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Why Opamp with potentiometer?

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

  1. sagh

    sagh Member

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    Can anyone tell me why we should use potensiometer when we have op amp in circuit?
     
  2. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    Assuming you mean a basic negative-feedback inverting amplifier circuit, a potentiometer allows you to adjust the gain. Depending on the fixed resistor value, a higher resistance on the pot gives you a higher gain, and lower resistance gives you a lower gain. The gain can be calculated by the following:

    Gain = -Rf/Rin

    where Rf is the potentiometer (connected between the output and the inverting input) and Rin is the fixed input resistor. The above formula is based on the following circuit:

    [​IMG]

    So simply put, using a potentiometer allows you to adjust the level of amplification.

    Hope this helps,
    Matt
     
  3. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    The short answer is, "Yes" :)

    A potentiometer is used when the user may have to adjust some parameter in the circuit. There are many different things.that may be made adjustable by the designer.

    As Matt pointed out, one use is to adjust the gain of the op amp stage. Another use is for adjusting the output offset voltage, commonly called a "null" adjustment. This was more common years ago. There are numerous other possibilities here though just as there are uses for regular resistors. It all depends on what the designer of the circuit wanted to allow the end user to adjust. Another simple example is an adjustment for the output of a voltage regulator circuit where the potentiometer adjusts the reference voltage for the error amplifier.

    For a better idea, take a look at some circuit with op amps and potentiometers and see what you think.
     
  4. dave

    Dave New Member

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  5. sagh

    sagh Member

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    Thanks for reply :), actually I don't know why we use a potensiometer to adjust the output offset voltage? In fact how does a potensiometer change the offset voltage?
    I also have another question, why do we apply zero volt to inputs of op amp to adjust the output offset voltage?
     
  6. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    The potentiometer in the circuit I showed you does not change the offset voltage. It changes the gain, which is entirely different. If you are referring to a potentiometer being used to adjust the offset, that is often used to, for example, modulate the pulse width coming from a square wave source.

    I think at this point it would be most helpful if you show us the circuit you are referring to. We may be referring to two completely different uses for potentiometers.

    Matt
     
  7. sagh

    sagh Member

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    The circuit that I referred to is exactly the circuit u showed in the first post, I assembled this circuit on breadboard. Now I want to adjust the offset ouput voltage(DC) to zero volt. I don't know how potentiometer works in this circuit :-(
     
  8. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    Using an op amp with an offset null input allows you to adjust the output offset voltage. It changes the offset by balancing one of the internal stages in the op amp.

    A op amp without a null adjustment means you have to input a small current into say the inverting terminal with a pot and resistor. The current is adjusted and that adjusts the output voltage toward zero. The values depend on the op amp gain in the circuit. For example, if you have a feedback resistor of 100k with non inverting terminal to ground and the output is offset by +0.1 volts, then using another 100k resistor on the inverting terminal to the center arm of a pot where the other two terminals of the pot are connected to plus and minus 10 volts, the center of the pot would be adjusted to 0.1 volts too because with the gain of 1 (matching 100k resistors) that 0.1 volts will subtract from the output offset 0.1 volts so you should end up with zero volts on the output.
    If the non inverting terminal is not at ground it's a little more complicated, or if the input requires a negative voltage and the system has no negative supply voltage then it gets a little more complicated too.

    There are many op amps made these days that have an automatic adjustment that keeps the offset very very low. These work very well for DC signals. If you do need offset adjustment it is probably better to go with an op amp that has that feature. They have two extra pins for that.
     
  9. sagh

    sagh Member

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    Thanks for ur help :), I uploaded a file shows the circuit that is being assembled on breadboard, I use op amp -tlo81- but I can't adjust offset output voltage to zero volt when
    I assemble the whole circuit. I don't know how I solve this problem!? :-((
     

    Attached Files:

  10. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Try fixing the polarity of the supply voltages first. It looks like both batteries supply a positive voltage when one should supply a negative voltage. Let us know if this helps or not.
     
  11. sagh

    sagh Member

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    It doesn't help :-(
     
  12. atferrari

    atferrari Well-Known Member

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    Hola sagh,

    Are you aware of this in the datasheet?
    Correction.PNG
     
  13. sagh

    sagh Member

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    Hi atferrari,

    Yes I know how to connect opamp tlo81 with potensiometer, I don't understand why I can't adjust offset output voltage to zero volt in my circuit?!
     
  14. atferrari

    atferrari Well-Known Member

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    OK, sagh. I did not see post No. 7 by MrAl. Yes you should solve that part first.

    What do you intend to do with that circuit?
     
  15. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Ok, then why does your schematic show BOTH batteries powering the op amps with the positive terminals rather than one with positive and one with negative? If that is a sim it will not work that way.

    Oh, is that because you specified them as minus voltages (the bottom ones) ? That's ok then.

    So then which op amp are you trying to adjust, the first one or second one ?
     
  16. sagh

    sagh Member

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    I want to use this circuit as a voltage amplifier.
     
  17. sagh

    sagh Member

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    I set the second one ( the op amp on the right) that's adjusted very well. In the next step when I assemble the whole circuit, I still have offset output voltage which isn't adjusted at all! :-(( I don't know what should I do to adjust it to zero!?
     
  18. sagh

    sagh Member

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    I think I don't know sth important about Op amp, because My problem isn't solved at all. :-( Please help me.

    Thanks in advance
     
  19. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    Did you try adjusting the first stage first? That would be a good idea.
    You may find that there is not enough adjustment range on the offset adjustment.

    Also, why is R2 so high in value, is that the right value? That seems to limit gain of the first stage with R1 being 1k.
    What is the desired overall gain of the circuit supposed to be?

    BTW, what is the advantage supposed to be for using those eight transistors with this circuit?
     
  20. sagh

    sagh Member

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    I modifed the circuit a little bit and I used 4 transistors instead of 8. why is the value of R2 limiting the gain of the first stage?
     
  21. sagh

    sagh Member

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    I think changing the value of R2 doesn't make a big difference on gain of the first stage. But I 'm not totally sure would u please tell me I'm thinking right?
     

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