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  18. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    How high is the amplitude at the output of the first op amp when it oscillates?

    One possibility is that when the op amp draws current from the power supply the current draws the voltage down and then the op amp draws a little less current then it goes back up then the current goes back up, etc. Look at the power supply rails with a scope maybe with AC coupling to see the changes.

    In your new circuit, dont you have to have one end of that 1k resistor (on the output of the lower current mirror) connected to ground? Then the output of the current mirror would also connect to the non inverting terminal of the right side op amp.
     
  19. sagh

    sagh Member

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    Sorry I don't get what you said. due to the symmetry, differential voltage should be zero I think one thing is wrong because by applying (-1v) to one of inputs I get zero volt
    at the output.
     
  20. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    In the lower section, if you apply a voltage the output current mirrors generate a current, and that current should run through a resistance so that it produces a voltage. In the top circuit this is no problem because the 1k goes to a virtual ground, but in the lower section the 1k goes to a very high impedance of 500k. That might not work. Remember the current mirrors generate a CURRENT not a VOLTAGE by their nature.
     
  21. sagh

    sagh Member

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    First of all would you please tell me why 1k in the upper section goes to virtual ground?
     

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